Vacuum Tube Radio Repair and Restoration Guide

by David V. Gonshor, BSEE, PE

1 Copyright 2008 by David V. Gonshor

All rights reserved. No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, file sharing or by any information storage or retrieval system without permission in writing from the Publisher. Inquires should be addressed to Permissions Department, 7121 S. Jellison St., Littleton, CO 80128.

2 Table of Contents Chapter 1. Safety...... 7 Safety in the Household...... 7 Safety of the Restorer ...... 11 Chapter 2. Vacuum Tube Electronics Basics...... 13 Power Supply Types...... 13 Tuned Radio Frequency Circuits...... 15 Superheterodyne Circuits...... 16 Audio Amplifier Circuits...... 18 How to Perform an Alignment on a Superheterodyne Chassis...... 19 Vacuum Tube Numbering System...... 22 Coil and Transformer Color Codes...... 22 Battery Cable Color Codes ...... 23 Resistor Color Codes...... 24 Vacuum Tube Characteristics...... 24 Dial Lamp Characteristics...... 25 Mica Capacitor Markings ...... 26 Ballast Resistor Data...... 26 Chapter 3. Starting the Restoration ...... 29 The Schematic...... 29 Cosmetic Restoration...... 30 Survey of Condition ...... 31 What to Plan for as a Minimum Restoration ...... 33 The Complete Restoration ...... 35 What Equipment Will You Need?...... 35 Chassis Identification ...... 37 Example Restoration ...... 38 Chapter 4. Repair and Restoration Techniques...... 41 Introduction...... 41 Replacing Power Cords Plus Re-wiring On/off Switches for AC/DC Sets...... 41 How to Replace Electrolytic and Paper Capacitors...... 42 Philco Bakelite Block Capacitors...... 45 Switch contact cleaning ...... 48 How to repair coils ...... 49 Speaker Troubleshooting and Re ...... 50 Resistance Line Cord and Ballast Tube Replacement...... 53 What to Monitor Upon Power Up...... 58 Battery Set Repair ...... 59 Vibrator Power Supply Repair...... 60 Selenium Rectifier Replacement...... 61 Chapter 5. Troubleshooting Techniques...... 63 Poor or no B+ (high voltage)...... 63 How to measure socket voltages ...... 64 More sophisticated techniques such as signal injection ...... 65 Chapter 6. Obscure Problem Troubleshooting...... 66 Mis-wiring...... 66

3 Bad Solder Joints...... 66 Distortion ...... 68 Intermittent Reception ...... 69 Fading ...... 70 Steady Hum...... 71 Modulation Hum ...... 72 Unwanted Oscillation ...... 73 Superheterodyne Alignment Problems: Lack of Sensitivity, Image Interference, Double Spot Reception...... 73 Chapter 7. Further Reading...... 76 Index...... 77

4 Preface

Welcome to the interesting world of vacuum tube radio and amplifier electronics. This book will help you get the most enjoyment out of your prized vacuum tube possessions. This book was written to be different and hopefully more useful to the vacuum tube circuit restorer than other books on the subject. Fluff has not been added in favor of supplying much technical information that is most useful to the restorer.

Even if you have little or no experience in electronics, you can do a great deal to extend the life of and promote safe operation of these relics of the golden age of consumer electronics. Contrary to today’s electronic products, old radios and amplifiers from the 1920’s through 1940’s were designed and built to be repaired. Using discrete parts and relatively simple circuits, the vast majority of problems could be fixed rather easily. This still holds true today. Most needed repair parts are readily available, including vacuum tubes, transformers, capacitors, resistors and products for cosmetics. In general, sophisticated test equipment is not necessary for performing many restorations. Naturally, the more one gets involved in this fascinating hobby, the more you may wish to add to your supply of parts and test equipment.

This book is intended for use by the novice as well as the more experienced restorer. Obscure troubleshooting techniques presented herein may be useful for the expert, if only to provide new ideas and new directions for solving some of the more cantankerous problems.

If you read only a part of this book, please be sure to include the chapter on safety. Because of higher voltages and less stringent safety standards of the time, safety is an extremely important aspect of restoring vacuum tube electronics. This includes both personnel safety while working on radios and amplifiers, as well as safety of using these in the home.

The scope of this book has intentionally been limited to exclude vacuum tube television sets. While many of the techniques presented here are appropriate for televisions, the use of much higher frequencies and much higher voltages in television sets

5 require a whole new set of repair techniques, test equipment and safety considerations. Repair of vacuum tube televisions is best left to the expert.

A special thanks goes to William Grimm for providing a technical review of this book and providing several good suggestions for content.

6 Chapter 1. Safety

If you read nothing else in the book, please read this chapter on safety. This is because using old tube electronics is unrewarding if it harms you in some way. There are two aspects of safety to be considered. First is the safety of the radio or amplifier in use in the household. Second is the safety of the restorer during restoration of the chassis.

Safety in the Household. Underwriters Laboratories Inc. (UL) is an independent product safety certification organization that has been testing products and writing safety standards for well over a hundred years. The overall objective of UL is to promote a safe environment for people. In 1922, UL tested the first vacuum tube radio. Subsequently in 1928, UL released the first Standard for radio. So, that means that these old radios and amplifiers were built to be safe, right? Unfortunately, the answer is no. Safety standards have progressed over the decades as more became known about hazards and mishaps. There are four primary areas of design that escaped the early efforts to build safe radios and amplifiers. These are fuses, resistance line cords, polarized power and line filter capacitors. Fuses. Fuses were not commonly used until the 1940’s. The chances are very good that a chassis built before World War II has no protection for shorting…A typical failure was the electrolytic

7 capacitors going bad (we’ll address this in detail in the following chapters). As a result of the electrolytic capacitors failing, a short could be placed across the power transformer, causing it to draw excessive current and fail. A fuse would not have prevented the electrolytics from failing, but would prevent the transformer from being damaged. Although somewhat remote, there is the possibility of this failure causing a fire. I have personally seen chassis’ that have been burned out from underneath. Adding a fuse not only improves safety, but protects expensive components such as power transformers. Resistance Line Cords. An innovation in the 1930’s to eliminate the need for a power transformer and therefore reduce costs, was to use the “AC/DC” power supply concept. It is called “AC/DC”, because the radio could be powered from either Alternating Current or Direct Current power. We use AC power today, but back in the 1930’s, DC was also used; hence another reason why the AC/DC power supply was used. In order to eliminate the power transformer, the tube filaments were run in series and connected to the AC or DC line. In early AC/DC sets, the tube filament voltages in total were much less than the line voltage, so additional impedance had to be inserted in series with the tube filaments. A big resistor was not an option because of the high power dissipation and heat generated (itself a safety hazard, especially if installed below the chassis). One solution to this problem was to use what is called a ballast tube, which plugs in like other tubes but contains a heating element to drop the voltage. Depending on how many tubes were used, the ballast tube itself became too hot. Another common solution was to insert a resistance wire in the line cord, to dissipate the heat over the length of the cord instead of a single location. These resistance line cords are often referred to as “curtain burners” because of the heat they generated. Fortunately, most resistance line cords nowadays have failed, and also fortunately, there are solutions which generate essentially no additional heat. It should be noted that any line cord, resistance or otherwise, that has worn spots, cracked insulation or has otherwise deteriorated, should be replaced. Another common safety issue related to the use of resistance line cords is the installation of the on/off switch on the chassis ground side of the power. The reason this was done is that the primary power to the power supply and the filament power through the resistance wire in the power cord were split apart in the power plug. The only way to switch both with a

8 single switch was to switch the return side connected to chassis ground either directly or through a capacitor. This creates a safety issue because once the switch is open, the power connection to chassis ground is lost, creating the possibility of a “hot” chassis if there is a failure such as a shorted capacitor. Polarized Power. In those AC/DC sets mentioned above, one side of the power line was often connected to chassis, or at least was connected to chassis through a capacitor. Depending on which way you plugged the power cord into the wall socket, full AC/DC power could be connected to the chassis, creating a shock hazard. One way, power neutral was connected to chassis, which is OK. Orientation the other way creates the shock hazard. Fortunately, today we all have provisions for polarized plugs in our wall sockets, so all that is needed is to install a polarized plug on the radio or amplifier so that power neutral is connected to chassis. A polarized plug is shown in Figure 1-1.

Figure 1-1. A Polarized Plug

Consistent with current technology, a three pronged plug could be used to connect the chassis to earth ground via the third prong.

9

One additional note on AC/DC sets is in order. Some AC/DC sets have components that are exposed to the AC line even when the on/off switch is in the off position. Some of the famous Zenith Transoceanic radios are noted for this. Inspect the schematic for the location of the on/off switch and note if there are components such as capacitors or rectifiers that are exposed to the AC line voltage even if the switch is off. If this is the case, it is highly recommended to never leave the set plugged in when not attended.

Line Filter Capacitors. Line filter capacitors were typically installed from the AC power input to the chassis for transformer sets, or line- to-line for AC/DC sets. Their purpose was to filter out interference that typically exists on power lines and also to protect the circuit from voltage spikes due to near-by lightning strikes. Capacitors used for this purpose both before and after World War II were the familiar wax/paper capacitors or mica/ceramic disk capacitors. The predominant failure mode of a line filter capacitor was to short. This then connects the input power directly to the chassis, creating a shock hazard. Not only should the original capacitors be removed, but new safety approved capacitors only should be installed. These new safety approved parts are capable of tolerating high voltage spikes and are designed to fail open, rather than shorting. These safety capacitors used for ordinary household use are designated X2 capacitors for line-to-line applications, and Y2 capacitors for line-to- ground applications. Safety capacitors are available in ceramic disc and metalized film versions. Remember that capacitors commonly sold for vacuum tube restoration are not safety rated capacitors. A safety capacitor is clearly marked with the UL symbol (in the USA) and will specify capacitance, voltage rating and either X2 or Y2. Only with these markings will you know you have an approved safety capacitor.

Details on installing all of these safety items are contained in the following chapters. Fires, serious injury or death caused by old radios and amplifiers were not very common. But nowadays, we have the smarts and technology to virtually eliminate these hazards, and doing so is strongly recommended. As a side note, in the mid 1970’s, the UL investigated around 10,000 incidents of TV fires which plagued the industry, resulting in the landmark federal television

10 standard which was adopted by the Consumer Product Safety Commission.

Safety of the Restorer. Old radios and amplifiers can be hazardous to repair because of the high voltages used in these circuits. In addition to the full line voltage (usually around 120 volts AC today), vacuum tube circuits have what is called “plate voltage” or “B+”, which provides the operating voltage for the tubes. In AC/DC sets, this plate voltage can be in the neighborhood of 120 volts DC, but in transformer sets, plate voltages over 400 volts DC are common. Furthermore, in order to generate such high plate voltages, power transformer secondary windings can produce up to 800 volts AC. These high voltages can be hazardous and safety precautions must be taken. Here are some rules to live by:

Rule #1. Always unplug the chassis when doing repairs. Unless you are troubleshooting and need the chassis powered to measure voltages, always unplug the chassis. Discharge any electrolytic filter capacitors unless there is a bleed path to discharge them. Rule #2. Never touch any high voltage leads or AC power connections with any part of your body. This may see obvious, but nonetheless be aware of the location of any dangerous voltages on the chassis so you can avoid touching them. Rule #3. Wear shoes with an insulating sole and never work on a wet or damp floor. Rule #4. Keep tools with insulated handles in good condition. Check the insulation for breaks or wear marks. Rule #5. Always use the “one hand rule”. Never place one hand on the chassis or a ground while using the other to probe voltages. A very good habit to get into is to place one hand in a pocket to prevent it from straying into a dangerous situation. When high current flows from one hand to the other, the heart is in the path between them. Rule #6. Don't wear any jewelry that could accidentally contact circuitry and conduct current. Rule #7. Don't work alone. Another person's presence may be essential in the event of an accident. Rule #8. If you must probe which circuits are live, put electrical tape over all but the last portion of the test probes to avoid the possibility of an accidental short which could cause damage to various components. Clip the reference end of the meter or scope to the

11 appropriate ground return so that you need to probe only with one hand. Rule #9. Use an isolation transformer if there is any chance of contacting power line connected circuits. A Variac(tm) (variable autotransformer) is not an isolation transformer (we will discuss the benefits of a Variac in a later chapter). Rule #10. Don't attempt repair work when you are tired. Not only will you be more careless, but your deductive reasoning will not be operating at full capacity. Rule #11. The use of a GFCI (Ground Fault Circuit Interrupter) protected outlet is a good idea but will not protect you from shock from many points in a line connected chassis. Rule #12. Finally, never assume anything without checking it first! Taking shortcuts can be deadly.

Although out of the scope of this book, television sets have voltages up to 35 thousand volts, and microwave ovens have voltages up to 5000 volts at high current levels. Either of these devices can be extremely dangerous to work on! One additional word of caution: Because of its excellent electrical and heat insulation characteristics, asbestos was often used inside the radio. Typically, it is found stapled or glued to a wood case either to the side of, above or underneath the chassis. If you come across a piece of asbestos, appearing as a heavy woven white cloth material, it is extremely important to NOT cut it, drill it, scrape it or in any way disturb it. If you do, asbestos particles can be released which are hazardous to the lungs. It should be disturbed as little as possible. Any attempt to remove it should be done by someone qualified in asbestos abatement. While we’re on the subject of toxic substances, selenium is toxic in large quantities, even though it is a vital substance in trace amounts. When a selenium rectifier fails, it emits an obnoxious smell and undoubtedly selenium. Refer to the section on replacement of selenium rectifiers. Cadmium was also used to many of the chassis used in old radios. Cadmium leaves a yellow-white powder on radio hardware. Cadmium is extremely toxic even in low concentrations. It should not be sanded where dust could be inhaled. Any skin contact should be immediately washed off.

12 Chapter 2. Vacuum Tube Electronics Basics

This chapter is not intended to be an introductory course in vacuum tube electronics theory. Such a course is the subject of an entire book. Good references are provided in the last chapter of this book for those interested in the theory side of vacuum tube electronics. The focus of this book is on restoration of vacuum tube electronics. This chapter provides valuable reference material which is essential in the restoration process. Included is a relatively brief introduction to the basics of vacuum tube electronics. Being able to read a schematic is extremely beneficial in the process of restoring vacuum tube electronics. If you cannot read a schematic, your abilities will be limited. However, the following introductory material should be useful to gain a basic understanding of vacuum tube circuits and begin the process of understanding a schematic.

Power Supply Types. The earliest radios contained no power supply other than a set of batteries. Supplying power from the AC wall socket had yet to be used. Commonly, three types of batteries were used, identified as A, B and C. The A battery supplied voltage to the tube filaments. There were two basic types of A batteries: a six volt lead-acid rechargeable battery for filaments that required a lot of power, such as the 01 tube, which used 1 amp of filament current per tube, later reduced to 0.25 amp per tube at 5 volts. Non-rechargeable carbon-zinc batteries were used for the X99 type tubes, which needed 3.3 volts at 0.063 amps per tube. The B battery supplied the plate voltage, and came in packages of 45 volt batteries. Typically, two 45 volt batteries were placed in series so that either 45 volts or 90 volts could be used by the various radio stages. In some early sets, 22 ½ volt batteries were also used. B batteries were later replaced by battery eliminators, which used socket power, a transformer, a cold cathode rectifier and a smoothing capacitor to supply the B voltages. A and C batteries were still required. Finally, a C battery supplied the negative grid bias. It was the simplest battery because very little current draw was required to negatively bias the tube grids. C batteries were found in packages of 4.5 volts.

13 The invention of the B battery eliminator foretold the conversion to full socket power for radios, which occurred in the later 1920’s. These new power supplies set the standard for many years to come. Figure 2-1 shows the basic full wave rectifier power supply which became the standard for the industry.

Figure 2-1. The Classic Full Wave Rectifier

Note that the power transformer will have more secondary winding(s) to supply filament voltage to the other tubes in the circuit. Figure 2-2 shows the filter circuit.

Figure 2-2. The Filter Circuit

The filter circuit provides the smoothing of the rectified alternating voltage coming from the full wave rectifier circuit. Note that the field coil of the speaker was often used as the filter choke (now called inductor) in the filter circuit of the power supply. Sometimes multiple filament voltages were required for the tubes in the set, so more than one filament winding in addition to the rectifier filament are sometimes found.

14 The power transformer was usually one of the most expensive parts in the AC powered radio circuit. Therefore, the industry devised a method by which a power transformer was not required. This was called an AC/DC power supply, because either AC or DC power could be used to power the set. Some areas of the country used DC power, so it was advantageous to have a set that could used either AC or DC. In order to make this new type of power supply work, the tube filaments had to be run in series. New tubes types with 25 volt filaments were invented so that the sum of the tube filaments was in the neighborhood of 75 volts. The audio output tube and the rectifier tube were selected to have the higher filament voltages. Ballast tubes or resistance line cords were used to provide the remaining voltage drop (replacement of these devices is the subject of a later section). Figure 2-3 shows a typical AC/DC power supply.

Figure 2-3. A Typical AC/DC Power Supply

A voltage divider, which was used in either transformer or transformerless sets, is shown at the output of this power supply. In later years, 35 volt and then 50 volt filament tubes were invented so that the ballast tube or resistance line cord were no longer necessary.

Tuned Radio Frequency Circuits. The simplest of all radio receivers is called the Tuned Radio Frequency (TRF) receiver. A block diagram of the TRF receiver is shown in Figure 2-4.

15 Antenna

Tuned RF Detector Audio Reproducer Amplification Amplifier

Figure 2-4. A Block Diagram of the TRF Receiver

The first stage, tuned RF amplification, may consist of one or more vacuum tube RF amplifiers (more than one is typical). Each amplifier has a tunable network to provide station selectivity. The detector separates the audio signal from the modulated RF signal coming from the tuned RF amplification stage. The audio amplifier increases the magnitude of the audio signal to sufficiently to drive a reproducer such as a speaker. This circuit was used in many battery sets from the 1920’s and also less expensive than AC powered sets in the 1930’s. This circuit is capable of producing very good fidelity reception.

Superheterodyne Circuits. Among the many inventions of Edwin Armstrong, the superheterodyne receiver is one of the most widely used. A block diagram of the superheterodyne receiver is shown in Figure 2-5.

16 Antenna Local Oscillator

RF Frequency IF Detector Amplification Conversion Amplification

Audio Reproducer Amplifier

Figure 2-5. A Block Diagram of the Superheterodyne Receiver

The first stage, RF amplification, was not always used and is not required to be part of the superheterodyne receiver. If used, RF amplification provides a receiver with better sensitivity. The next stage, frequency conversion, converts the received signal to a set frequency, called the intermediate frequency (IF). It does this by mixing a signal from what is called a local oscillator with the incoming received signal to produce the IF. This conversion is done such that the same intermediate frequency results, no matter what the frequency of the received signal. The IF signal is then amplified and detected. Note that the amplification of the IF always occurs at the same frequency. Detection, audio amplification and reproduction follow just as in the TRF receiver. Frequency conversion is often done using a single vacuum tube with two functions: frequency mixing and generating the local oscillator signal. Or, usually a separate vacuum tube is used to generate the local oscillator. Sometimes, frequency conversion is referred to as the mixer/oscillator or the “first detector”. The detector that follows the IF amplification is then called the “second detector”. At first blush, the superheterodyne receiver may seem unnecessarily complicated compared to the TRF receiver. However, the superheterodyne receiver has two major advantages: selectivity and

17 sensitivity. Remember that in the TRF receiver, the RF amplification is usually performed using more than one tuned amplifier. Each amplifier then must be tuned to the same frequency, requiring ganged selective circuits. In the superheterodyne receiver, the amplification of the received signal occurs after frequency conversion, always using the same intermediate frequency. This arrangement results in better selectivity and sensitivity.

Audio Amplifier Circuits. Before we get into the subject of audio amplifier circuits, let it be said that this book deals only with the basic operation of audio amplifiers, not esoteric features that audiophile experts know and debate vigorously. In the audiophile community, considerable attention is paid not only to the basic configuration of the amplifier, but on the type of resistors, capacitors, audio transformers, triode or pentode vacuum tubes, etc. that are used. These considerations are often up to the personal taste of the listener and will not be explored here.

The basic triode audio amplifier circuit is shown in Figure 2-6.

Figure 2-6. The Basic Triode Audio Amplifier Circuit

The output of the amplifier is fed to a load resistor (RL) through an output transformer. The purpose of this transformer is to impedance match the tube to the load, in this case the voice coil of a speaker. In this triode example, the bias for the tube is provided by a bias resistor in the cathode circuit. This resistor makes the grid of the tube more negative than the cathode, which is a required feature of a vacuum tube amplifier. Alternately, the cathode may be grounded to B-, and

18 the negative bias provided in the circuitry for the input signal. The input signal to the amplifier is designated as es. The battery in the diagram is the B+ supply, which may be provided by an AC power supply as described earlier.

The “push-pull” amplifier is a method of obtaining greater power output than can be obtained from a single vacuum tube. A push-pull amplifier uses two identical tubes operating together as a single stage of amplification. A typical push-pull audio amplifier is shown in Figure 2-7.

Figure 2-7. A Typical Push-Pull Audio Amplifier

In this circuit, the tubes are biased such that one tube amplifies the positive going portion of the signal and the other tube amplifies the negative going portion of the signal, thus doubling the power applied to the load as compared to a single tube amplifier. The outputs are summed and applied to the load (speaker) through the output transformer. The input signal to the grids of the push-pull amplifier tubes are applied through an input transformer, as shown in the figure, or resistance-capacitance coupled.

How to Perform an Alignment on a Superheterodyne Chassis. The process of aligning a superheterodyne chassis is not necessarily a complicated process, but it is somewhat tedious and methodical. The accepted standard sequence in performing an alignment is to first align the various tuned circuits of the IF amplifier to each other

19 and to the correct IF frequency to which the IF amplifier is designed. Next, the oscillator circuit should be adjusted at the higher end of the tuning range so that is tracks properly with the RF circuits. This is repeated at the lower end of the dial, again to ensure that the oscillator tracks properly. Finally, the RF (preselector) and first detector alignments are performed. Because an alignment starts with the IF amplifier circuits, we will discuss IF alignment first. The frequency of the test oscillator must be accurately set to the proper IF frequency. To do this, a frequency meter is required. Fortunately nowadays, a frequency counter is commonly included in the functions of a solid state multi-meter. A method must be devised to measure the output of the receiver in order to determine when the proper alignment has been achieved. An output meter is necessary because the human ear is just plain not sensitive enough to hear anything better than a doubling or halving of the sound power level. A meter is required that will respond to the audio output of the final audio amplifier. A normal AC voltmeter will meet this need. The meter should be connected across the voice coil terminals or alternately to the plate of the last audio output stage provided. If a connection to the plate is chosen, make sure to capacitively couple the meter to the plate, unless the meter itself contains capacitive coupling. A 0.1 microfarad capacitor is recommended. The antenna should be disconnected from the receiver. Inject the modulated IF signal to the grid of the mixer/oscillator (first detector) tube. Starting with the last IF amplifier stage before the second detector, adjust the IF transformer trimmers to achieve the highest reading on the output meter. Figure 2-8 shows the trimmer screws on a typical IF transformer can.

20

Figure 2-8. Trimmer Screws on a Typical IF Transformer Can

Use the smallest amplitude IF signal as possible which gives an acceptable output reading so that any Automatic Volume Control (AVC) action does not occur. Start adjustment with the secondary of the IF transformer, followed by the primary. If the receiver uses more than one IF stage, continue with the next stage, moving back towards the mixer/oscillator. Adjustment should be made with a fiber or bakelite tool so that the transformer coupling is not affected and, in some cases, so the tool does not short B+ to the can. Again remember to keep the oscillator output at the minimum level that can be detected by the output meter so that any AVC action does not occur. The test oscillator is now moved to the antenna terminal of the receiver. The oscillator is first adjusted for correct tracking at the high end of the band. The radio dial is tuned to a convenient frequency such at 1400 kHz, the test oscillator adjusted to 1400 kHz, and the oscillator trimmer is adjusted for peak response of the output indicator. If a low frequency tracking “padder” is employed in the set, the tracking is adjusted at the low end of the band, usually 600 kHz.

21 This adjustment should be made as the received dial is “rocked” slightly above and below the 600 kHz setting. In the case where the set does not have a low frequency padder, the low frequency adjustment is made first by loosening the dial shaft and rotating the dial until the 600 kHz position aligns with the oscillator or just moving the dial pointer to the 600 kHz position. Subsequently, the high end of the dial is adjusted as described above. Finally, the tuned circuits of the RF (pre-selector) stage, if employed, and the first detector or mixer stage are now aligned with each other at the high end of the band (1400 kHz for the broadcast band). This completes the alignment of the receiver.

Vacuum Tube Numbering System. Early tubes made in the USA were simply numbered in sequence starting with 01. Western Electric tubes were an exception to this and some tubes were numbered with a letter prefix. The following designation system was used in the USA beginning about 1930 and continuing thereafter. The first number was the filament or heater voltage. Then came a set of letters. The letters U, V, W, X, Y and Z indicate rectifiers. Combination like AB, AC, and AD were used when all of the single letters were used up. S as a second letter indicates single ended construction. The next number indicates the number of useful elements. In metal tubes, the shell is considered an element. Additional letters that follow are: G for glass bulb type ST-12 to ST-16 GT for glass bulb type T-9 GT/G is a glass bulb type T-9 and is interchangeable with type G Refer to a tube manual for the dimensions of glass bulb types ST-12 to ST-16, and T-9.

Coil and Transformer Color Codes. The Radio Manufacturers’ Association adopted a standard that was used by most manufacturers for the color of leads for transformers. The following Table identifies those colors.

Table 2-1. Color Coding of Leads of Transformers Transformer Winding Function Color Color 2 Center

22 Type 1 Tap Color Power Primary Black Black - Secondary Rectifier Yellow Yellow Yellow Filament and Blue Secondary Rectifier Red Red Yellow Plate and Red Secondary Filament Green Green Green Winding #1 and Yellow Secondary Filament Brown Brown Brown Winding #2 and Yellow Secondary Filament Slate Slate Slate and Winding #3 Yellow IF Primary Blue Red - Transformer Secondary Green Black Green or and Black White Audio Primary Blue Blue or B+ Red Transformer Brown Secondary Green Green Black or Yellow Note: Dual colors are usually a 50/50 striped design.

Battery Cable Color Codes

The following table lists the color coding of battery cables.

Table 2-2. Color Coding of Battery Cable Leads Battery Type Color A+ Red A- Black B+ Blue B- Yellow Intermediate B+ White C+ Brown

23 Intermediate C- Orange C- Green

Resistor Color Codes. For small carbon type resistors, numbers are represented by the following colors:

0 Black 5 Green 1 Brown 6 Blue 2 Red 7 Violet 3 Orange 8 Grey 4 Yellow 9 White

Three colors are used on each resistor to identify its value. The first two colors represent the value, for example, yellow-black is 40. The third color represents the multiplier. For example, yellow indicates that 4 zeros are added to the number given by the first two bands. There are two methods of placing the color identification on a resistor. In the first method, the body color represents the first figure of the resistance value. The end or tip color represents the second figure, and a colored band or dot near the center of the resistor represents the multiplier (number of zeros) following the first two figures. In the second method, the colors are indicated by a series of bands generally placed at one end of the resistor. The colors are read starting at one end and moving to the center. An additional color code has been established covering the tolerance of the resister. A fourth band placed on one end of the resistor indicates the following tolerances:

Gold, +/-5% Silver, +/-10% None, +/-20%

Vacuum Tube Characteristics. Vacuum tube characteristics are contained in receiving tube manuals published by RCA, General Electric, Sylvania and others. These manuals are readily available either in reprint form or in original form. Vacuum tube characteristics are also available on line at http://www.nostalgiaair.org/Tubes/. Many times a type substitution

24 can be made with no change in circuit operation. Books are available called Tube Substitution Guide Books that identify which substitutions can be made. These resources are an invaluable reference for the vacuum tube electronics restorer. Most of the time, a tube with the same type number except a different envelope (glass, metal, etc.) can be interchanged. One specific exception to this is the metal envelope type substituted for a glass type. On metal tubes, pin one is connected to the metal shell of the tube. If on the glass equivalent type, pin 1 was not ground or was used as a terminal point for other connections, then a metal-type substitution cannot be made. Keep in mind that there are industrial type equivalents and also foreign type equivalents for American tubes, and vice versa. For example, the 1622 industrial tube is the equivalent to the 6L6. Also, the GZ34 foreign tube is the equivalent to the 5AR4. Also pay attention to restrictions in acceptable substitutions, such as series only or parallel only filament connections.

Dial Lamp Characteristics. The following Table gives the characteristics of miniature dial lamps.

Table 2-3. Miniature Dial Lamp Characteristics Lamp No. Volts Amps Bead Color Base 40 6.3 0.15 Brown Screw 40A 6.3 0.15 Brown Bayonet 41 2.5 0.5 White Screw 42 3.2 0.5 Green Screw 43 2.5 0.5 White Bayonet 44 6.3 0.25 Blue Bayonet 45 3.2 0.5 Green Bayonet 46 6.3 0.25 Blue Screw 47 6.3 0.15 Brown Bayonet 48 2.0 0.06 Pink Screw 49 2.0 0.06 Pink Bayonet 49A 2.1 0.12 White Bayonet 50 7.5 0.2 White Screw 51 7.5 0.2 White Bayonet

25 Mica Capacitor Markings Mica capacitors usually are marked with three or more colored dots and with an arrow indicating the sequence in which the dots are to be read. The capacitance values are in pico-farads (micro-microfarads) and the color code is the same as the one for resistors. The three dot color code shown in a) in Figure 2-9 is used for capacitors with a working voltage of 500 volts, a tolerance greater than 10%, a value less than 100 pF and the value has only one or two significant figures. The first dot indicates the value of the first significant figure of the capacitance. The second dot indicates the second figure. The third dot indicates the multiplying factor. For capacitors having three significant figures, the five and six dot systems shown in b) through e) in the Figure are used. In these systems, the third dot indicates the value of the third significant figure, the fourth dot indicates the multiplying factor, the fifth dot indicates the tolerance in percent, and the sixth dot indicates the DC working voltage in hundreds of volts.

Figure 2-9. The Color Code System for Mica Capacitors

Ballast Resistor Data. Plug-in ballast resistors (often called ballast tubes) are coded as follows. The sequence of coding is letter(s), a number, a letter, and finally letter(s), for example K49B. The first letter refers to the type of pilot lamp used in the circuit:

K denotes a #40, 6.3 volt, 0.15 amp pilot light L denotes a #46, 6.3 volt, 0.25 amp pilot light M denotes a #51, 6.3 volt, 0.2 amp pilot light

26

A letter B prefixing K, L or M indicates ballast action on pilot light section. A letter X following K, L or M denotes a 4-prong base type mounting.

The number indicates the voltage drop across the entire resistor unit, including the pilot lamp sections, at the standard current of 0.3 amps.

The next part of the code is again a letter A, B, C, D and so on representing the circuit arrangement as defined by Figure 2-10 below.

Figure 2-10. Ballast Resistor Circuit Arrangements.

Finally the fourth section of the code is a suffix G, MG or J, where

G indicates a glass envelope MG indicates a metal-glass envelope J indicates a direct zero resistance connection between prongs 3 and 4, 6 and 7 or 5 and 3 of the base.

27

Voltage drop values and tube complements are as indicated in the following Table.

Table 2-4. Ballast Resistor Voltage Drop Values and Common Tube Complements Voltage Drop Number of 6.3 V Filament Number of 25 V Tubes Filament Tubes 80 2 1 55 2 2 49 3 2 42 4 2 36 5 2 23 3 3

28 Chapter 3. Starting the Restoration

First and foremost, never plug in an un-restored radio or amplifier. Plugging in an un-restored chassis can cause permanent damage because there may be (and often is) a short of some kind that will cause the power transformer or other parts to fail. These shorts are most often caused by failure of the electrolytic filter capacitors in the power section of the chassis. The risk of a short is moderate, and the consequence is very high, as power transformers are expensive and may be difficult to find. It is recommended that a complete inspection is performed, obviously bad parts are replaced and the electrolytic filter capacitors replaced before any attempt is made to power-up the chassis.

The Schematic. Fortunately for the hobby of collecting vacuum tube electronics, schematics are readily available for the vast majority of makes and models. It is always a good idea to have a copy of the schematic before starting any restoration. Schematics from the early days of radio into the 1950’s are contained in what are called the Rider’s Perpetual Troubleshooter’s Manuals, published by John F. Rider. This is a huge 23 volume set containing schematics of radios, amplifiers and phonographs. You can purchase original volumes, quality scans on compact disk, individual scans, or get them free on- line at http://www.nostalgiaair.org/. Older schematics are also available in a series of books called the Most Often Needed Radio Diagrams and Servicing Information compiled by M. N. Beitman. Newer electronics schematics are found in the Sams Photofact Sets, published by the Howard W. Sams & Co, Inc. Either Riders or Sams may also be found in your public library. A word about terminology used in these old schematics is in order. Back then (1920’s through the 1940’s), capacitors were called condensers, inductors were called chokes, frequency in Hertz was called cycles per second (CPS or just cycles), and pico Farads of capacitance was called micro-micro Farads. The most confusing terminology change has to do with resistance values. Old schematics referred to a thousand Ohms as M Ohms instead of k Ohms. So remember, if you see M Ohms on an old schematic, it means thousand Ohms (not megaOhms). MegaOhms was spelled out in old

29 schematics rather that using M Ohms as we do today. Are you thoroughly confused?

Common schematic symbols are shown in the following Figure 3-1.

Figure 3-1. Schematic Symbols

Cosmetic Restoration. Cosmetic restoration should be performed first. Cosmetic restoration includes: cleaning grime and dirt from the chassis, removing rust from the chassis, blowing dust and dirt from

30 underneath the chassis, painting parts such as the power transformer, cleaning the dial, etc. A clean chassis will allow you to spot other defects easier. Regarding rust removal, steel wool is very effective in removing rust and polishing metal. There is risk however that the fine strands of steel can make their way to the tuning condenser, causing shorts. These shorts can be very difficult to find and remove. If steel wool is used, an effort should be made to protect the tuning condenser during use and blowing off the chassis before the tuning condenser is uncovered.

A note of caution: some dials were made with water soluble ink. Cleaning them with water or alcohol will result in a nice clean dial, with no markings on it! It is best to do a small spot check first before cleaning a dial.

Survey of Condition. Next comes a survey of condition of the chassis. This inspection is looking for obvious problems that can be detected with the naked eye. Here are some examples of what to look for. Wax capacitors that have dripped wax indicate that the capacitor has overheated and is therefore “leaky” and must be replaced. See Figure 3-2.

Figure 3-2. A Obviously Leaky Paper Capacitor Oozing Wax

31 Black tar oozing from the bottom of the power transformer indicates it has become too hot due to a short on the secondary side of the transformer. This may or may not indicate that the transformer is bad, but it certainly indicates that the short that caused the overheating (most likely a bad electrolytic filter capacitor) must be found and fixed. Bad solder joints are often the cause of intermittent or difficult to diagnose problems. When in doubt, re-solder any suspicious connections. Use a high enough wattage soldering iron to ensure that the solder flows freely and the solder connection is solid. A 40 Watt minimum soldering iron is recommended for vacuum tube circuits. When restoring a radio, inspect the tuning condenser (see Figure).

Figure 3-1. A Tuning Condenser

A tuning condenser consists of a rotor (the plates that rotate) and a stator (the plates that are stationary). If during rotation of the rotor, the plates touch at any time, the tuning condenser will short and the radio will not work. Using a magnifying glass, view the plates to see

32 of one or more are bent and are causing shorting. Also inspect for dirt or foreign particles between the plates. This takes practice, as the gap between the plates is very small. To measure a tuning condenser for shorts, use an Ohm-meter. Place one prod on the U- shaped base of the condenser (this connects to the rotor). Place the other lead on the wire lead coming from the condenser (this connects to the stator). Rotate the condenser through the full range of motion and monitor the meter for shorts. The meter should read an open circuit. Any other reading indicates a short. The plates of the rotor can be gently bent to clear a short. This can sometimes be a tedious and difficult procedure, so try to be persistent and patient. A tuning condenser may have only two or up to four sections. Be sure to inspect/test each section.

What to Plan for as a Minimum Restoration. There are three things that usually cause a radio or amplifier to not work properly, or not at all: capacitors, capacitors and capacitors. The majority of restoration work consists of replacing capacitors. Three types of capacitors are commonly found. They are shown in the Figure below. On the left is a vintage electrolytic can, usually mounted so the can protrudes above the chassis. Next is a type of vintage electrolytic that is mounted below the chassis. The third from the left is a modern electrolytic capacitor used to replace the first two. In the middle is a wax paper capacitor, notorious for failing. The third from the right is a modern replacement capacitor, in this case a metallized polyester capacitor. Second from the right is a mica capacitor. And finally, on the far right is a ceramic capacitor. The vintage mica and ceramic usually do not have to be replaced.

33

Figure 3-2. Examples of the Three Types of Capacitors: Electrolytic, Paper and Mica/Ceramic

First and foremost, any original electrolytic filter capacitors should be replaced almost without exception on any radio or amplifier built before World War II and many built after the war. Filter capacitors contained within a power supply assembly potted with black tar, however, seem to have survived the ages well, undoubtedly due to the tar keeping moisture from the capacitors. If you have difficulty getting to the electrolytic capacitors because of potting with black tar, the chances are good that these capacitors will not have to be replaced anyway. Some paper capacitors should automatically be replaced, such as the coupling capacitor to the final audio stage (here, a “leaky” capacitor will effect the grid bias of the audio output tube and cause distortion). Vacuum tubes should be checked for emission and shorts, although most of the time you will find that the tubes are good. Finally, safety items should be added or replaced. As discussed in the chapter on safety, these items are:

- Adding a fuse to protect vital parts of the chassis and enhance safety

34 - Polarized plugs should be added to AC/DC (transformerless) chassis, with the neutral connected to the chassis side of the line - New line cords should be installed whenever the old or original cord has cracks, worn spots or has generally deteriorated

The Complete Restoration. The complete restoration consists of replacing all electrolytic and paper capacitors. In addition, resistors are checked and those that are substantially out of tolerance should be replaced. Carbon resistors have a natural tendency to drift to a higher value. This does not necessarily mean that they need to be replaced. In fact, many circuits will work properly with a wide range of resistor values (and capacitor values too). One only needs to review the many different designs using the same tube for the same function; you will find a wide range of component values that are used to get essentially the same results. If a circuit is not working properly, as determined by operation or by troubleshooting techniques such as measuring socket voltage (next chapter), consideration should be given to replacing widely out-of-tolerance resistors.

What Equipment Will You Need? The only equipment considered absolutely necessary is a voltmeter to measure the B+ voltage at initial power up. If you have a voltmeter, you probably then have a multi-meter, so you also have the ability to measure resistance values and AC voltages. Another necessary piece of equipment is an isolation transformer for use when working with AC/DC sets, which do not have the benefit of isolation through a power transformer. Figure 3-3 shows a typical isolation transformer, so called because the primary side and the secondary side windings are isolated, thus providing some protection from a line to ground shock.

35

Figure 3-3. A Typical Isolation Transformer

A tube tester is nice for checking emission (or trans-conductance) and shorts of the vacuum tubes. Tube testers are limited in their usefulness. They do not check tubes under rated load conditions and often show a tube as being good when the tube does not function properly in the circuit. A tube tester will not detect inter-element capacitance being too high, or gas (in some testers) or high voltage shorts between elements. The best test of a tube is to use the substitution technique. This is a technique where a known good tube is substituted for a suspected bad one. A simple resistance test across the filament pins of the tube will also tell if the tube has an open filament. Other optional equipment can come in handy, such as an audio and/or radio frequency (RF) signal generator for the purposes of signal injection, a Variac ™ for slow power up of a

36 chassis, a capacitor tester to determine capacitor value and leakage and finally an oscilloscope to look at waveforms. Figure 3-4 shows a picture of a Variac ™.

Figure 3-4. One Example of a Variac

Many restorers prefer to use vintage vacuum tube equipment when working on vintage tube electronics. Also, old vacuum tube test equipment if often reasonably obtainable as events such as ham fests or flea markets. My personal experience is that the vintage vacuum tube test equipment works just fine, except that I regularly use a modern solid state multi-meter instead of a passive multi-meter or a vacuum tube volt meter (VTVM) because it provides additional functions such as a true RMS reading AC voltmeter, a frequency counter and one-scale-reads-all resistance measurements. One disadvantage of using a modern (FET input) multi-meter is that often values are given on the schematic for a specific meter loading, usually 1000 Ohms/volt. The FET input multi-meter will give a higher voltage reading because the input is far greater than 1000 Ohms/volt and the meter will not load down the circuit as much as an older passive meter.

Chassis Identification. Occasionally, the restorer comes across a chassis that has no identification of make or model. This may make it

37 difficult to perform a restoration, although much can be done by reading part values from the parts themselves in order to select replacement parts. One can sift through Riders or Sams to find a similar chassis with the same tube complement and power supply type. This will get a schematic that will be similar to but probably not exactly the same as the un-identified chassis you have. Finally, a document called “The Locater” is an index by tube types of all two through eight tube AC and AC/DC schematics found in all 23 volumes of the Rider’s Perpetual Troubleshooter’s manuals. By just following the tube types and power supply type, The Locator will tell you all the makes and the location of the Riders schematic that matches.

Example Restoration. Figure 3-5 shows the underside of a chassis under restoration.

Figure 3-5. A Chassis Under Restoration

Several items can be described about the progress so far. This is a ten tube Zenith chassis. Note the fuse that has been installed to protect the power transformer. This is especially important on this set because it uses dual 6X5 rectifiers, which are known for shorting and damaging the power transformer. A fuse keep the power transformer from being damaged. Note the installation of two power supply filter

38 electrolytic capacitors in the upper right corner. The old electrolytics are completely removed from the power supply circuit. Note also the existence of many wax/paper capacitors. In a complete restoration, these would all be replaced by modern capacitors. A partial restoration would at least replace the wax/paper capacitors that are used as bypass capacitors and coupling capacitor to the audio output stage. In examining Figure 3-5, one can see why Zenith radios of this era are a favorite to work on. Nearly all components are readily accessible due to the wide-open layout under the chassis. Figure 3-6 shows the top side of the same chassis.

Figure 3-6. Top Side of a Chassis Under Restoration

The power transformer is located on the lower left of the chassis. Note the two IF transformer cans, one located on the bottom center of the chassis and a second just to the upper right of the first. The intermediate frequency of 455 KC (kilo Hertz) is clearly marked on the cans. This ten tube chassis uses a combination of metal and glass tubes. Three of the tubes use shields, and important feature to note

39 as missing tube shields can and probably will cause unwanted oscillations.

40 Chapter 4. Repair and Restoration Techniques

Introduction. How to proceed with the restoration is somewhat a matter of personal taste. However, certain things should be done before a power-on test is ever attempted. These are: check and replace the power cord if necessary, replace electrolytic capacitors, and clean the switch contacts on any switch decks the chassis may have. Individual sections below cover these topics. Once these tasks are performed, an initial power up test can be performed. It is not recommended to perform a complete restoration of replacing all capacitors and resistors until a power-up test is performed. It is best to get a baseline of performance, however poor it may be, before a total restoration is attempted. The reason for this is that a total restoration, such as replacing all paper capacitors, is difficult to perform without making an error in the wiring. The preferred approach, based on performing hundreds of chassis restorations, is to use the following sequence; - check/replace power cord if necessary - replace electrolytic capacitors - clean switch contacts, volume and tone potentiometers - perform initial power up to verify existence of good plate voltage - if the radio doesn’t perform, conduct troubleshooting (next chapter) - repair or replace any components necessary to get the chassis working - now begin replacement of all paper capacitors, one, two or three at a time - check chassis operation after the small number of capacitors have been replaced - continue until complete Using this approach, it is much easier to determine when a wiring mistake occurs. If one replaces all of the paper capacitors at once and then discovers that the chassis is not functional, then all of the replaced capacitors will have to be examined for errors which is no small task.

Replacing Power Cords Plus Re-wiring On/off Switches for AC/DC Sets. For safety reasons, it is imperative that a high integrity power cord and plug are used. Any cracked insulation, worn spots,

41 or corroded wires produce a safety hazard. The type of cord used for replacement is up to the discretion of the restorer. Many sets made in the late 1920’s and 1930’s used cloth covered power cords. For originality, many restorers prefer to re-wire the power cord using cloth covered cord. Cloth covered cord is available from many hardware stores and antique radio supply shops. New power plugs are then attached to the cord. As a cost savings measure, modern power cables commonly used for appliances, lamps, television sets, etc. can be used. This type of cord may also closely replicate what was originally used on the chassis made in the later 1930’s and newer. The advantage of this type of cord when restoring AC/DC radios is that they use a polarized plug. As mentioned earlier, a polarized plug is necessary on AC/DC radio for safety reasons. The neutral prong on the plug, identified by the wider spade, is wired directly to the chassis or to the return side of the power circuit if isolated from chassis by a capacitor. The on/off switch was often originally installed on the chassis ground side of the power, which is a safety hazard. Therefore, when installing a polarized power cord on an AC/DC chassis, the on/off switch must be moved to the “hot” side, not the neutral side, of the power. When this is done, placing the switch in the off position removes power from the chassis and leaves the chassis at neutral potential.

How to Replace Electrolytic and Paper Capacitors. When replacing electrolytic capacitors, we again find there are different approaches depending on the personal taste of the restorer. Electrolytic “cans” originally installed on the top side of the chassis should be left in place for cosmetic reasons. It looks odd to have a big hole in the chassis where an electrolytic can once was. Many of us save old electrolytic cans from chassis we scrap-out for the purpose of filling a hole in a chassis. The can may be “gutted’ leaving room to install new electrolytic capacitors. Modern electrolytic capacitors are much smaller and are far more reliable than original equipment. They are readily available in voltage ratings up to 450 volts DC for transformer sets, or voltage rating of 160 volts DC for transformerless sets. The process of “gutting” the old electrolytic cans is tedious and necessary only for authenticity, if that is a priority for the restorer. Otherwise, the old electrolytic capacitor is removed entirely from the circuit (leaving the can installed on the top side of the chassis) and a terminal strip is installed on the underside of the

42 chassis for the purpose of wiring in the new electrolytic capacitors. Most restorers prefer this approach as the restoration is hidden under the chassis and the wiring is perfectly rigorous. Many AC/DC (transformerless) sets had the original electrolytic capacitors installed on the underside of the chassis. In this case, replacement with modern capacitors is both authentic and straightforward. It is most important to emphasize that the old electrolytic capacitors must be removed from the circuit electrically so that any leakage they may, and probably did, have is no longer present in the circuit. There is a process for re-forming electrolytic capacitors. It works only on aluminum electrolytics that have been in storage for a long time. It will not work on capacitors where the electrolyte has dried out. This process does not always work and can be dangerous if the capacitor ruptures and leaks electrolyte. Furthermore, a replacement new stock capacitor should cost in the neighborhood of only a couple of dollars, hardly enough to warrant spending a lot of time trying to get an old part to perform as required. Therefore, re-forming electrolytic capacitors is not recommended and may actually be dangerous.

A common mistake made when replacing the electrolytic capacitors is to connect them with the polarity reversed or to reference the negative (-) side of the capacitor to the wrong point on the circuit. Not all electrolytic capacitors are connected to chassis ground. Careful inspection of the schematic may show that the negative terminal is connected to the secondary B+ winding of the transformer or to some other point where the voltage has been resistively dropped to a lower level. Failure to install the electrolytic capacitors in the correct manner may cause the capacitor to fail (burn up or vent) or for the power supply to generate excessive hum.

Replacement of paper capacitors is yet another area where the personal preference of the restorer comes into play. A “purist” will insist that the old capacitor leads are completely removed and the new capacitor is installed in it’s place. This is sometimes a very difficult procedure when a capacitor is tied to a terminal or a socket having multiple wires. It is all too easy to break the terminal strip or the tube socket by pulling on a capacitor lead in order to remove the capacitor. An alternative is to use a hook method, whereby the old capacitor leads are clipped, allowing sufficient length to form a loop at the end of the wire. The new capacitor lead is then looped in a

43 similar fashion and the union is soldered. This approach is shown in the Figure 4-1.

Figure 4-1. The Hook Method of Replacing a Resistor or Paper Capacitor With a Modern Part.

As long at the solder joint at this location is proper, this is an acceptable approach, although not authentic in the eyes of the purist. Non-polarized capacitors, usually constructed with paper and wax, are notorious culprits in non or poorly functioning chassis. They just plain were made cheaply and readily degraded over time. They are often seen with wax drips on their ends due to the leakage and heat these capacitors develop. As a minimum, certain critical paper capacitors should be replaced. A prime example is the coupling capacitor to the grid of the audio output stage. Leakage of this paper capacitor will allow plate voltage to bleed onto the tube grid, causing distortion. Any paper capacitor connected between B+ and ground is suspect in causing problems. A simple voltage survey performed with the chassis powered, using good safety precautions, is used to identify these candidates for replacement. Some restorers prefer to replace all paper capacitors to head off problems down the road.

44 Modern replacements for non-polarized capacitors are readily available from many suppliers. These capacitors are rated at 630 volts DC, are non-inductive, self-healing and most importantly built to be leakage free for many years to come. “Orange Drop ™ and metallized polyester construction are very popular. Several exotic types are also in favor for audiophiles and guitar amplifier enthusiasts. The familiar “bumble bee” capacitors, named because of their black body and colored bands, are most likely paper capacitors and therefore should be replaced. This type of capacitor is even replicated in a foil-in-bakelite fashion with the familiar bumble bee appearance.

Be sure to use safety rated capacitors for line filter capacitors. A safety rating of X2 for line-to-line and Y2 for line-to-chassis is required. These safety capacitors will prevent a failure that creates a shock hazard. Refer to the first chapter on safety.

For some of us, the sight of a wax/paper capacitor sends shutters up our spines, for they are notorious trouble makers in old vacuum tube electronics. For others, a restored chassis does not look right unless the capacitors have the appearance of the old wax/paper capacitors. In order to preserve the appearance of originality, the old capacitor, can be removed from the circuit and heated to allow the paper shell to be removed. With sufficient heat, the shell will slide right off. Then, a modern capacitor is inserted into the shell and held in-place with hot melt glue. The capacitor then looks old, but performs like a modern capacitor. This is strictly a cosmetic process, and does nothing to help the circuit perform better or safer. Whether or not you do this depends on your desire for the appearance of originality and your available time.

Philco Bakelite Block Capacitors. Philco bakelite block capacitors are a special topic because of their unique usage on Philco radios of the 1930’s. Figure 4-2 shows a typical Philco bakelite block.

45

Figure 4-2. A Typical Philco Bakelite Block

These devices were used to house capacitors and sometimes resistors in a convenient form so that the terminals on the block were used for point-to-point wiring. They came in a wide variety of configurations. They made the assembly of the chassis much simpler for Philco, but unfortunately make restoration much harder. There are two basic ways to deal with these bakelite blocks that often contain leaky capacitors. In the first method, the connections to the internal components of the block are severed, allowing new parts to be installed using the terminal points of the block. Figure 4-3 shows a close-up of the connection from the internal parts to the terminals via a small wire coming through an eyelet.

46

Figure 4-3. A Close-up Showing the Connection from the Internal Components to the Terminal Point Through the Eyelet

This wire must be removed by unsoldering it and pulling it out of the eyelet hole, thus severing the connection to the internal bad components. The second more difficult but preferable method is to physically remove the innards of the block. This is done by heating the underside of the block to melt the black wax that encapsulates the internal components. The internal components can then be removed. Figure 4-4 shows the underside of a block that has been evacuated.

47

Figure 4-4. A Philco Bakelite Block with Internal Components Removed.

New components are installed inside the block and the block installed as it was originally. It is a messy process but the finished result is excellent. Sometimes, the block can be un-bolted from the chassis and turned over without de-soldering the wires, allowing the internals to be heated and removed with little disruption.

Switch contact cleaning. Switch contact cleaner is available from many sources. A brand called Deoxit is a favorite among restorers, although effective and less costly alternatives are available. Contact cleaner should be used to clean contacts of band switches by spraying the switch decks with the power removed from the chassis. The switch should be rotated through it’s full motion while the contact

48 cleaner is still wet. The cleaner should be given time to thoroughly dry before powering-up the set. If this is not done, and the switch deck switches high voltage, arcing can occur which will carbonize a path and destroy the switch deck. Contact cleaner is also useful for cleaning on/off switches which do not switch properly. Finally, contact cleaner can be sprayed inside potentiometers (volume or tone) to remove scratchy or intermittent operation. Sometimes it is necessary to disassemble the potentiometer in order to get the cleaner inside. A somewhat risky alternative is to drill a small hole in the casing, being most careful to not penetrate the potentiometer and damage the internal parts.

How to repair coils. Coils, whether they are oscillator coils, radio frequency coils or intermediate frequency coils, are constructed with very fine wire. Figure 4-2 shows and example of this.

Figure 4-2. An Example of Very Fine Coil Wire

49 If the coil proves to be defective following a resistance test, the fine wire should be inspected for breaks or burn-through. A magnifying lens is highly recommended. Repair of these breaks is as much an art as a science. A steady hand and patience are required. Once the break in the wire is found, the end is cleaned with fine sandpaper to remove the varnish coating. Usually, it is necessary to add a new section of wire to bridge the gap. Twist the ends of the wire together and solder. It may be necessary to completely replace the winding. If that is the case, it is important to not only count the turns as you remove the old wire, but to remember the direction the wire was wound (clockwise or counter clockwise). Direction of rotation is important in coils that are phase sensitive such as oscillator coils.

Speaker Troubleshooting and Repair. Speaker troubles are very commonly encountered when repairing old radios. Aside from a split, torn or otherwise damaged paper cone (in which case the cone should be replaced if a repair is not possible), the most common problem encountered is a rubbing voice coil. The voice coil consists of several turns of wire wound around a cylindrical form at the apex of the speaker cone. The voice coil typically measures a few ohms in resistance. The cylindrical form on which the voice coil is wound fits into a circular air gap with a very small amount of clearance. If it is not centered properly, the coil rubs on the pole and creates distortion. Such a condition can be easily detected by performing a simple test. Gently and uniformly force the cone downwards and listen carefully. The speaker should be grasped around the top cone frame and the thumbs used to depress the cone. The cone should be pushed down with an even force around the circumference of the cone. The cone should be able to move in and out without generating a rubbing or scratching sound, which will be amplified by the cone. If the cone does not move silently, it is rubbing. There are two types of voice coil centering arrangements; either an internal or external “spider” is used. The spider is the structure that connects the apex of the cone to the speaker frame or pole piece. In order to re-center the voice coil, it is necessary to make the spacing between the voice coil and the pole piece the same at every point. To do this for an internal spider, loosen the screw which passes through the center of the spider and screws into the center of the pole piece. An internal spider is shown in Figure 4-3.

50

Figure 4-3. An Internal Speaker Spider

Insert three thin cardboard or plastic shims of proper thickness in the cutouts in the spider, so that they fit in between the voice coil and the center pole piece. Plastic shims seem to work better as they are more stiff than paper. This is shown in Figure 4-4 for an external spider (the spider is not seen in the picture).

51

Figure 4-4. Speaker Shims Center the Voice Coil in the Air Gap

This forces the voice coil to take a centered position in the air gap. For an internal spider, tighten the center screw, remove the shims and check for rubbing as above. In the case where the spider is external and connects between the outside of the voice coil and the speaker frame, fastening screws are usually provided. In a manner similar to that described above for the internal spider, the voice coil is re-centered and the fastening screws are re-tightened. Again, the shims are removed and the voice coil is checked for rubbing. If this procedure fails to align the voice coil, repeat the procedure. If the voice coil still rubs, it is possible that the speaker cone has become distorted, possible due to getting wet. In this case, the spider should be aligned as above, tightening the fastening screw(s), but leaving the shims in place. The speaker cone should then be cut with a razor blade around its perimeter. The cone will then align with the center due to the action of the shims. A bead of silicone caulk or other suitable adhesive is then place around the cut in the cone. Allow to dry overnight and test for centering. Replacing a bad voice coil or a damaged cone is best left to the expert, as replacement parts are not readily available. If you have the necessary materials or are willing to

52 sacrifice another speaker to get the parts you need, re-attachment of the cone can be done using the general procedures described above. Another case where an expert should be employed or a new speaker found is when the field coil of the speaker is open circuit. It is theoretically possible to disassemble the speaker by removing the cone, drive the pole piece out of the frame and removing the field coil. The field coil can then be replaced (provided parts are available) or repaired (see the section on repairing coils), re-installed in the speaker and the speaker re-assembled. This is a tedious process not for the faint of heart!

Resistance Line Cord and Ballast Tube Replacement. Elimination of the original resistance line cord or ballast tube is often necessary because they no longer work, replacement parts cannot be found and/or a reduction in heat generated is desired (either for safety reasons or to protect the cabinet from the effects of excessive heat). Fortunately, there are ways that the equivalent of a resistance line cord or ballast tube can be had with modern replacement parts. Two common ways are using a solid state diode or using a capacitor. A typical resistance line cord filament schematic is shown in Figure 4-5.

To Rectifier Plate Resistance 6.3 volt Line Cord 25 volt 6.3 volt 6.3 volt Lamp 150 Ohm filament filament filament

6.3 volt 6.3 volt AC 0.05 filament filament Input MFD Lamp Resistor

On Off Chassis Switch Ground

Figure 4-5. A Typical Resistance Line Cord Filament Schematic.

53 The primary objective is to reduce the AC line voltage (typically around 120 Volts AC) so that the voltage applied to the series filament string is correct. It is important to understand that the reduction in voltage is to be applied only to the series filament string, not the rectifier that produces the B+ voltage. Some people have trouble understanding that a capacitor can reduce the AC line voltage just as well as a resistor. Both a capacitor and a resistor produce an impedance to alternating current, and therefore introduce a voltage drop. The advantage that a capacitor has is that it produces a voltage drop with no heat generated. All one needs to do is find a capacitor of the correct value and correct voltage and insert it into the filament string. A polarized capacitor such as an electrolytic is not correct for this application. A DC rated unpolarized capacitor is better, but they are difficult to find in the 5 to 10 microfarad range of values at a high enough DC voltage rating to cover the peak-peak swing in the AC voltage. The preferred capacitor is an continuously rated AC voltage type of capacitor. These are used as motor run capacitors and are available in several values. They are usually rated at 250 working volts AC, so have plenty of margin for radio use. Choosing the correct value of capacitance is a bit more tricky. First, sum up all the values of the filament voltages, and include any pilot lamps in the sum. Unfortunately, one cannot just subtract this sum from 120 VAC to determine the correct value for the capacitance voltage drop. The reason for this is that the voltage across the filaments and the voltage across the capacitor are not in phase. Without going into some complicated mathematics, Figure 4-6 provides a graph to be used to determine the correct capacitance value based on the sum of the filament (and pilot lamp) voltages.

54

12

11

10 MicroFarads

9

8

Capacitance, 7 50 60 70 80 90 100 Sum of Filament and Pilot Lamp Voltages

Figure 4-6. Use This Graph to Select the Correct Value of Capacitance Based on the Sum of the Filament and Pilot Lamp Voltages

The capacitor is then inserted in place of the resistance line cord or ballast tube. The schematic is shown in Figure 4-7. Note the on-off switch has been moved to the high side of the line. A polarized plug should be used to keep the chassis at neutral potential.

55 Zener Diode To Rectifier Plate 7 MFD On Off 250 VAC 25 volt 6.3 volt 6.3 volt Switch Capacitor filament filament filament 6.3 volt Lamp 6.3 volt 6.3 volt AC 0.05 filament filament Input MFD Lamp Resistor

Chassis Ground

Figure 4-7. The AC Rated Capacitor Replacement of the Resistance Line Cord or Ballast.

An example of an AC rated capacitor is shown in Figure 4-8. This capacitor is approximately 2 inches by 1 inch by ½ inch.

Figure 4-8. An AC Rated Capacitor

56 A final check of the restoration is made by measuring the filament voltages and comparing them to the intended values. Measured values within ten percent of ideal are acceptable. Two things are recommended here. First, a Variac ™ is recommended to slowly bring up the AC voltage. If something is not right, this will prevent you from burning out tube filaments. Second, a “True RMS” reading AC voltmeter is needed to make the correct readings.

The second way to eliminate the need for a resistance line cord or ballast tube is to use a solid state diode. The diode should be rated above the series filament current, typically 0.3 Amps. A diode with a 1 Amp or higher rating is recommended. The venerable 1N4007 diode will work well for 0.3 Amp filament tubes. The effect of the diode is to cut the AC line voltage in half (approximately). So, 120 VAC line voltage becomes approximately 60 VAC. As an example, let’s say a radio has two 25 volt filament tubes, and four 6.3 volt filament tubes, for a total filament voltage of roughly 75 volts. It would seem the use of the diode will result too low a voltage for the filament string (60 volts provided compared to 75 volts needed). However, the tube filament voltages tend to be self regulating, because a low filament current will result in a lower resistance of the filament, thus increasing the current. In reality, the example above will provide very close to the correct filament voltages. Power-up precautions as for the capacitor solution above should be used. The diode solution tends to give poorer results as the required filament voltage gets higher than the example of 75 volts. In that case, the capacitor solution is recommended. A schematic of the diode replacement of the resistance line cord is shown in Figure 4-9. In the circuit shown in Figure 4-9, the sum of the filament and lamp voltages is 56.5 volts, so the diode solution will work well.

57 Zener Diode To Rectifier Plate

On Off 25 volt 6.3 volt 6.3 volt Switch Diode filament filament filament 6.3 volt Lamp 6.3 volt 6.3 volt AC 0.05 filament filament Input MFD Lamp Resistor

Chassis Ground

Figure 4-9. The Diode Replacement of the Resistance Line Cord

One drawback of these remedies is that pilot lamps tend to suffer. The reason for this is that as the tube filaments warm up, their resistance increases. Initially, there is a surge of current because the filaments are cold. The resistance line cord or the ballast tube tended to mitigate this surge. The diode or capacitor do not mitigate the surge, and the pilot lamp will temporarily be overdriven. This will result in substantially lower lamp life. The easy solution to this problem is to place a zener diode across the lamp, thus limiting the voltage. Because of how the zener diode behaves on an AC waveform, a diode with a clamping voltage above the lamp voltage will provide the best results. For a 6 volt bulb, use a 9 volt zener diode rated at five watts or more.

What to Monitor Upon Power Up. Never power up without replacing the electrolytic capacitors first. Place a DC voltmeter across the last electrolytic capacitor in the power supply. Power up is best performed using a Variac™ by slowly increasing the AC line voltage until the rectifier begins rectification and B+ is generated for the circuits. Using this method, one should see B+ voltage generated by the power supply at a level of a few volts, slowly increasing as the rectifier warms up and the AC voltage is increased to a level of several hundred volts. Check the schematic to find out what the

58 proper B+ voltage will be. Remember that power line voltages have increased throughout the years. A chassis designed to operate on 110 VAC will produce higher B+ voltages when operated at 125 VAC. If the power supply fails to generate proper B+, the chassis should be quickly powered-off and troubleshooting should begin. Do not let a shorted condition which prevents proper B+ to exist any longer than is absolutely necessary before power is shut off. If you do not have a Variac™, don’t despair. Another acceptable technique is to again instrument the power supply with a DC voltmeter prior to power-on. Place the meter across the last electrolytic capacitor in the power supply. Turn the power on and monitor the voltmeter, being ready to quickly turn the power off if necessary. You should see the B+ voltage begin to rise as the rectifier tube warms up. The voltage should rise within a few seconds to several hundred volts, but usually not much over 400 volts. As the various stages of the chassis begin to operate and draw current from the power supply, the B+ voltage will gradually decrease until the B+ specified on the schematic is reached (make allowance for the modern higher AC line voltages). Again, do not let a shorted condition which prevents proper B+ to exist any longer than is absolutely necessary before power is shut off. If the power supply fails to generate proper B+, the chassis should be quickly powered-off and troubleshooting should begin.

Battery Set Repair. The typical 3-dial battery sets of the 1920’s require almost no repair to operate properly, with one exception. Separate A, B and C batteries are provided to power the set, so there is no power supply to worry about. Most of these sets are TRF sets; only a few were superheterodynes and these are nearly always RCA brands as RCA held the superheterodyne patent. The TRF set circuitry was simple: three stages of tuned RF amplification, detection and audio amplification for a five tube set. Due to the simplicity and ruggedness of the parts, only one trouble is commonly found. That is an open circuit on one of the windings of the audio inter-stage transformers. A typical inter-stage audio transformer is shown in Figure 4-10.

59

Figure 4-10. A Typical Inter-stage Audio Transformer

The section on repairing coils above contains information which may be useful on repairing an open winding on this type of transformer. However, it often becomes necessary to substitute a new transformer. Small replacement transformers are available that are small enough to fit inside the original can, thus preserving cosmetic originality. The original transformers are usually a 3:1 winding ratio to increase amplification of the audio going to the audio amplifier. However, the winding ratio and the winding impedances are not critical, and almost any winding ratio will work.

Vibrator Power Supply Repair. Radio sets operated from 6 volt or 12 volt lead acid batteries were common for automobile use and also for “farm” use, where the tractor battery served a dual purpose when it was brought in at night to power the radio. These sets used mechanical vibrators to periodically interrupt the battery voltage, so that a transformer could be used to step-up the voltage for use as a B+ supply. A picture of a typical vibrator is shown in Figure 4-11.

60

Figure 4-11. A Typical Vibrator

The mechanical vibrator incorporates points similar to old fashioned automobile ignition points. When the set is turned on, a humming sound is heard as the vibrator armature opens and closes the points. The points are subject to wear caused by arcing as the voltage is interrupted. This is often the cause of failure in old car and farm radios. These vibrators are no longer manufactured (and haven’t been for many years) so finding a replacement mechanical vibrator can be very difficult. Fortunately, modern solid state replacement vibrators are available in a wide variety of configurations and use transistors to switch the voltage. Therefore, they are silent in operation. Replacement of filter capacitors as for an AC powered radio is also necessary. In addition, a “buffer” capacitor connected across the secondary of a vibrator transformer to suppress voltage surges that might otherwise damage other parts in the circuit should always be replaced whenever the vibrator is replaced. Special high voltage (1600 volt) capacitors are available for this purpose.

Selenium Rectifier Replacement. A selenium rectifier is an early type of semiconductor diode. They are commonly found in many

61 types of equipment before the modern silicon diode came into use. Figure 4-12 shows a typical selenium rectifier.

Figure 4-12. A Typical Selenium Rectifier.

When a selenium rectifier fails, it emits a very obnoxious odor, plus possibly selenium itself. Selenium in trace amounts is necessary for our cells to function, but in large amounts is toxic. So, it is advisable to replace selenium rectifiers with silicon diodes. A common 1N4007 diode will usually suffice. A selenium rectifier provides a much larger forward voltage drop than a silicon diode. Therefore, it may be necessary to add a voltage dropping resistor to reduce the plate voltages to design levels. A simple voltage test is all that is needed to determine if B+ voltages are much higher than specified in the schematic. If so, add a voltage dropping resistor of the required wattage.

62 Chapter 5. Troubleshooting Techniques

Poor or no B+ (high voltage). If after replacing the electrolytic capacitors in the power supply section of the transformer-type chassis and good B+ or high voltage is not obtained, the power transformer needs to be checked. The easy way to do this is to remove the rectifier tube and measure the socket voltages. This can be done with a Variac ™ to reduce the voltages so that they are not quite as dangerous. Remember that the AC voltage across the B+ winding of the power transformer secondary can be as high as 800 volts AC (unloaded). If a Variac ™ is not available, special care must be taken, as described in Chapter 1, when the measurements are made. A high B+ voltage is applied to the plates (anodes) of the rectifier tube. This voltage should be on the order of several hundred volts AC. With the power off, connect the AC voltmeter to the anode pins of the rectifier socket and measure the AC voltage. Also measure the filament voltage to the rectifier tube and compare it to that required for the rectifier tube type. Then measure the filament voltages to the other tubes. If the transformer fails to perform using these test methods, it must be replaced. This can be an expensive undertaking and is why special care is always taken to ensure that a short is not presented to the power transformer, which could cause it to fail. If the transformer checks out properly, then the problem of poor B+ is probably due to a short somewhere on the chassis. Disconnect the high voltage lead coming from the last electrolytic capacitor in the filter section. Power the set and measure the voltage across the last electrolytic capacitor. This voltage should be several hundred volts DC. If not, suspect the electrolytic capacitor (was it installed with the correct polarity?). Assuming this voltage is good, gradually reconnect the various parts of the high voltage circuits, monitoring B+ for correct DC voltage. The objective here is to reconnect portions of the circuit until the B+ voltage is loaded down to a low level, indicating a short condition. Remember to allow the short condition to exist for as little amount of time as possible to prevent the power transformer from failing. Once the branch of the high voltage circuit causing the short is identified, look for the failed part causing the short. This can be done probing with your ohm-meter. Suspect capacitors first, then inadvertent shorts due to foreign material (such as a solder blob) or mis-wiring.

63

How to measure socket voltages. Measurement of socket voltages is an effective way of identifying what portion of the circuitry is causing the radio or amplifier to not work properly. Socket voltages are sometimes but not always supplied along with the schematic or on the schematic itself. An example is show in Figure 5-1.

Figure 5-1. An Example of Socket Voltages Shown on the Schematic

If not, check the schematic itself to see if socket voltages are annotated there. As a last resort, refer to a tube manual to identify what is typical for the tube type under consideration. Remember two things when measuring socket voltages. First, the measured voltages will undoubtedly be higher than the schematic indicates because the AC power voltage is now higher than when the set was manufactured. Higher voltages do no indicate a problem. Second, the tube pin voltages given in a tube manual are not expected to be precisely met. In fact, a considerable variation often exists, such as twice or half the voltage given in the manual. With the power off, attach a clip lead to the negative side of the DC voltmeter and to chassis (or another point if so indicated on the schematic). Using this technique allows you to use only one hand to probe the chassis, while the other hand is kept in a safe place, such as your pocket. The socket voltages of interest are usually the plate voltage, screen grid voltage(s) and finally the grid voltage. Filament voltages usually do not have to be measured as one can quickly tell visually if a filament is not lit. Once an errant socket voltage is identified, inspect the parts in that portion of the circuit. Measure resistors, looking for one that has become open circuited or drifted substantially up in value. Look for shorted capacitors using an ohm-meter to measure

64 the resistance across the capacitor. Look for open coil windings such as the primary of an intermediate-frequency (IF) transformer. If all socket voltages are normal, proceed on to more sophisticated techniques.

More sophisticated techniques such as signal injection. Signal injection is used to identify portions of a circuit that are not functioning properly by providing correct signals to the various stages of the circuitry. An audio and radio frequency (RF) signal generator is required. However, there are some things you can do if you do not have a signal generator. Placing a finger on the grid cap of the second detector tube will cause a loud buzz to occur if the audio output stage is working properly. This can also be made to happen if a probe lead is touched to the volume control. If probe lead connected to the grid of the mixer/oscillator tube results in reception of stations, this shows that the first RF stage (if present) is not working properly. Beyond this, there is not much that can be done without a signal generator.

The recommended sequence for performing signal injection follows. Signals are injected starting with the last stage (the audio stage) moving backward towards either the antenna for a radio, or the inputs for an audio amplifier. When a new injection point is selected and the circuit fails to perform properly, then you have identified the offending stage. First start by verifying that the audio output stage is working properly by injecting an audio signal (400 Hz or 1 kHz will work just fine) into the grid of the final audio output tube. Next, inject an audio signal into the grid of the first audio tube. Then verify that the IF amplifier is working by injecting the IF signal into the grid of each IF amplifier and then the mixer oscillator. Remember to modulate the IF signal, so the detector has something to respond to. The correct IF frequency must be selected. The IF frequency is always indicated on the schematic. Many modern multi-meters have a frequency counter capable of measuring IF frequencies. Failures in the IF section are frequently due to badly out-of-alignment IF transformers, or IF transformers having an open winding. Once the offending stage is identified, inspect and test for open or badly out-of-tolerance resistors, bad capacitors, open coils and incorrect wiring (especially if a total re-cap was done).

65 Chapter 6. Obscure Problem Troubleshooting

Obscure problem troubleshooting tips provided in this chapter are intended to help the restorer come up with new ideas. No matter how experienced the restorer, we all get stuck from time to time and need fresh ideas to find the cause of the problem. Before we get into specific problems, two general topics will be discussed: mis-wiring and bad solder joints.

Mis-wiring. Sometimes we find ourselves chasing a problem that seems to have no cause. All parts check out okay or have been replaced. All adjustments have been correctly made. All voltages are measured as correct. Yet, the circuit fails to operate as intended. Mis-wiring can be caused by two events: a past restorer did the mis- wiring for whatever reason, or you did the mis-wiring when replacing parts. A total “re-cap”, that is replacing all the capacitors, is a tedious job and wiring errors are bound to occur from time to time. If you suspect mis-wiring, the only thing to do is to compare the chassis wiring against the schematic in the area where re-wiring was done. Sometimes, it helps to let the project sit for a day or two, so a fresh set of eyes can spot a problem. It helps to see it as it is, rather than how you want it to be.

Bad Solder Joints. Bad solder joints are often the cause of intermittent or difficult to diagnose problems. The term “cold solder joint” refers to a solder connection that was either not heated enough during manufacturing or repair, or where parts were moved before the solder had a chance to solidify. To locate cold solder joints, use a strong light and magnifier and examine the solder joints of components for hairline cracks in the solder around the pin. Wiggle the component if possible (with the power off). A movement at the joint indicates a problem. If you suspect an intermittent or bad solder joint is causing a problem (after the initial restoration), with the power on, gently prod suspect joints with an insulated tool to see if the problem can be affected. An ohmmeter can also be used to test for bad solder joints; remember though that if the joint is intermittent, it may test out good with an ohmmeter. When in doubt, re-solder any suspicious connections. Use a high enough wattage soldering iron to ensure that the solder flows freely and the solder connection is solid.

66 A 40 Watt minimum soldering iron is recommended for vacuum tube circuits. Bad solder joints do not necessarily result only from repair work. Bad solder joints often were made in the factory and did not cause problems for years and years of use. Sometimes it is necessary to check and re-solder nearly every connection in the receiver to locate the faulty joint. A picture of a good solder joint is shown in Figure 6-1.

Figure 6-1. A Good Solder Joint

Notice the flow of the solder completely around the wire as well as the terminal. An example of a bad solder joint is shown in Figure 6-2.

67

Figure 6-2. A Bad Solder Joint

Notice how the solder fails to flow onto the terminal. This solder joint is bound to cause problems.

The following paragraphs discuss specific obscure problems, their causes and how to remedy them.

Distortion. Locating the causes of distortion is indeed an important task, for the radio or amplifier performance will be unsatisfactory due to distortion, no matter how sensitive it is. The following paragraphs cover the most common causes of distortion and what can be done to fix them.

The first thing to suspect when localizing the cause of distortion is the audio tubes, especially the output tubes. In the case of a push pull amplifier, one tube with poor trans-conductance will cause the audio output to sound distorted. Another cause of distortion in the audio output tubes is “gas”. When gas (air) enters a vacuum tube that is supposed to be at near perfect vacuum, the introduction of unwanted grid current causes the input resistance of the tube to decrease which results in distortion. Some tube testers are capable of measuring the presence of gas in a vacuum tube by measuring the amount of grid current with the bias near zero. Note that gas in a vacuum tube does not necessarily manifest itself by a glow (other than the filament glow)

68 in the glass envelope. The best test for gas of course is to substitute known good tubes for the suspect ones.

Another cause of distortion is incorrect biasing of the output tubes. This typically occurs when the coupling capacitor becomes “leaky”, allowing plate voltage from the previous stage to leak to the grid of the output tube. A similar result occurs when the bias resistor bypass capacitor becomes leaky. This problem is easily corrected by eliminating the leaky wax paper capacitors. Incorrect bias is more difficult to measure, because the meter may load down the extremely high impedance grid circuit. When in doubt, replace the old capacitors, as this will at least prevent future problems even if the present distortion is not remedied.

Distortion can also occur due to a problem with the speaker. The problem could be a mis-alignment of the voice coil in the air gap, resulting in the voice coil rubbing on the pole piece. Refer to chapter 4 to investigate this condition. Inspection of the speaker may also show that the cone spider has broken, allowing the cone to move in incorrect ways. Spider replacement is then required. Or, the speaker cone may come literally unglued around the circumference of the cone. In this case, re-center the voice coil with speaker shims and glue the cone back into position, allowing the glue to dry before the shims are removed. Plastic cement is good for this purpose.

An incorrect alignment can also cause distortion, due to cutting of the sidebands. Incorrect alignment procedures are often a cause of distortion in high fidelity receivers, where the IF bandwidth is variable. Refer to the following section on alignment problems and how to correct them.

Micro-phonic tube “howls” are also a form of distortion. Loose elements within the vacuum tube cause distortion when vibrations are present. An easy test for micro-phonic tubes is to gently tap the tube with the eraser end of a pencil. A definite response to the tapping will be heard in a tube that is micro-phonic.

Intermittent Reception. Intermittent reception is defined when the receiver reception cuts in and out periodically. This can occur at any rate, fast cut outs or long term cut outs. Intermittent reception and

69 fading are closely related topics. Intermittent reception is often due to poorly soldered connections and intermittent shorts within bypass and coupling capacitors. Intermittent connections can sometimes be found by inspection, or by the use of an insulated tool used to tap on parts. Leads and connections should be probed with an insulated tool to attempt to replicate the intermittency. Gently move or tap parts in the suspected area.

Bad vacuum tubes are often the cause of intermittent reception. A broken heater wire may result in the tube testing satisfactorily in a tube tester, but failing in the radio after a warm up period. As the tube heats up, the broken points lose connection, causing the emission to fail; as the heater cools down, the broken points again make contact and the tube begins to function again. The cycle continues.

Fine wire connections on the lugs of RF and IF coils may become broken, making contact intermittent. Careful inspection of the fine wire connections should be done to locate this potential cause of intermittent reception. Here, a magnifying glass helps.

Leads of parts that have been disturbed enough to cause intermittent shorting should be ruled out by careful examination of the underside of the chassis.

Finally, a tuning eye tube, tuning meter or shadowgraph can be used to localize the intermittency to either the RF or audio portion of the circuit. If the audio portion of the circuit is at fault, the tuning indicator will show no variation in signal strength. If it does, then the RF portion of the receiver is suspect.

Fading. Fading is a form of intermittent reception, where the reception gradually fades out, rather that cuts out abruptly as discussed above. Please note that a gradual fall-off of signal strength is commonly encountered in short wave reception due to changes in the propagation path. This fading is not the topic of this section. Fading is usually not caused by poor solder connections, broken wires and the other causes of intermittent reception discussed above.

70 The conditions which surround the episode of fading should be examined. Does switching a light switch on/off cause the fading? Does switching the receiver off and then back on cause the faded signal to return? In this case, a leaky or intermittent bypass capacitor is usually at fault. A faulty volume potentiometer can also cause these symptoms. An extremely frustrating part of this investigation is that disturbing anything to take measurements may cause the problem to disappear.

When fading occurs only after a long period of warm up, the part causing the fading can be identified by using a localized source of heat. This is a laborious process but one that will yield results. A localized source of heat can be a heat gun or even a hair dryer or a soldering iron or gun.

Defective tubes can also cause fading. This cause is usually not discovered in the typical tube tester. Only substitution with a known good tube will identify this problem.

Steady Hum. By far the most common cause of a steady hum is bad filter capacitors in the power supply. Assuming that the filter capacitors have been replaced and installed correctly, the more obscure causes of a steady hum are addressed here. If the power supply filtering circuit uses a filter inductor (choke), it may have developed a short circuit, thereby loosing the filtering effect of the inductor. Another cause for a steady hum is an open circuit on one side of a center-tapped filament resistor (if used). Normal voltage checks will not show this problem. Individual ohm-meter checks of the center-tapped filament resistor should be made. If the filament resistor checks out OK for continuity, then make sure that it is adjusted for optimum balance and minimum hum. A steady hum can also be caused by a bad tube that checks out good on a tube tester. Poor insulation between the heater and cathode of an indirect heater type tube results in leakage and excessive hum. The best test for this condition is to swap the suspect tube with a known good one. Also, a used-up or “gassy” rectifier tube will cause hum, but this condition will be noted by lower than normal plate voltages. Some power supply designs used a capacitor to “tune” the filter inductor to a certain frequency. If this capacitor short circuits, the filter inductor will loose its effect. If this capacitor open circuits, the pitch of the hum as

71 well as the magnitude will increase. Finally, excessive hum can be caused by loose laminations of a power transformer or filter inductor core. Excessive vibrations can be felt with the hand and remedied by tightening the clamping screws for the core.

The cause of excessive hum can be localized by performing a series of tests, starting with the output stage. The audio output stage is checked by shorting the control grid(s) of the tube(s). Elimination of the hum shows that its source is within the audio output stage. Similarly, if the amplifier employs another stage of audio amplification prior to the final output stage, it should also be checked by shorting the control grid of the tube. If the hum source has still not been located, continue shorting the control grids of the preceding stages until the source of the hum is located. Each component of the offending stage should be checked, especially by-pass capacitors.

Modulation Hum. Modulation hum, as opposed to steady hum, only occurs when a station is tuned in. It is also called tunable hum. The primary cause of modulation hum is too close proximity of the AC filament leads to the RF or IF amplifier grid leads. The result is that the carrier wave becomes modulated by the power source frequency (60 Hz). This type of hum is passed on to a following stage only when a station is tuned in, because the RF or IF amplifier is not designed to pass only the raw power source frequency. AC filament leads may have been re-routed or otherwise disturbed during restoration efforts such as capacitor replacement. Careful separation of AC filament leads and grid leads will cure this problem.

By-pass capacitors which have deteriorated allow the by-pass point in the circuit to pick up power source frequency by induction, thus causing modulation hum.

In AC/DC receivers, modulation hum is caused by power lines modulating the signal voltage and re-radiating to the antenna or to other circuits. This problem is due to ineffective line filter capacitors on the input power line, either line-to-line or line to ground. It is important to use safety rated capacitors for line filter capacitors (refer to the chapter on safety).

72 Unwanted Oscillation. In this context, unwanted oscillation is an indication of instability, rather than an intentional design of the circuit such as the oscillator in a superheterodyne radio. For those radios designed to operate with a length of wire for the antenna, as opposed to a loop antenna, unwanted oscillation may occur if the antenna is too short. In this case a small antenna does not place a sufficient load on the receiver input, thus making the stage vulnerable to regenerative feedback. Bad by-pass capacitors in the plate or screen grid circuits are a very common cause of unwanted oscillations. Insufficient plate or screen grid by-pass capacitance will result in one stage of RF or IF having energy belonging to another stage. This results in energy circulating from one stage to another. This inter- stage coupling results in unwanted oscillations. The solution, of course, is to replace the by-pass capacitors. If an inductor (choke) is used in a plate circuit, and the inductor becomes short-circuited, then inter-stage coupling will occur again because energy belonging in one stage is allowed to enter another stage. A resistance check of the plate inductance should reveal if it is shorted. If so, replacement or re-winding of the coil is required.

Another frequent cause of unwanted oscillations is the break-down of the grounding system for shield cans of the vacuum tubes or transformers. Inspect the direct contact points for corrosion or, if a wire ground is used, inspect the wire for breaks or bad solder joints. Remove corrosion or re-solder wires as required.

Finally, if it is apparent that a replacement RF or IF transformer has been installed, the separation of leads may be insufficient to prevent unwanted oscillations. Separate leads which differ in high-frequency potential as far as possible should eliminate the unwanted oscillations in this case.

Superheterodyne Alignment Problems: Lack of Sensitivity, Image Interference, Double Spot Reception.

Lack of Sensitivity. If the receiver seems to be operating, evidenced by typical inter-station noise, but no stations are received, the oscillator is suspect. An easy way to test for a defective oscillator, if you have a signal generator, inject a signal into the mixer tube. The frequency of the injected signal should be higher than a received

73 station by an amount equal to the intermediate frequency. Should this test result in reception of stations, then a problem in the oscillator circuit is evident. Check coil winding resistances looking for an open winding. Substitution of a known good mixer/oscillator or oscillator tube will determine if a vacuum tube is at fault. At times, a tube which checks good in a tube checker will not perform properly in a radio (typically performing at some frequencies but not at others).

Faulty solder joints can also be the cause of poor sensitivity. This problem is especially indicated if there is reception at the low end of the tuned frequency range, but poor or no reception at the high end of the band.

Finally, a bad cathode bypass capacitor in the RF or IF sections will cause poor sensitivity.

In a Tuned Radio Frequency (TRF) receiver, poor sensitivity at the lower end of the band is an indication that a primary or secondary winding of and inter-stage transformer is open circuited. Capacitive coupling across the transformer allows the higher end of the band to work, sometimes seeming very normal. An open in the secondary of the RF inter-stage transformer is easy to detect, because the plate voltage of the preceding tube will be zero. An open in the primary of the RF inter-stage transformer can be detected by using an ohm- meter. Often, lightning is a cause for an open antenna coil or RF inter-stage transformer.

Image Interference. Any signal that mixes with the oscillator frequency to produce a signal equal to the intermediate frequency will be detected and amplified by the audio amplifier. Also keep in mind that receivers are designed so that the oscillator frequency is always higher than the signal frequency by an amount equal to the IF frequency. However, there is another condition that may occur where the signal frequency is higher than the oscillator frequency by an amount equal to the IF frequency. If this signal is strong enough to get to the mixer tube, then interference will occur between the two received signals. Note that the interference signal is always higher than the received signal by an amount equal to twice the IF frequency. The interference signal is called the image frequency of the desired signal. Image interference occurs when there is a

74 problem with the pre-selector stage that is designed to attenuate the image frequency. Failure of the pre-selector circuit resulting in image interference can be caused by: - A very strong interfering signal - Incorrect adjustment of the image frequency rejecter circuit trimmers - Incorrect trimmer adjustments on IF transformers - Incorrect tracking of RF, oscillator and mixer tuning capacitors - Ineffective shielding of the RF, mixer or oscillator circuits.

These difficulties are remedied by carefully following the manufacturer’s directions for alignment of the receiver. In the case of a very strong interfering signal, then a “trap” circuit tuned to the interfering signal frequency must be added.

Double Spot Reception. Double spot reception in a superheterodyne receiver occurs when a single station can be tuned-in at two points on the dial. Double spot reception always occurs at dial settings which are twice the value of the intermediate frequency. For example, if a receiver used an IF of 455 kHz, then the two reception points on the dial will be separated by 910 kHz. Double spot reception is a case of image interference discussed above and remedies are the same.

75 Chapter 7. Further Reading

The following additional resources cover a wide range of topics for the vacuum tube electronics restorer. Some are new publications, others are original publications which must be purchased on the used book market.

Antique Radios, the Collectors Resource, Antique Radio Forum (good source for advice)

The Locator, Copyright Gerald Larsen, 7841 W. Elmgrove Drive, Elmwood Park, IL 60707.

Modern Radio Servicing, Alfred A. Ghirardi, Murry Hill Books, 1935 Essentials of Radio, Slurzberg and Osterheld, McGraw-Hill Book Company, 1948

Radio Physics Course, Alfred A. Ghirardi, New York City Radio and Technical Plulishing Co., 1932

Fundamentals of Radio, Frederick Terman, McGraw-Hill Book Company, 1938

Everybody’s Radio Manual, How to Build and Repair Radio Receivers, Prepared by the Editorial Staff of Popular Science Monthly, Grosset & Dunlap Pubilshers, 1942

Television and Radio Repairing, John Markus, McGraw-Hill Book Company, 1953

Riders Perpetual Troubleshooter’s Manuals, John F. Rider Publisher, 23 volumes covering the 1920’s through the 1950’s.

Most Often Need Radio Diagrams, Compiled by M. N. Beitman, Supreme Publications

76

Index

AC rated capacitor 56 AC/DC power supply 15 Alignment 19 Audio amplifier 18 Ballast resistors 26 Battery sets 59 Chassis identification 37 Coil repair 49 Color codes - battery cable 23 Color codes - coil and transformer 22 Color codes - mica capacitors 26 Color codes - resistors 24 Cosmetics 30 Dial lamp characteristics 25 Distortion 68 Double spot tuning 73 Electrolytic capacitors 42 Fading 70 Full wave rectifier 14 Fuses 7 Ground fault circuit interrupter 12 Hum - modulation 72 Hum - steady 71 Image interference 73 Intermittent reception 69 Inter-stage audio transformer 23 Lack of sensitivity 73 Line filter capacitors 10 Microphonic tube 36 Mis-wiring 66 On/off switches 41 Oscillation 73 Paper capacitors 10 Polarized plug 42 Power supplies 14 Power up 35 Push-pull audio amplifier 19 Resistance line cords 8 Safety capacitor 10 Safety in the household 7 Safety rules 11 Schematics 29 Signal injection 65 Socket voltages 64 Solder joints 32

77 Speaker repair 50 Spider 50 Superheterodyne 16 Switch contact cleaning 48 Terminology 29 Test equipment 37 Tuned radio frequency 74 Tuning condenser 32 Vacuum tube characteristics 24 Vacuum tube gas 36 Vacuum tube numbering 22 Vibrator power supply 60 Zener diode 58

78