Measurements 2015A

MEASUREMENTS AND AN INTRODUCTION TO FAULT FINDING GOLBORNE 2015 This years’ talk is on Measurements and an Introduction to Fault Finding. I’ll start by going over my background in electronics then revisit the 2012 talk to go over various aspects of measurements and how to interpret them. This is probably one area where beginners have difficulty. I’ll also describe the effect on the unit being tested of the measurements as in some cases the act of connecting a meter can either cause the unit to fail or to start working and go on to describe various components and their failure mechanisms. In the second part I’ll go over the basics of fault finding. Now everybody will have their own views on the best way to fault find, usually built up over many years of experience and in many cases in a commercial service environment. Others will have little experience and will need significant guidance. Witness the Vintage radio forums where beginners are often guided through the fault finding and repair process remotely. In some cases it can take many posts to make a measurement and interpret the result whereas a more experienced person would probably have taken a few minutes to make and interpret the same measurement. It’s all part of the learning experience but there’s nothing better than bringing a radio or TV back to life using your own efforts. It’s probably a greater thrill for a beginner than for someone who’s been doing it for years. How many of us can remember their first successful diagnosis and repair? And finally I’ll talk about replacing components and finding alternative parts. Background Before I start I’ll briefly describe my experience in both fault finding and general electronics. I’ve been interested in electronics for most of my life and in the 1960s started doing repairs to radios and TVs for friends and relatives to supplement my pocket money. I had decided that I would rather design equipment rather than repair it so after A levels I did an electronic engineering degree at university then moved out into the big wide world earning a living in the military, commercial and most recently consumer electronics fields. During the design, and more importantly, the testing phase, my previous experience of diagnosing and repairing faults came in very useful. It is much more difficult to diagnose a fault on a brand new prototype piece of equipment than on a mature piece of equipment. Failure Analysis In my last job one of my tasks was failure analysis. This is different to repairing equipment where the aim is to find the fault, repair it and get it back to the customer as soon as possible. In failure analysis the aim is not necessarily to repair the item, although quite often the item is repaired, but to find the fault and investigate the cause. If the same fault keeps re-occurring then further investigation is undertaken and measures can be put in place to correct the manufacturing or test process in the factory. In many cases a single unit being investigated will be damaged irreparably during the course of the investigation. Of course the investigation can also show faulty components being fitted. I’ll mention a few cases I’ve been involved in to show how failure analysis and normal fault finding differ. 1 We had a case where a rectifier in the power supply went short circuit shutting the unit down. The first case took some time to investigate but it soon became a “stock” fault with many failures being recorded. I could diagnose the fault within seconds of powering the unit up. It actually took longer to open up the unit to confirm the fault than to do the diagnosis. We had information on the build dates of these units and the numbers of failures so I plotted the build date and number of failures and it soon became apparent that there were two peaks relating to certain build months. Further investigation showed the majority of faulty rectifiers had two date codes. We sent faulty rectifiers back to the manufacturer and to an external test house for investigation and eventually the manufacturer was audited by our parent company and their manufacturing process for those rectifiers was considered as not up to the required standard. The manufacturer was removed from the list of suppliers following that audit. We changed to a different rectifier manufacturer and the failure rate for units built after that date due to failure of the rectifier dropped to zero. The Safety Warning Before we go any further here’s the usual warning about the high voltages used in radios, especially valve radios and TVs. Most of the equipment described in this talk is powered from the mains, typically 230V. This voltage can be fatal if used improperly. Also the HT side of valve radios can also give you a nasty shock. Always take care when working on a mains powered unit, switching off and disconnecting from the mains before replacing components. Inevitably the unit must be powered up to make measurements and under these circumstances it will often have been taken out of its case. It is recommended that you keep one hand in your pocket when making measurements and make the connections to the test points before switching on. The use of an isolating transformer when working on AC/DC sets, this includes TVs, is recommended as in some cases only a single pole switch was fitted by the manufacturer which can result in the chassis becoming live when switched off. Note that only one item should be connected to an isolation transformer at a time. See last years talk on Power Supplies for more details. Ensure the transformer is rated for the units connected to it. 200VA is probably adequate for most radios and 1950/60s B & W TVs but up to 1000VA may be needed for first generation colour TVs which can consume up to 400W. CRT colour TVs also have a switch on surge due to the degaussing coils operating to demagnetise the CRT so the transformer must be rated to cope with that. Some of the earlier 1930s/40s TVs used mains derived EHT which will be fatal if touched. For a beginner these are probably best avoided until more experience is gained. Take care with the mains and treat it with respect for, unlike James Bond, you only live once. Hopefully this hasn’t put you off so we’ll begin with Measurements. 2 Test Equipment and Measurements This section is the environmentally friendly part of the talk as some of it is being recycled from the 2012 talk on test equipment and describes the effect of component tolerances and instruments on the measurements. I feel it’s worth repeating as these two factors can have a major influence on the measurements and highlights the need to interpret the results of any tests. Before I go over the measurements what test equipment do you need? There are many items of test equipment that can be useful for fault finding, such as signal generators and oscilloscopes but the basic minimum is a multimeter. Multimeters come in two basic flavours, analogue and digital. Each has its own advantages and disadvantages and it’s down to your own preferences. Analogue meters, such as the AVO 8, are useful where voltage are varying such as when aligning a radio whereas digital meters can give a more precise reading. Most analogue multimeters have DC and AC voltage ranges, usually up to 1000V, DC current ranges up to 10A and resistance ranges. Some analogue multimeters have AC current ranges. Range selection is usually manual. The cheaper digital multimeters are usually manual range selection with similar voltage, current and resistance ranges but often have a simple transistor tester. Higher spec digital multimeters are often auto ranging and can have additional features such as frequency and capacitance measurements. Whatever meter you have, take time to get used to it and how to operate it. Component Tolerances and Measurements All electronic components have a tolerance in their values. The most common examples are resistors and capacitors where you’ll see the values quoted as say 10kΩ ± 5% or 100µF ± 20%. When components such as resistors are manufactured no two parts will come out with the exactly the same value even though they have been made from exactly the same material and have been through the same processes. There will be a slight variation in the values. In the early days manufacturers used to measure the resistor values, and place them in various bins according to the measured value. Those that were closest to the required value, within 5%, were then marked as 5% parts (gold band) those that were within 10% were marked as 10% parts (silver band) and the rest that were within 20% of the required value had no band. The 5% and 10% parts were sold at a premium. That is why if you measured a new 20% resistor it would be between 10% and 20% of the marked value. As manufacturing processes improved components, such as resistors, could be made to much closer tolerances and in many cases the actual values could be trimmed to within 1% or better of the required values. These days 1% and 2% resistors are readily available and are relatively cheap. However the voltage rating of the most commonly used resistor types is generally unsuited to valve circuits. For most manufacturers of valve consumer electronics 20% resistors were perfectly adequate and, more importantly, were cheaper than the closer tolerance parts, which were only used in cases where the value was critical.

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