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By Associate Professor Graham Parslow the Philips 148C Was One

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By Associate Professor Graham Parslow the Philips 148C Was One By Associate Professor Graham Parslow Photo courtesy of Kevin Poulter, President of 1-IRSA The Philips 148C was one of a series of portable radios produced by Philips under different brand names, including Mullard and Fleetwood. All had the same construction and broadly similar circuitry. Their common case was an interesting design, with all controls and the dial concealed by a shutter. Finally, the case panels were made from aluminium, a fairly exotic material in the later 1940s and 1950s. We now take the aluminium can the 1940s. It is surprising that Philips The Philips 148C is a full-featured for granted since billions of them are were one of the few radio manufac- sup erhet with an RF stage. With the made every year. However the first turers to use it for making a case, al- exception of the full-wave rectifier aluminium products were extremely though RCA in America manufactured (which is a 6V4 in my set but could be expensive. The original Auld Mug of a nearly identical case. an EZ82), all of the miniature valves 1857 (The America's Cup) was made Regardless of the lightness of alu- are battery types such as 1T4 and 1R5. of aluminium and predated indus- minium, the weight of the Philips 148C Its RF stage makes it a sensitive per- trial production that commenced is a substantial 6.3kg. even without former and it readily pulls in weak sta- in 1888. batteries. In fact, it is only the outer tions. While it might be thought that Two world wars necessitated quan- panels which are made from alu- the aluminium panels would offer a tity production of aluminium for air- minium while the end panels are of degree of shielding and would thus craft. Aluminium gathered mystique Bakelite. The chassis is steel and the reduce signal pickup by the aerial from its use in aircraft while becoming internal construction is fairly conven- coils, they have little practical effect ever more available for other uses in tional for the time. in this regard. siliconchip.com.au Celebrating 30 Years MARCH 2018 91 changing the volume control on one of these radios because of the way the shutter-operated switch made access to the volume pot so difficult. The front end has two separate loop coils for the aerial and these are built into both ends, using the Bake- lite mouldings as retainers. Although the ends look the same externally, the left and right differ internally to accommodate the coils and their ter- minations. As can be appreciated from the ac- companying photographs, having two aerial coils added to the challenge to set up this radio on the workbench, after the casing was disassembled for restoration. The short fly leads from the radio to the aerial coils must be unsoldered to remove the chassis. This means that temporary wiring is needed to connect the aerial coils for working at the bench. Circuit details The aerial coils are wired in series Since the Philips 148C has its two aerial coils integrated into the case, temporary wiring was needed to connect the coils up for testing. and act with the first gang of the three gang tuning capacitor (C1) to provide Opening the shutter on the front of achieved by inserting the mains plug RF input to the 1T4 RF preamplifier the case reveals the two controls (one into the socket on the rear of the case. valve. A second tuned circuit involv- for volume and one for tuning), at op- These switches are worked by ing (C2 and L5) feeds signal to the grid posite ends of the slide-rule dial. springs and levers that add complex- of the 1R5 mixer-oscillator valve. The The shutter actually operates the ity to the mechanical construction of 1R5 additionally receives tuned input On-Off switch while the changeover the radio. A friend who ran a radio re- from the local oscillator formed by C3 from battery to mains operation is pair shop told me that he spent a day and L8. EXTERNAL AERIAL & EARTH 020 3V4 1T4 1S5 tl 2.50TIN c¢ 9LuE L6 L7 L. I 1R5 V2 V3 V4 V5 Iti2 LIS L14 C „Let LS . • ( R 9 7 -3 C at 1 5 e/ rr C 4 / RS R G7 C31 CIS cm, III •-11• •-11—• 027 ,....--- 19itR20 2 • • , LC. TRIMMER sy gz OSC TRIMMER IV6' R16 117 ; 200. i0v TRIMMER AC mmHS C29 LAYOUT coutiEcro SE LI OSC COIL BATTERY Pun PADDING ADJ. .04 LOOxING AT SWITCH Al SHOWN IN 45-S- &c/f! PINS. BATTERY POSITION AERIAL 'TRIMMER POSITIONS BATTERY CHASSIS EARTH ONLY, A.C.MAINS CASE IS 1501.ATEO FROM KB THIS SWITCH HAS SO°. CHASSIS . Si ,S2_, OPERATED By DIAL suuTTeR MOVEMENT Apart from the full-wave rectifier which enabled operation from the 240VAC mains, all the valves are miniature battery types and it is a conventional superhet with a tuned RF amplification stage. 92 SILICON CHIP Celebrating 30 Years siliconchip.com.au The intermediate frequency is 455kHz and is selectively passed on by the first IF transformer to the IF am- plifier valve, another 1T4. The two IF transformers are of the relatively min- iature type that Philips developed in the early fifties. The preceding model, type 148 of 1950, featured full-size cy- lindrical IF coils. The tuning capacitor is also rela- tively small, with brass plates (not aluminium). Even so, this is still a cluttered layout with difficult access to many components, as can be seen from the picture of the front of the chassis with the speaker removed. The 1S5 detector and audio pream- plifier has only one diode that serves the double function of detection and providing AGC to the first two valves via resistors R6 & R1 and inductor L5. The volume control potentiometer changes the signal level fed to the grid The unrestored aluminium case with the dial shutter closed and badge removed. of the 1S5. The preamplified audio The radio does not have an on/off switch, but instead is turned on by opening signal then passes to a 3V4 pentode the shutter. output stage proving a modest level of 250mW or so. In practice this is quires 3V for the filament, however cover with a hole (see picture of the quite satisfactory when coupled to the this is two 1.5V filaments in series front of the chassis before restoration) 6-inch Rola type H speaker. and in parallel the filaments can work was replaced with acetate sheet, heat- Oddly, Philips reduced the speaker from 1.5V. moulded to shape. to a 5-inch unit in later variations of This was challenging because the this radio (model 168). To my ear, the Restoration work width needs to be precise to fit into the later ones do not sound as good as The anodised aluminium case had this one. developed a patina of green and looked The one series valves (1T4, 1R5 and tired. After cleaning with a degreaser 1S5) are the standard set for portables it was resprayed with an automotive of the late 1940s and the 1950s, used chrome finish. by almost every manufacturer. In com- Editor's note: some readers may bination with the 3V4 output pentode object to using a sprayed metal these valves were a proven combi- finish rather than restoring the nation for performance and efficient original aluminium finish. How- battery usage. They are not rare but ever, after many decades of use, the 1R5 and particularly the 3V4 are surface corrosion, deep scratches prone to fail and are becoming harder and pitting make it very diffi- to obtain. cult to obtain anything like the The radio ran at 90V HT with a original finish. Anodising drain of 10mA and 9V LT with a drain is not a simple process of 50mA. For portable operation, the and it involves dyes. This battery was a combination type incor- is a satisfactory and prac- porating 90V and 9V sections, such as tical compromise. the Eveready 753, and was connected The Bakelite end via a single plug. pieces were polished For mains operation, the 6V4 full with car wax. The wave rectifier produces the HT and yellowed dial the LT, using ballast resistors to re- duce the voltage to 9V for the series- connected valve filaments. The "one" This end view prefix on the valves indicates a nomi- shows part nal filament voltage of 1V, but these of the mains valves barely operate at one volt. on/off switch Greater than 1.2V is needed to en- which was operated by the sure efficient emission from the fila- shutter at the ments. The 3V4 (V5) nominally re- front of the case. siliconchip.com.au Celebrating 30 Years MARCH 2018 93 ing). For reasons that I suspect relate to operating at a relatively low volt- age, portable valves have a high fre- quency of failure of pin connections and I routinely clean the pins before powering up. A signal tracer at the volume control showed that detected (rectified) audio was being delivered from the 1S5. Since there was absolutely no sound from the speaker, it was detached and its voice coil checked for continuity. It was OK. Detaching the speaker also gave ac- cess to many components otherwise inaccessible (see the picture of the front of the chassis). The next fault possibility suggested by the low HT current was an open-cir- cuit primary in the output transformer. This annoyingly common fault proved to be the case. When the out- put transformer is open-circuit there This view shows the front of the chassis with speaker removed.
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