This is a five-valve superhet housed in a wooden (veneered ply) cabinet, manufactured 1950. B8G based valves were used. The pins on these valves are very thin and can cause noise due to poor contact. A good clean of the pins with fine wire wool plus the usual clean and lubrication of the holders usually cures any problems.   Having done 'cold' resistance checks and found no problems evident from switch-on with the series lamp, I took the plunge and connected direct. Valves lit, faint hum from speaker, then a plume of smoke from below chassis.

 There are two 1k carbon stick resistors in series linking the reservoir and smoothing capacitors, acting as a choke. Chokes are wound components, much more costly than resistors, so you can see the logic here. Why place two in series, you ask? Could it be that the makers were using up existing stocks? Possible, and sometimes it happened, but in this case no.

You would be forgiven for thinking that carbon stick resistors were quite high in power dissipation because they are physically quite large, but in fact they are only capable of dissipating a maximum of around 1 watt safely.

If the designer had specified a single 2k, it would have run hot. Fitting a wirewound resistor might have been an option but the location, below chassis, was obviously considered to be restricted and consequently heat generation a possible problem. The solution is neat and thoughtful. Each resistor drops half the voltage therefore half the power, so the heat source is spread and limited in extent and the lower cost stick resistors work fine.

Except in this case! Both these resistors had cooked. Meter tests showed a dead short across C25, the smoothing capacitor. I had previously checked the capacitor, and it had showed OK, so had it given up the ghost between my test and the switch-on? Unlikely, I thought, so I isolated the HT connection. The capacitor now read OK again. Moving down, the output valve anode voltage read reassuringly high, so the valve wasn't to blame. Following the HT line I came to C13, a typical wax-coated 0.05µF HT RF decoupler – which was short-circuit. I replaced this with a modern equivalent of 0.047µF.

Local oscillators may refuse to function if the HT is down. This gives the symptom of a 'dead' set - one where the heaters and dial lamps are lit, but little else happening. It is always worth a quick check of the available HT voltage before delving too deeply into circuitry. A handy test point is often the tags of the A F output transformer.

General chassis renovation work included restringing the dial, replacing dial lamps, checking all capacitors for condition and replacing where suspicious, thorough clean and polish of the whole chassis. To do this meant removal or partial removal of the mains transformer, the output transformer, the complete dial assembly and the tuning capacitor. The tuning capacitor was removed and washed in a solution of washing up liquid and water, dried with a hair dryer and the brass earthing fork carefully cleaned before lubrication of the bearings. The component was re-mounted by means of the original anti-vibration rubber grommets. Thankfully, these were in good condition but if they hadn't been, I would have used new standard grommets, spacing with extra grommets or parts of grommets. I would not have bolted the component rigidly to the chassis.

Rigid mounting of the tuning capacitor can cause mechanical feedback, for example from the loudspeaker. This causes in turn 'microphony' -odd tinny sounding instability due to sound from the loudspeaker physically vibrating the air-spaced vanes. For the same reason, you may find the frequency-changer valve mounted in a flexible holder, mechanically isolated from the rigid chassis by rubber bushes. This helps prevent chassis vibrations from vibrating the internal electrode structure of the valve.

Cabinet work 

The polish to the cabinet exterior was in a very poor state, deeply scratched and gouged. Beneath all of this I could see that the veneer was particularly fine, especially the bottom third of the front of the set, which had been veneered in rosewood as a contrast to the medium mahogany of the rest of the cabinet. A full strip and refinish was undertaken, using Danish Oil. The loudspeaker fabric was well past it's best and resisted all attempts to come clean, so this was replaced with a fabric in keeping in style with the original.

Knobs were cleaned and polished. The whole now looks very smart indeed, and performs faultlessly. It really is a well-made and attractive looking set, with a logical layout, good big transformers and well-rated components. Although the new loudspeaker fabric looks greyish in the above photo, it is in reality a gold weave, very suitable for the period. 

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