This "Recent Repair" was kindly contributed by Ranulph Poole.
This set dates from the 1946. It is a compact 'no frills' LW/MW table receiver for AC/DC mains. Although in a somewhat distressed state, it was basically sound, with little evidence of previous 'repair' work. The case had lost some of its polish and one side was split, both as a result of the heat generated by the dropper resistor. As to be expected, the electrolytic capacitors had seen better days, and there was a considerable amount of disintegrating rubber-insulated wiring.
The case is constructed of veneered plywood with a green-painted bakelite front panel on which is mounted the loudspeaker baffle and glass tuning scale. Volume, wave-change and tuning controls are provided underneath the scale. There is a cardboard cover for the back and a removable panel on the underside. The external appearance is something of an acquired taste. Internally, the layout appears rather cramped, thanks to the use of six octal valves.
The electronic design of the front-end is very conventional, with the possible exception of the frame aerial. More curious is the audio amplifier, which incorporates a phase-splitting auto-transformer and push-pull output stage. In theory, large output powers should be available, but the designers decided to under-run the valves: both the HT and heater voltages are well below those specified by the valve manufacturers, and the output power is only 1 W at most. It is not clear why the designers adopted this approach.
Repair Work Carried Out
Apart from the above, the components - including the wax-covered capacitors - appeared in good condition.
Modification Work Carried Out
The set was given to me last year as a birthday present. I confess I was less enthusiastic about repairing it than I have been about other sets: it isn't exactly a good-looker, and the sound quality isn't great either. However, apart from the large quantities of rubber-insulated wiring, very little needed replacement.
Work is awkward, partly because of the cramped layout, and partly because the hot and horrible mains dropper is separate from the chassis. Unfortunately, the various trimmer capacitors are not accessible unless the chassis is removed from the cabinet - in which case the dropper must come out as well.
The curious push-pull audio amplifier and phase-splitting transformer had a very poor frequency response. I'm afraid I did add a bit of negative feedback to improve matters.
The one real headache I had was with the
IF transformers. The trimming adjustment is by means of a short length of brass
studding to which is attached an iron dust core. If the core cannot move freely
for some reason, it breaks away from the studding. Repair is fiddly, to say
Text Copyright © 2003 Ranulph Poole.