This "Recent Repair" was kindly contributed by Ranulph Poole.

General Description

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

• Everything thoroughly cleaned with vacuum cleaner and paintbrush.
• Rubber-covered cable replaced.
• Electrolytic reservoir capacitors C18 and C22, and decoupling capacitor C26 replaced.
• Stopper resistor R14 replaced, as its value had increased from 10 k to 18k.
• Tuning mechanism freed up and drive cord replaced.

Apart from the above, the components - including the wax-covered capacitors - appeared in good condition.

• Loudspeaker grille, etc, and tuning scale carefully washed. Case cleaned, the crack filled, and the appearance improved with several coats of 'Durax' clear polish.
• Complete RF/IF realignment carried out. Two of the IF tuning slugs broke away internally, and required gluing back on to their brass studs. The IF transformers are a curious design, each rather resembling two pot-cores placed end-to-end. The cores move inside paper sleeves, and can seize in position with unfortunate results if alignment is necessary.
• Pilot lamps replaced.

Modification Work Carried Out

• Mains lead replaced with colour-coded flex.
• 100R and 0.1uF bias components added between cathode of V2 and earth. (This modification was carried out by the manufacturers.)
• 6 dB negative feedback applied to the bottom end of the volume control by means of a 180R/22R potential divider across the loudspeaker output. The motivation for this was to improve the very poor low-frequency response of the audio amplifier (-3 dB at 300 Hz). The following changes were needed to keep the feedback stable.
• 10 nF capacitor added in parallel wit C15.
• 33 k resistor added in series with C31.
• 2.7 k resistor added in series with C27
• 1nF capacitors added from V4 and V5 anodes to earth.

Comments

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 the least.

Text Copyright © 2003 Ranulph Poole.

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