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

General Description

According to the information on the service sheet, this set dates from 1945. It is a quite handsome LW/MW/SW table model for AC mains. Despite active service in a workshop and a lengthy retirement in a garage, the set appeared to be in reasonable condition underneath its coat of grime. There were no signs of earlier repairs.

As typical of the period, the case is constructed of (walnut?) veneered plywood. The front is taken up with the cloth-covered loudspeaker baffle on the left and glass tuning scale on the right. An anodised aluminium strip separates the two, and an ivory plastic 'picture frame' surrounds them. Volume, tone, wave-change and tuning controls are provided in a row underneath. There is a cardboard cover for the back and a removable panel on the underside. No internal antenna or 'magic eye' tuning indicator is provided. Internally, the layout is spacious, thanks in part to there being one less valve than usual.

The electronic design of the front-end is very conventional, except that the detector diodes are contained within the output valve. There is no triode audio amplifier; the output valve is driven directly from the volume control. Despite the resulting loss of gain, the designers were still able to include some negative feedback from the output anode to the bottom end of the volume control. The switchable tone control is built into the feedback, and allows two levels of treble-cut and one of bass-cut. This is a neat idea, except that it cannot work if the volume control is set to maximum. There is a directly heated rectifier and energised loudspeaker magnet.

One cannot help wondering whether the pick-up input was of any use - about 20 V peak-peak would be needed to load the output stage fully. The audio from the detector diode is larger than usual, thanks to an increased delay voltage for the AGC detector. (This delay is developed across the cathode resistor of the output stage.)

Comments and Repair Work Carried Out

• Everything thoroughly cleaned with vacuum cleaner and paintbrush.
• There was no need to replace any cable: only the loudspeaker connections were rubber-covered, and even these were OK.
• Although the mains cable was sound, I changed it for (legal) 3-core flex.
• Electrolytic reservoir capacitors C29 and C30 were replaced within the original 'can'.
• Cathode bypass capacitor C28 was replaced with one of similar physical size (but rather larger in value and working voltage).
• Local oscillator anode resistor R3 had increased from 47 kW to 58 kW, and so was replaced.
• The loudspeaker field-coil resistance is 1 kW, not 3 kW as stated on the circuit diagram. (The voltages given in the service sheet bear this out.)
• One of the IF transformer tuning slugs broke away from the brass adjustment stud during alignment. This is a problem I have encountered before and does not have an obvious cause. It is a fiddly job taking the transformer apart and gluing the slug back on.
• Tuning mechanism cleaned and lubricated.
• Loudspeaker socket repaired by using the innards of a modern component - the original had corroded away.
• Generally the components appeared to be in good condition.
• Loudspeaker grille, etc, and tuning scale carefully washed. Unfortunately, the cloth did not survive this treatment. I have never had any trouble with the grilles of 1950s sets, which were made of a plastic material, but this set used real cloth held in place with a water-soluble adhesive. One lives and learns! I obtained an attractive replacement cloth from Sid Chaplin.
• The aluminium strip between the loudspeaker grille and the tuning dial was badly oxidised. It was revived with the help of a stainless steel pan scourer - brutal but very effective.
• Case cleaned and the appearance improved with several coats of 'Durax' clear polish.
• Control knob engraving picked out by rubbing in some gold paint.

Modification Work Carried out

• Mains lead replaced with 3-core, colour-coded flex.
• Linearity of the detector improved by adding a 10 MW resistor between the screen-grid of V2 and the detector diode - my favourite modification!

Otherwise the set seemed well designed. It gave a lively performance despite the lack of an audio amplifier stage. The appearance is good as well.

Text and Photo Copyright © 2003 Ranulph Poole

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