This "Recent Repair" was kindly contributed
by Ranulph Poole.
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.
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)
- 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
- 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
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
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The types of equipment discussed on this
website may contain high voltages and/or operate at high temperatures.
Appropriate precautions must always be taken to minimise the risk of accidents.
Last updated 14th April 2006.