cossor 501
bakelite
Makers:
A
C
Cossor Ltd.
Approx.
year of manufacture: 1949
Initial checks
Valve
holders,
switch noisy. Possible low HT line - power rectifier? Volume low even at
full setting of control. Howls, instability. Suspect decoupling
capacitors. Mains lead lethal.
The
cabinet
filthy but intact. Output stage needed much work, including output valve
replacement.
Chassis
restoration
in brief
The
chassis top was vacuumed whilst still in cabinet before removing to complete. The knob attached
to the tuning spindle was rusted solid and took patient work with WD40
to finally free and remove without damage. The chassis, once cleaned of
the layers of dirt, proved to be in good general condition with only a
small amount of rusting.
The
(typically) very dangerous mains lead was replaced.
A
sleeved grommet was fitted to carry the new circular three-core cable
through the chassis rear wall. I attached a self-tightening strap around the cable behind the
exit point, leaving a little slack, to prevent inadvertent
disconnection.
A
13
A
plug
completed that side of the work.
The
HT when tested was very low and was indeed due to a low-emission
rectifier valve V5. This was replaced, bringing the HT up to a decent
level but low volume and instability problems remained until
the output valve
V4 was replaced. This gave much improved volume and
almost cleared the howling and instability.
A
general clean up – valve-holders, volume control, tone control,
wave-change switch – helped to further reduce the problem. To
completely rid myself of the trouble, I replaced C27, 0.002 tone control
circuit, the output valve cathode bias electrolytic C 30, 22, and 0.02
A
F
coupling C24. This last is marked on my circuit as a 0.005, but the one
I removed – and I’m sure it was original – was definitely a 0.05.
Values in these
A
F
circuits are not especially critical, though I would have thought the
larger value was to be preferred in any case.
(values are those quoted in the relevant Trader sheet)
The
surface of the chassis
was wire brushed to remove rust and an anti-rust compound was then applied.
After
this had dried the IF cans and the
smoothing capacitor were polished then the chassis was masked off at
the valve-holders, aerial and loudspeaker socket strips. The control spindle
nuts were loosened and the spindles taped to prevent paint adhering. The
chassis was then painted with smooth Hammerite.
Cabinet
Restoration
The
loudspeaker frame and cone were carefully cleaned and the wooden panel
carrying the badly stained and damaged loudspeaker grille fabric was
stripped of the old and fresh fabric attached using spray adhesive. The
cabinet itself was thoroughly degreased using foam cleanser, dried
and brought up to a high sheen using metal polish.
The
back panel with its frame aerial was cleaned by using a damp cloth and
washing-up liquid.
Conclusion
A
quite attractive set in Bakelite, increasingly a collectable material. Works
well, with good tonal quality and plenty of power.
All rights reserved. © VRW 2006/2012
