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Old 11th Nov 2021, 9:20 pm   #1
mister valve
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Join Date: Aug 2009
Location: Blackpool, Lancashire, UK.
Posts: 311
Default Cossor, 378 "Super-Ferrodyne" Receiver, Restoration

I have just restored a 1936 Cossor model 378 T.R.F. "Super-Ferrodyne" receiver. This type of receiver was made in A.C. mains, universal mains, and battery versions.

This set is an A.C. Mains only set with full isolation double-wound mains transformer. The Valve line-up should be :- 'MVS/PEN', 'MS/PEN', 'PT41', '442BU'. Basically it is three Pentode valves + Rectifier. The 'MVS/PEN' is a variable-mu Pentode, tuned R.F. Amplifier, with gain controlled by altering the cathode bias (i.e. the negative grid voltage). The 'MS/PEN' is a leaky grid detector, 'straight' Pentode. 'Reaction' is provided in this stage which makes it very sensitive & selective. There is no reaction 'back-lash', (back-lash can be a nuisance in some T.R.F. sets). The output valve is a directly heated Pentode which provide plenty of volume.

This set has been in my possession for nearly two years, waiting for restoration. It was in a filthy condition, covered in a black 'soot' like substance. this needed cleaning off before considering restoration.
The Mains Transformer was tested and found to be good (much to my relief).
However, the Output Transformer had an open circuit primary and the loudspeaker was out of centre and the air-gap full of 'crud'. The speech coil, hum bucking coil, and energising coil were all OK, but the cone paper had a small tear in it, but this was easily repaired.

The speaker was completely dismantled and and cleaned, and the Output Transformer replaced with a 'multi-tapped, pri. & sec.' type which made matching the recommended 8,000 Ohm Anode load to the 3 Ohm speaker load easy. Rebuilding & re-centring is quite easy with the energised type speaker, and it now moves freely. Unfortunately the 'dust-bag' fitted over the speaker, had totally disintegrated into a 'rag' full of holes ! I have not as yet replaced this.

The chassis top was stripped down and repainted as seen in the pictures, the underside was cleaned, Electrolytics & Paper Capacitors were replaced. All the paper caps., were electrically leaky, and the Electrolytics 'dried up'. None of the Mica capacitors needed changing, and only one Resistor needed changing, R6, a 200 Ohm resistor had gone O/C, I replaced this with a 220 Ohm resistor (the value is not critical in that position). The rest of the Resistors were well within tolerance - which surprised me and pleased me simultaneously

The set was gradually run up on my variac, checking for smoke, flashovers etc. whilst monitoring the H.T.
Then I heard a voice in the loudspeaker ! Everything was working....

This receiver is very sensitive and the reaction control is very smooth with no back-lash. The selectivity is good too. Not quite as good as the "Philips Superinductance" range of T.R.F. sets, but never-the less it gives very credible results.

The cabinet has not been tackled yet (apart from re-gluing it back together). I will seek advice on the cabinet restoration. It is not in bad condition, but will need re-finishing.

I'm going to try and upload some pictures of the renovated chassis....
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