07-11-2016, 08:54 PM
Not wanting to steal someone else's thread, I'll start a new one on a similar topic.
Bought at Gnosall last month, for too much really, a Cossor 524 bakelite table radio.
It looked OK at the auction, had all the knobs ( they are known to be fragile ) case intact but needed a polish, dial glass good, old 2 core bell wire mains lead, and a back. What could go wrong?
Well, plenty as it happens. On picking it up a crack in the bottom became apparent, not serious as it doesn't show. The internal FM dipole aerial wires held in the socket with matchsticks. So homeward bound, on the bench in the morning.
Back off, all valves present, fresh EL84 and EZ81 all others Cossor possibly originals. Bit of rust here and there, but oh dear, speaker baffle riddled by the worm, straight through the fret fabric as well. So it needs immediate stripping and treating rather than risk the rest of the collection.
Tone and volume concentric knobs come off easily, both have small cracks, superglue to the rescue, wavechange comes off in good condition but the outer tuning knob is very tight. Fortunately the card bottom is part of the back so access is possible to the rear of the knob boss to give it a careful levering off, unbroken.
Unplug the speaker, the ferrite rod aerial and remove the baffle for treatment. Thankfully the speaker cone is un-nibbled. Whilst the solvent is dispersing, clean and glue the cracked bottom rail, re-enforce with a plastic card too.
Chassis on the bench, check for shorts on mains, transformer windings, wiring, reservoir/smoothing caps etc. Set to 250v input. Connect bench speaker and AM aerial. Hang ferrite rod back on temporarily.
Find QMB mains switch permanently o/c, get some spray into it and work it till it operates again. Check continuity on mains and output transformers.
All seems reasonable, fit a plug with 1 Amp fuse, into lamp limiter fed by isolation transformer, power up.
All OK, drawing some current, valves lit. Run for a while, cap can staying cold. Grease tuning drive whilst waiting, find some sound on medium and long waves. Give it full beans and tune all AM bands, including short wave, very good so far. Electrolytic caps seem to be reforming OK. Check output valve grid coupling capacitor by shorting previous valve anode to chassis whilst monitoring the HT line, slight increase seen so new cap fitted. Chop 3 broken Hunts Mouldseals whilst the irons hot.
Now the problems start, no FM at all, totally dead. Check that the ECC85 in the FM can has correct voltages, all near enough so try a fresh valve, but still nothing. Several hours spent checking components, replace another Hunts cap found in the FM head. Nothing restores the FM, so signal generator warmed up, 10.7MHz modulated injected into the alignment point on the FM, nothing.
Puzzled as the FM uses the AM mixer as 1st IF amp, the whole AM works but nothing when injecting into the FM anywhere. Must be in the RF FM box, not my favourite area to work on.
Spend many hours head scratching, doing other things, thinking, looking at the FM coils and components.
Attend Golbourne meeting on Sunday, and listen to Keith's (Keithstv ) talk on valves, excellent address. Have a chat with the font of all knowledge afterwards, and take note of his suggestions.
Today I Found It! Green spot corrosion on the primary of the 1st IF transformer, the copper was o/c in the centre of the winding, on unwinding it, the single cotton covering held it together till I had the winding off and tensioned the wire.
Now the ticklish part, rewind the coil in situ onto the bobbin and resolder the ends to the vertical support wires. I did consider removing the whole transformer but they are double IFs with the AM one as well in the same can, lots of unsoldering in a crowded underchassis. Rebuild and check, we have continuity but still no FM so new ECC85 in and I have a good FM output across the whole band. Slight distortion on strong signals which is likely to be the alignment being a bit off. Melt out the wax on the core I have been fixing, tweak proves it needs re-alignment. Another little job with wobbulator signal generator and 'scope.
Success!
It works well, put it all back together tomorrow. Thanks to Keith, a fresh set of brain cells and a bit of patience wins the day. Another set saved from destruction, it performs well, will clean up nicely, worth the effort.
Bought at Gnosall last month, for too much really, a Cossor 524 bakelite table radio.
It looked OK at the auction, had all the knobs ( they are known to be fragile ) case intact but needed a polish, dial glass good, old 2 core bell wire mains lead, and a back. What could go wrong?
Well, plenty as it happens. On picking it up a crack in the bottom became apparent, not serious as it doesn't show. The internal FM dipole aerial wires held in the socket with matchsticks. So homeward bound, on the bench in the morning.
Back off, all valves present, fresh EL84 and EZ81 all others Cossor possibly originals. Bit of rust here and there, but oh dear, speaker baffle riddled by the worm, straight through the fret fabric as well. So it needs immediate stripping and treating rather than risk the rest of the collection.
Tone and volume concentric knobs come off easily, both have small cracks, superglue to the rescue, wavechange comes off in good condition but the outer tuning knob is very tight. Fortunately the card bottom is part of the back so access is possible to the rear of the knob boss to give it a careful levering off, unbroken.
Unplug the speaker, the ferrite rod aerial and remove the baffle for treatment. Thankfully the speaker cone is un-nibbled. Whilst the solvent is dispersing, clean and glue the cracked bottom rail, re-enforce with a plastic card too.
Chassis on the bench, check for shorts on mains, transformer windings, wiring, reservoir/smoothing caps etc. Set to 250v input. Connect bench speaker and AM aerial. Hang ferrite rod back on temporarily.
Find QMB mains switch permanently o/c, get some spray into it and work it till it operates again. Check continuity on mains and output transformers.
All seems reasonable, fit a plug with 1 Amp fuse, into lamp limiter fed by isolation transformer, power up.
All OK, drawing some current, valves lit. Run for a while, cap can staying cold. Grease tuning drive whilst waiting, find some sound on medium and long waves. Give it full beans and tune all AM bands, including short wave, very good so far. Electrolytic caps seem to be reforming OK. Check output valve grid coupling capacitor by shorting previous valve anode to chassis whilst monitoring the HT line, slight increase seen so new cap fitted. Chop 3 broken Hunts Mouldseals whilst the irons hot.
Now the problems start, no FM at all, totally dead. Check that the ECC85 in the FM can has correct voltages, all near enough so try a fresh valve, but still nothing. Several hours spent checking components, replace another Hunts cap found in the FM head. Nothing restores the FM, so signal generator warmed up, 10.7MHz modulated injected into the alignment point on the FM, nothing.
Puzzled as the FM uses the AM mixer as 1st IF amp, the whole AM works but nothing when injecting into the FM anywhere. Must be in the RF FM box, not my favourite area to work on.
Spend many hours head scratching, doing other things, thinking, looking at the FM coils and components.
Attend Golbourne meeting on Sunday, and listen to Keith's (Keithstv ) talk on valves, excellent address. Have a chat with the font of all knowledge afterwards, and take note of his suggestions.
Today I Found It! Green spot corrosion on the primary of the 1st IF transformer, the copper was o/c in the centre of the winding, on unwinding it, the single cotton covering held it together till I had the winding off and tensioned the wire.
Now the ticklish part, rewind the coil in situ onto the bobbin and resolder the ends to the vertical support wires. I did consider removing the whole transformer but they are double IFs with the AM one as well in the same can, lots of unsoldering in a crowded underchassis. Rebuild and check, we have continuity but still no FM so new ECC85 in and I have a good FM output across the whole band. Slight distortion on strong signals which is likely to be the alignment being a bit off. Melt out the wax on the core I have been fixing, tweak proves it needs re-alignment. Another little job with wobbulator signal generator and 'scope.
Success!
It works well, put it all back together tomorrow. Thanks to Keith, a fresh set of brain cells and a bit of patience wins the day. Another set saved from destruction, it performs well, will clean up nicely, worth the effort.
Boater Sam.
