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Old 1st Aug 2020, 5:56 pm   #1
kestrelmusic
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Default Cossor 501 oscillator puzzle

I'm working on a Cossor 501, the earlier model with Loctal (7S7 etc.) valves and am much puzzled.

The local oscillator is running hot and strong on MW and at least adequately on SW, but there's not a whisker of oscillation on LW.

I've given the wavechange switch a good clean and checked that all the contacts are making properly. The coils all seem OK with the DC resistance values believable.

C13, C15 and C16 (manufacturer's sheet, attached relevant bit) are all close to correct value. C9 and C10 must be about right otherwise presumably it would not oscillate in any range.

The triode anode voltage is a bit puzzling. The service sheet gives 96V; on MW my set gives 106V (but this is explainable as the valve emission is a little on the low side) but on SW and LW it's 68V.

I've tried a substitute valve which has 100% emission and mutual conductance and it makes no difference.

Any helpful thoughts would be much appreciated! I know these are regarded as "nothing special" but it's a nice set in a pleasing case, and I'd like to get it back to full working order.
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Last edited by kestrelmusic; 1st Aug 2020 at 5:57 pm. Reason: Typo
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Old 1st Aug 2020, 6:51 pm   #2
Cobaltblue
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Default Re: Cossor 501 oscillator puzzle

This series of chassis does seem to suffer from the Aerial coils going "Low Q"

Although less of a problem I suspect the same malaise may affect the oscillator coils.

I have not investigated this due to lack of time, but over the years I have seen the problem of the aerial coils several times but I have had enough spares from scrap sets and indeed have supplied a coil to at least one forum member. The LW seems to be the first affected because of it's higher inductance and lower Q in the first place.

Don't rule out the low value mica capacitors in the oscillator they rarely fail but they can still fail and cause odd faults like this .

C15 133pF is only in circuit on LW

Cheers

Mike T
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Last edited by Cobaltblue; 1st Aug 2020 at 6:58 pm.
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Old 2nd Aug 2020, 6:27 pm   #3
kestrelmusic
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Default Re: Cossor 501 oscillator puzzle

Many thanks for this. When you say the "low value mica capacitors", do you mean C9/C10 (100pF each) or the trimmers? I assume since the oscillator is running in MW and SW, C9 and C10 are OK, or would it be worth hiking them out and checking them or trying replacements?

I've already checked C15 (133pF) and it's very close to the correct value - certainly within 10%, probably better. That was the first thing I suspected as it's the only one in circuit only on LW.

The only other thing I can think of - a long shot? - is that the set has clearly been exposed to very damp conditions for a prolonged period; the chassis has a lot of rust top and bottom, and the IFTs and HT capacitor are quite corroded. I wonder if this may have affected the coil. If so, presumably replacement is the only option.
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