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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 12th Aug 2019, 3:04 pm   #1
Malcolm G6ANZ
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Default Cossor 524 poor FM

This was my fathers radio bought new in 1955. It was use as the family radio for about 15 years until we moved house. It was never used on FM, probably as there was no need as all programmes were on MW/LW.
It was then stored in a dry attic until I decided to revive it and use as an FM radio.
I have gone through it and changed the dodgy looking caps and out of spec resistors. It performs well on long and medium but the FM is insensitive and difficult to get an undistorted output.
I know that the coils and preset caps have not been touched so my thoughts are now edging towards the FM front end (6AQ8) and/or the discriminator (6AK8).
The 6AQ8 can be got from various suppliers as tested ,but this only shows DC conditions not at 100MHz.
Am I in the right area or is there anything else to check?

Malcolm
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Old 12th Aug 2019, 3:16 pm   #2
Cobaltblue
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Default Re: Cossor 524 poor FM

Like most sets of the period the HT is switched to the VHF front end after decades of non use results in cathode poisoning of the 6AQ8/ECC85.

Usually they are completely dead

IMHO if they test good DC wise they should be OK.

The ones that are poisoned have virtually no emission.

Cheers

Mike T
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Old 12th Aug 2019, 4:21 pm   #3
Nuvistor
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Default Re: Cossor 524 poor FM

It’s worth changing if you haven’t already done so C38 5ufd capacitor in the ratio detector circuit., using makers manual reference.
Also the 6AK8/EABC80 has two diodes that are only used on FM in the ratio detector, any large imbalance in the emission of the diodes will give distortion.

Langrex have NOS ECC85 valves.
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Old 13th Aug 2019, 5:20 pm   #4
Malcolm G6ANZ
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Default Re: Cossor 524 poor FM

Thanks for the suggestions, C 38 was one of the ones I replaced.
I have ordered a 6AQ8/ECC85 from langrex and a 6AK8/EABC80 from Watford valves. Langrex didn't have a EABC80 in their list.
Further reports when the valves arrive.
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Old 14th Aug 2019, 6:28 pm   #5
Malcolm G6ANZ
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Default Re: Cossor 524 poor FM

The new ECC85 arrived today from Langrex, very prompt service. I fitted it to the front end and switched on. Wow what a difference Radio three at correct volume and minimal distortion
Hopefully the residual distortion will be removed when the EABC80 is replaced. I may give the ratio det coil L25(Trader Sheet) a small tweak to see if this improves the sound after the new valve is fitted.
Thanks all.

Malcolm
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