Cossor 494AC Mains Hum - UK Vintage Radio Repair and Restoration Discussion Forum

gary_crutchley · 16th Jan 2016, 2:15 pm

Hi all,

The Cossor 494AC that we have been working on is performing well. However, one issue is that it has a constant low volume mains hum, it's not too much of a problem but it's there.

Any ideas if there is anyway to address this please?

Regards,

Gary

Herald1360 · 16th Jan 2016, 4:18 pm

If the smoothing caps have been replaced already, it's possible that the hum is original so to speak, though there are other possibilities. The set has a hum cancelling output transformer arrangement which will depend on the capacitor values being the same as original. If the TX has been replaced by non original, this could be the cause.

Does shorting the output valve grid to chassis get rid of the hum? If not, it's likely HT smoothing related, if it does, it could be a pickup problem in the triode wiring or even a faulty DDT valve.

gary_crutchley · 16th Jan 2016, 6:16 pm

Hi there,

I will try the output valve grid test. The smoothing capacitors are original, as is the transformer as far as I can tell.

Regards,

Gary

gary_crutchley · 16th Jan 2016, 6:21 pm

Quote:

Originally Posted by Herald1360

Does shorting the output valve grid to chassis get rid of the hum?

Can you confirm which pin the grid is connected to please?

Regards,

Gary

Herald1360 · 16th Jan 2016, 6:31 pm

7C5 g1 is pin 6

http://frank.pocnet.net/sheets/155/7/7C5.pdf

16th Jan 2016, 2:15 pm	#1
gary_crutchley Heptode Join Date: Apr 2013 Location: Bloxwich, West Midlands, UK Posts: 554	Cossor 494AC Mains Hum Hi all, The Cossor 494AC that we have been working on is performing well. However, one issue is that it has a constant low volume mains hum, it's not too much of a problem but it's there. Any ideas if there is anyway to address this please? Regards, Gary

16th Jan 2016, 4:18 pm	#2
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,536	Re: Cossor 494AC Mains Hum If the smoothing caps have been replaced already, it's possible that the hum is original so to speak, though there are other possibilities. The set has a hum cancelling output transformer arrangement which will depend on the capacitor values being the same as original. If the TX has been replaced by non original, this could be the cause. Does shorting the output valve grid to chassis get rid of the hum? If not, it's likely HT smoothing related, if it does, it could be a pickup problem in the triode wiring or even a faulty DDT valve. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

16th Jan 2016, 6:16 pm	#3
gary_crutchley Heptode Join Date: Apr 2013 Location: Bloxwich, West Midlands, UK Posts: 554	Re: Cossor 494AC Mains Hum Hi there, I will try the output valve grid test. The smoothing capacitors are original, as is the transformer as far as I can tell. Regards, Gary

16th Jan 2016, 6:31 pm	#5
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,536	Re: Cossor 494AC Mains Hum 7C5 g1 is pin 6 http://frank.pocnet.net/sheets/155/7/7C5.pdf __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O