wayne
Society Members
Posts: 138
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Mullard
Oct 20, 2021 18:55:20 GMT 12
Post by wayne on Oct 20, 2021 18:55:20 GMT 12
Hi all, I'm trying to restore Mullard 652. I've replaced all the capacitors including the filter caps. When I try and tune in a station I get very loud humming and whistling sounds but when I put the tip of an insulated screwdriver on the top of the wired cap on the EF39 valve which wired to what I believe is a IF transformer. (Please correct me if I'm wrong as trying to learn. ) the noises just about go away. Just incase it matters one of the 25uf filter caps has been replaced with a new old 25uf only because I haven't got any new ones at the moment Any help would be appreciated Thanks Wayne
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peter
Tinkerer
Retired, collector of vintage radios and test equipment for restoration and repair.
Posts: 97
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Mullard
Oct 21, 2021 21:52:45 GMT 12
Post by peter on Oct 21, 2021 21:52:45 GMT 12
Of the top of my head and without the schematic I'd suspect the IF transformer being open circuit - with the set OFF check the ohms from the cap to ground with a multimeter.
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wayne
Society Members
Posts: 138
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Mullard
Oct 21, 2021 21:53:08 GMT 12
Post by wayne on Oct 21, 2021 21:53:08 GMT 12
I did some research and found to my understanding that the EF39 has a red metal coating on it which on my valve there is not much left so I made and fitted a shield and along with a longer Arial it has made difference Wayne
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wayne
Society Members
Posts: 138
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Mullard
Oct 21, 2021 21:56:53 GMT 12
Post by wayne on Oct 21, 2021 21:56:53 GMT 12
Thanks Peter. I will check that Regards Wayne
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wayne
Society Members
Posts: 138
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Mullard
Oct 21, 2021 22:03:18 GMT 12
Post by wayne on Oct 21, 2021 22:03:18 GMT 12
What result should I get when checking Thanks
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peter
Tinkerer
Retired, collector of vintage radios and test equipment for restoration and repair.
Posts: 97
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Mullard
Nov 1, 2021 22:33:13 GMT 12
Post by peter on Nov 1, 2021 22:33:13 GMT 12
So long as it's not open or infinity it's probably OK - I'd expect a few 100 ohms to a few thousand at the top.
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