alkermes Posted October 23, 2012 Report Share Posted October 23, 2012 Hoping that JKent may be able to help out with this problem....but of course I would love to hear from anyone who can shed some light on this problem.A few months ago I picked up a Model Twenty One radio. The speaker is stamped with a November 1968 date code, so it's coming up on it's 44th birthday. It makes a lovely sound but there is a low-level hum that is always in the background especially at lower volumes. I thought replacing the filter cap was in order, so I replaced it with a 2200uf 35v cap. While I was in there I also recapped the amp board, and replaced the two diodes as well.However the hum remains. All of the wiring seems OK inside. I am at a loss as to what to do. Is there anything else I can do? Link to comment Share on other sites More sharing options...
JKent Posted October 23, 2012 Report Share Posted October 23, 2012 Sorry but you've done what I know about already--I'm not really a tech.Is the hum coming through the speaker or could it be coming from the transformer? If you plug an RCA plug into the Extension Speaker jack I "think" that will turn off the internal speaker and you can listen for hum on the chassis. If it's definitely coming from the speaker then I'm thinking a bad ground somewhere.Sorry I'm not of more help. Maybe others will know more. If all else fails send it to Tim at Bristol Electronics in Ho-Ho-Kus NJ. He's very familiar with these and can definitely fix it.Or, if a Service Manual would help PM me. I had some printed up a couple of years ago.Kent Link to comment Share on other sites More sharing options...
alkermes Posted October 23, 2012 Author Report Share Posted October 23, 2012 Thanks Kent. It's definitely coming out of the speaker. I'm not really sure where to check for possible bad grounds, I'm no tech just a hacker. All of the wires seem firmly attached, all of the screws are firmly attached and there is no corrosion.I did find a copy of the service manual, in fact there's one here at the Classic Speakers website, I'm assuming it's the same as what you have?Thanks for the tip on Bristol Electronics, if all else fails then I will go there. Maybe I'll try the antique radios forum first. Link to comment Share on other sites More sharing options...
JKent Posted October 24, 2012 Report Share Posted October 24, 2012 Thanks for the tip on Bristol Electronics, if all else fails then I will go there. Maybe I'll try the antique radios forum first.Tim loves the Twenty-One. He has a couple in his house and is currently repairing 2 for me.Oh yeah--forgot about the Service Manual posted here. I sent it to David, he put it up here http://www.classicspeakerpages.net/IP.Board/index.php?showtopic=5666Kent Link to comment Share on other sites More sharing options...
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