Delco Car radio from the 1950s plays weak when AVC tube is in circuit

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Delco Car radio from the 1950s plays weak when AVC tube is in circuit

Post by Fid on Fri Dec 11, 2009 11:59 am

I've been repairing vacuum tube car radios for years and have been doing many more in just the past 2 years. I have come upon a problem which I'm having trouble diagnosing. It's in a 6 volt Delco radio with the typical 6BA6, 6BE6, 6AV6 and 6V6 tubes. It has two 6AV6 tubes; one is the Detector and first AF amp, the other is Phase Inverter and AVC. I replaced all the wax caps and the electrolytics, tried all new tubes from a working example and it still plays very weak. If I remove the 6AV6 AVC tube, then it plays pretty well so I'm guessing for some reason the AVC amp has too much gain. All components associtated with that circuit appear to be OK. Any suggestions?

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Re: Delco Car radio from the 1950s plays weak when AVC tube is in circuit

Post by sprman on Fri Dec 11, 2009 11:27 pm

Did you check tube socket resistances and voltages for all?Mica caps can go bad .Make sure nothing in the other stages has changed value.Remember AVC will be in IF and RF stages.Car radios get heat/cold cycles ,so make sure all tube sockets and pins are ok .Sprman. : )

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Re: Delco Car radio from the 1950s plays weak when AVC tube is in circuit

Post by Fid on Sun Nov 07, 2010 8:09 pm

Here's what I found out - the audio output transformer had shorted coils. Strange symptom from that but replacing that transformer (which had some wax dripping out) took care of it.

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Re: Delco Car radio from the 1950s plays weak when AVC tube is in circuit

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