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Zenith 10S464 issue - Printable Version

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Zenith 10S464 issue - murf - 08-28-2022

Hello fella's
Have an issue with a Zenith 10S464 that I just re-capped etc.
When powered up it plays excellent with sound quality and volume.
After about 15 minutes the volume will drop to about half of what it was when first powered up.
If I turn the volume up a little it reacts as it should.
Then after a while the volume will go up again for a while and then go lower again for a while.
Have checked the tubes and they are all in the acceptable range.
Seems to me like a tube issue.
Out of spec resistors were also replaced as well.
I did order a couple 6V6 tubes.
Could it possibly be an issue with the 6A8 or 6J5 tubes that test ok?
Any suggestions would be appreciated.
Thanks,
murf

RE: Zenith 10S464 issue - RodB - 08-28-2022

Hi Murf, it might be either a tube cathode that is getting phlaky or bad tube pin connection. I would look at the 6K7 first, wiggle the tubes a little, poke around with a wood stick then measure voltages when it happens. Also the grid caps should be looked at. When the change is distinct like this the cause should show itself.

RE: Zenith 10S464 issue - murf - 08-28-2022

Will take a look again.
I put a new 6K7 in there and always clean the tube pins and pin sockets.
Put a new wire and grid cap on the 6K7 also.
Did not do anything with the 6A8 grid cap.
Last resort, I will try to monitor voltages if the new tubes I ordered don't cure it.
Thanks,
murf

RE: Zenith 10S464 issue - MrFixr55 - 08-28-2022

Hi Murf.

If the Radio has an Aux / Phono, (Schematic shows a "Television" Connection) Inject an audio signal there. You can make up a cable to inject a computer or CD Player headphone level signal into this point. Switch to that to determine if direct Audio sounds good. "Calibrate" by switching while radio level is good. If Aux Audio level did not change when the radio level did change, that exonerates the "Audio" section. troubleshoot the "radio" section. If the Aux level also dropped, then troubleshoot the audio and power supply sections. Possible issue areas in the audio and PS sections may be the B+ / C- Resistors R17 through R20, the voltage and signal divider resistors for the phase Inverter (R14, 15, 13 and 2 numbers that I can't read of the NA Schematic). Of course, transformer windings, the speaker rectifier and audio tubes are suspect. In the radio section all is suspect including cathode resistors, IF transformer secondaries, etc.

Good luch

RE: Zenith 10S464 issue - murf - 08-28-2022

Thanks will give it a shot.
murf

RE: Zenith 10S464 issue - murf - 08-30-2022

Found the culprit guy's,
Seems the 6K7(new) was causing the problem.
Tests good, but acts up after about 10 minute warmup.
Thanks for all the help!
murf