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Zenith K731 - Printable Version

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Zenith K731 - Paul Philco322 - 01-03-2016

Well I have been cleaning up the bench, fired up my Z brand K731 that had always played well. No FM today. Icon_sad It has been recapped and has played both bands well. Could not find a schematic at Nostalgia Air. 12DT8 possibly?

Any thought? I am much obliged.

Paul 

RE: Zenith K731 - Phlogiston - 01-03-2016

Band switch?

RE: Zenith K731 - Radioroslyn - 01-03-2016

This should get you in the ballpark.
http://www.rsp-italy.it/Electronics/Radio%20Schematics/Collections/Beitman/1963.pdf
pg 179

Terry

RE: Zenith K731 - Paul Philco322 - 01-03-2016

Bandswitch always a sore spot appears to be fine, has been cleaned and is operating, maybe a tube.

Paul

RE: Zenith K731 - David - 01-03-2016

Paul, I have a similar Z set the 12DT8 tube is a pain. I have had several NOS fail after maybe 50-100 hours  went to using 12AT7, ECC81 which are current production. The BIG DIFFERENCE is pin 9 on the ECC81 is center tap to the heater, on 12DT8 pin 9 is internal shield to ground. One must remove the ground connection from pin 9 of the tube socket or kiss your new tube good bye in a flash. 

RE: Zenith K731 - John Bartley - 01-03-2016

Not a Zenith expert, but I have worked on one of these. I also had a problem with the FM, mine being a cut out and cut in problem. It turned out to be silver mica disease in the first FM IF. Unlike the same issue with AM, this SMD caused no thunder/crashing noises on FM (yours may be different).

I will try to attach a schematic (not sure it is the correct one, but it might help).

cheers

John

EDIT :: nope, I messed up the attachment ... gimme a minute ....

try this : http://oldradio.ca/radio/Info/Zenith_K731/K731-7M07schematic.gif

RE: Zenith K731 - titan1a - 01-03-2016

I have one in my bedroom that the selenium rectifier was replaced by a diode and filter capacitors replaced. Also the electrostatic tweeter was replaced by a crystal tweeter. It still plays nice.

Rick Ethridge

RE: Zenith K731 - VntgRads - 01-04-2016

Found mine at a flea market with a giveaway price as only the AM worked.  Cosmetically it is near perfect.  Started with the tubes and found 3 that were marginal/bad.  Replaced those 3 and it worked perfectly (and still does today). Picks up lots of stations.  Simplest fix I have come across yet!  It was a while ago and I don't remember the tubes I replaced.  Sure wish this model had a dial light.

Craig R