[mancave]

I have replaced all electrolytic and paper capacitors. Selenium rectifier replaced with diode and dropping resistor; B+ voltage is correct. Tubes 19t8 and 12be6 tested very weak and had partial shorts, so replaced those also. The radio plays very well on both AM and FM. Sensitivity is superb. Sound is good until the radio has been playing 15 to 20 minutes, after which it becomes distorted. 

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What should I look for next?

[morzh]

Check tubes. For anything. Leakage, gas....change one by one, start from the output

[David]

35B5 is not one of my favorite tubes! David

[Paul Philco322]

I may have some of those tubes as I have a few Zeniths if you get stuck.

Paul

[462ron]

I’ve had a similar symptom on one of my sets a few years ago. I monitored the control grid voltage on the audio out put tube and found the voltage slowly creeping up after a full recap after several minutes. I disconnected the coupling cap from the previous audio stage and it still kept going up. Turned out the tube itself was bad!

Ron

[mancave]

Thanks all. Two replacement tubes ordered.

[mancave]

The radio now plays well at low volume but when the volume is turned up more than a third of the way, the sound is distorted. This happens on both AM and FM. I connected a different speaker and the same thing happened. What should I look for next?

[morzh]

Is this behaviour different from before? Does warm-up affect anything?
When you first listed the problem, the radio was playing well for 15-20 minutes. Was it also playing well at full volume, or, like now, only before 1/3 way up?

[RodB]

Depends on what test equipment you have. You will want to narrow it down to one of the audio stages. If you have a scope and audio generator feed audio at the top of the volume control then look at the signal as it passes through each section, looking for the distortion. If you don't have test equipment, use headphones or small speaker with a cap in series and listen for the sound as it travels through. I would look at a couple points in particular. One is the volume control wiper. Maybe some more cleaner there. The second is what looks like a negative feedback circuit made of C16 and R12. Open the circuit feeding to the volume control and see if the distortion goes away. If you haven't found it, temporarily replace C14. These do fail occasionally. I don't see a cathode bypass cap on the output tube, try a 10 - 20mfd 25 volt cap. I assume the 2 tubes were verified. Check bias voltages.

[RodB]

If you have just an audio signal source then feed it at different points, using a series capacitor, starting at the output transformer and working forward. Watch out for the high voltage!

[mancave]

Thanks. This describes a newish behavior that occurs when the radio is first turned on, and continues in a consistent fashion no matter how long it has been left on. I don't have an oscilloscope but will follow through on the other suggestions provided. You are all a great resource. I won't have time to work on this until after Easter but will let you know what happens.

[morzh]

what, other than the tubes, has been changed?
If you put the old tube(s) in, does the old behavior return?

[David]

Take a look at the 19T8 and any an all things connected to the tube socket. Have had NOS tubes fail within a hundred hours. David

[Jayce]

I agree on the 19T8 about NOS tubes failing. I have had at least two develop shorts after only a handful of hours of play and the were NOS. I've also had 12AT7s suddenly just go kaput on me. It also wouldn't hurt to swap in another 35C5 or even a 50C5 as the radio won't mind with today's higher voltages to see if that helps. From my experience I have had tubes that test great, but make every radio I try them in sound bad. That's about all I can offer.

[John Bartley]

Some years ago I worked on a newer but somewhat similar Zenith AM/FM set which had similar symptoms in that it would start fine when cold and over a short time it would distort and stop playing.

Both the O.P.'s set and the one I worked on have slug-adjustable I.F. transformers and in mine I found an AM I.F. with silver migration issues. I fixed that I.F. transformer and it cured the problem.