FM Tuner Drift

[Winky Dink]

I put together this radio with the best parts of an H725 and a J725. The H725 was the model my Dad had in his bedroom for 50 years, and I needed an FM radio to listen to while I worked. Esthetically (my personal taste) it's a radio that only a mother could love.



It works well and sounds good, but the FM tuning has to be very precise and often I can't listen for more than a few minutes before it drifts into distortion. This is my first foray into FM, and all I did on this set was replace (as needed) caps, resistors, tubes, and selenium rectifier.

What should I look for to regain stable FM tuning?

[zeno]

My father also bought the same radio new. Was a fixture
in the house. Never broke, just cleaned the controls. My
brother uses it now.
Anyhows the drift is prob a bad tube. Start with any that are
FM only if the AM is OK. Tube will prob test leakage.
Then do the IF tubes. Filter can also a possability.
Drift is vague, describe better if you can.

This radio uses PTO in the FM for tuning, DO NOT mess with any
coils etc. PTO is only seen on the highest end communications
gear, I dont think anyone else used it at the consumer level.
Its a good example of balls out quality we used to make.
If you go to junk shops or flea market in these parts you
will see lots of them still working along with other 1950's
Zenith radios, rarely any other brands. They are damn near
immortal.

73 Zeno

[Jon A.]

Originally Posted by Winky Dink

Esthetically (my personal taste) it's a radio that only a mother could love.

Actually, I think it's pretty neat, even though these are too old for me to want to pursue. And the condition it's in... wow.

[bob91343]

The drift is probably temperature related. Maybe more cabinet ventilation for starters anyway. Is the drift always in the same direction? Does it abate after a while?

You can try putting in some negative temperature coefficient capacitors in the oscillator circuit. Sometimes there will be a bad solder joint, lousing up the oscillator tank Q.

You could get fancy and install a temperature control circuit.

[Winky Dink]

Thanks, guys. That's a lot to look into. The temperature factor is particularly interesting because the 35B5 gets so hot that the inside of the cabinet is insulated in that area. I'll take the back off, get some air circulation, and see if that helps. Then I'll follow up with tube testing etc.

[zeno]

Originally Posted by bob91343

The drift is probably temperature related. Maybe more cabinet ventilation for starters anyway. Is the drift always in the same direction? Does it abate after a while?
You can try putting in some negative temperature coefficient capacitors in the oscillator circuit. Sometimes there will be a bad solder joint, lousing up the oscillator tank Q.
You could get fancy and install a temperature control circuit.

Heat related yes, also have to consider line voltage. Seen multi
band radios that worked but when the line dropped the osc
would quit at hi freqs. IIRC Zenith TO's would do this, bad osc
tube that checks good. Bad cons can be eliminated, this is 100%
hand wired. BUT dirty tube sockets are a strong possibility.
As I said drift is a bit vague, I assume freq. drift. The set does
have AFC & if its freq drift its pretty bad...... The other thing
that comes to mind is we did replace IF cans in these,
they would crackle then crap out.

73 Zeno

[Winky Dink]

After determining that heat wasn't the issue, I started tube checks with the 19T8 FM discriminator. Got confusing, differing short-tests on this tube and a spare 19T8, using two emissions testers but performance improved when I reinserted either the original tube or the spare. Cleaned the pins and sockets thoroughly for all tubes. Problem solved.

Thanks, everyone. Now I can get back to the AM transmitter that wouldn't work a couple of years ago.

[DavGoodlin]

Good Job Winky Dink. I was also going to suggest using freeze spray on some of the resistors and caps under the chassis.

I have a newer model A825 with the ConelraD symbols on AM, with its original box. It seemed these similar-looking models were made over several years.

Then came the wood-cabinet "Interlude" models , no less impressive with the same tuning Zeno described.