I have a Y723 here that was doing something I have never seen before. I would tune it to a station at 91.5. It played well with good sensitivity. I thought I was done. After a while, though, the station would go up the band so I had to tune to about 92.5 to get it. After a while it would pop back. It would do this twice or more until settling to the tuning it had on start up.
The problem was the 12AT7 which in this radio is both local oscillator and mixer. Apparently the local oscillator section is adding and subtracting capacitance at a set amount as it warms up.

The tube is a Zenith brand replacement with date code indicating manufacture in 1973. I understand that modern 12AT7 won't work well in RF circuits since they are optimized for audio. Is this the kind of thing that they do?

Best Regards,

Bill Grimm

I doubt the tube is doing this. I once had an FM tuner that would sometimes suddenly change stations. The problem turned out to be the tuning capacitor. It was intermittent from the rotor plates to the frame. A good cleaning fixed it. It took weeks to find this as the problem would come and go, sometimes disappearing after I had made some "fix".

I haven't looked up your radio to see if it was cap or slug tuned but I would carefully look for some kind of intermittent conection. This is the kind of problem that can drive you nuts since it may decide to go away just after you have changed something and then come back days or even weeks later. It also might be a good time to try some "cold" spray on the tuning components.

:I have a Y723 here that was doing something I have never seen before. I would tune it to a station at 91.5. It played well with good sensitivity. I thought I was done. After a while, though, the station would go up the band so I had to tune to about 92.5 to get it. After a while it would pop back. It would do this twice or more until settling to the tuning it had on start up.
: The problem was the 12AT7 which in this radio is both local oscillator and mixer. Apparently the local oscillator section is adding and subtracting capacitance at a set amount as it warms up.
:
: The tube is a Zenith brand replacement with date code indicating manufacture in 1973. I understand that modern 12AT7 won't work well in RF circuits since they are optimized for audio. Is this the kind of thing that they do?
:
:Best Regards,
:
:Bill Grimm
:

Funny you should mention this. I just turned on the radio and it did it again, jumped higher than ever before! It then started tunig itelf to various stations. Maybe it doesn't like classical music.
So much for the Frequency Agile 12AT7. It is just a good tube.

By the way this is a slug tuned radio. The problem is clearly in the Local Oscillator.

Best Regards,

Bill Grimm

:Funny you should mention this. I just turned on the radio and it did it again, jumped higher than ever before! It then started tunig itelf to various stations. Maybe it doesn't like classical music.
:So much for the Frequency Agile 12AT7. It is just a good tube.
:
:By the way this is a slug tuned radio. The problem is clearly in the Local Oscillator.
:
:Best Regards,
:
:Bill Grimm
: