Since I haven't posted anything here for a while I figured I would toss this one out for some of you old school vacuum tube radio guys to ponder on.
Earlier this year I picked up a heathkit FM -4 tuner from a scrap pile and got it working. Mostly working anyway.
The only issue I have now is the AFC doesn't seen to either work or if it does it does not have the range of self adjustment needed to keep it on channel as the old radio warms up.
What it does is when cold the channel has to be tuned in on the high side then as it warms up (20 - 30 minutes) it has to be tuned lower and lower on the FM band to stay on the channel. That implies the issue is with the RF amp/Osc mixer tube and it related circuit having the issue but so far I have not been able to determine what exactly would be the heat sensitive part of it that would cause so much drift.
Best guess is the capacitors are old enough to be that thermally sensitive now and if so which ones are most likely the problem point or is it something else entirely and just normal for a tube based FM radio? It's the first complete and working tube radio I have never had so it's new territory to me.
Here's a link to the radios specs and schematics.
https://www.w6ze.org/Heathkit/Heathkit_045_FM4.pdf
Earlier this year I picked up a heathkit FM -4 tuner from a scrap pile and got it working. Mostly working anyway.
The only issue I have now is the AFC doesn't seen to either work or if it does it does not have the range of self adjustment needed to keep it on channel as the old radio warms up.
What it does is when cold the channel has to be tuned in on the high side then as it warms up (20 - 30 minutes) it has to be tuned lower and lower on the FM band to stay on the channel. That implies the issue is with the RF amp/Osc mixer tube and it related circuit having the issue but so far I have not been able to determine what exactly would be the heat sensitive part of it that would cause so much drift.
Best guess is the capacitors are old enough to be that thermally sensitive now and if so which ones are most likely the problem point or is it something else entirely and just normal for a tube based FM radio? It's the first complete and working tube radio I have never had so it's new territory to me.
Here's a link to the radios specs and schematics.
https://www.w6ze.org/Heathkit/Heathkit_045_FM4.pdf