Quote:
Originally Posted by Adam
Now for the sound, first I thought to put the signal generator on the grid of the 1st audio IF tube, and just tweak all the IF transformers for loudest volume, but it was still pretty weak.
Then I hooked up the full blown early 1950s RCA sweep generator alignment set up I have and was able the get the picture on the scope for the discriminator transformer alignment just like it should be (ignoring the fact that for some reason the marker-adder decided it only wanted work set to reverse polarity) But, when I moved back to the 3rd IF transformer I could see the peak was set to twice the IF frequency, and I couldn't get it down much below 31MC. The sound section is in the 3rd schematic I posted...
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Don't know if you've checked this yet, but while I was working on my 12" porthole I discovered the same issue in the sound IF's that I had already found in Zenith's table model sets of that vintage. They used silver mica wafer caps in the IF cans that started to break down and leak(silver mica migration). I had to disassemble two of them and remove the damaged mica sheets and then add external mica caps to the base of the IF cans. That solved the problem in my case.
Bob