The 1950 version of this classic, G725 has the same slug tuned FM local oscillator circuit. So does the 1955 A825 (T/Y825), which has no AM or FM but has an aux input
. A third set I'm working on is a 1960 model C730 (marked G730), which has no audio. This set also has an aux input.
Here is what I found after tracing out the FM circuit (in red on Zenith schematic) and replacing the drifted 10 K resistor and 50 pf cap on 12AT7 pin 2, got the oscillator going. Trouble is, no matter how many pf caps I tried, the FM will not tune below 99 MC, with slug fully in. 108 MC is mid scale, so osc runs too fast. The 50 pf was the tiny tube style cap, a different style mylar film and disc was tried, no improvement. Tried replacing the other little tube, a 22 pf across the coil. Also swapped in larger values than 50; 68 & 70pf. 24 and 30pf for 22. No real change in frequency, disc caps aren't same as tubular ones? RF sure is wacky.
Zenith G725 FM osc.docx.jpg
I initially tried a metal film resistor for the 10K and that was not good, with overload on strong local, appearing all over the range. Shango has advised against using metal films in RF and IF circuits, I agree that carbon is better in this application.
The AM works fantastic, a great DX set but the FM is only good between 99-108. The set is in great physical condition, so when do you give up on trying to perfect FM to add an auxiliary Jack to make it sellable?