04-03-2024, 11:29 AM
I always knew that if I kept playing with radios I would run into an oscillator issue.I’m working on a Zenith 7S-363 that has an oscillator that is deader than three o’clock in the morning.
Schematic can be found here
http://www.nostalgiaair.org/Resources/355/M0025355.htm
I brought this up on another forum and I have tried all the suggestions except substituting a 6J8 tube. That discussion can be found here
https://www.antiqueradios.com/forums/vie...p?t=434443
DC voltages in the radio are mostly on the high side. Voltages on the 6A8 are now as follows; Pin 3 is 323 volts, Pin 4 is 44 volts, pin5 is -0.2 volts, pin 6 is 11 volts,. The voltages shown on the schematic are 235, 30, -1.6 and 190. The voltages shown on the schematic are 235, 30, -1.6 and 190. Measurements were not taken with the specified 1000 ohms per volt meter resistance.
All out of tolerance resistors and all paper caps were replaced. Three different 6A8 tubes were tried and voltages remained approximately the same. Oscillator coil measured 3.1ohms thru the entire lineup of windings and the tickler was about an ohm. Band switch has been cleaned, 6A8 tube socket was replaced due to loose pins,
While I had the tuning condenser out, I pulled the oscillator coil out for inspection and measurements. There were a few dark spots on the coil but it did not look like burn marks, but I could be mistaken, and there was continuity in all coils. I stuck in a generic 455kHz oscillator coil, still no luck and tube voltages had not changed much.
I used a signal generator to inject a signal that was 455kHz above the stations that I can tune during the day and only the station at 920 lHz could be tuned. Measured the oscillator resonance and found that the oscillator peaked out at 1435 kHz which means the radio cannot tune above 980.
Does anybody have any other ideas I can try before using a substitute tube or possibly rewinding the oscillator coil?
Schematic can be found here
http://www.nostalgiaair.org/Resources/355/M0025355.htm
I brought this up on another forum and I have tried all the suggestions except substituting a 6J8 tube. That discussion can be found here
https://www.antiqueradios.com/forums/vie...p?t=434443
DC voltages in the radio are mostly on the high side. Voltages on the 6A8 are now as follows; Pin 3 is 323 volts, Pin 4 is 44 volts, pin5 is -0.2 volts, pin 6 is 11 volts,. The voltages shown on the schematic are 235, 30, -1.6 and 190. The voltages shown on the schematic are 235, 30, -1.6 and 190. Measurements were not taken with the specified 1000 ohms per volt meter resistance.
All out of tolerance resistors and all paper caps were replaced. Three different 6A8 tubes were tried and voltages remained approximately the same. Oscillator coil measured 3.1ohms thru the entire lineup of windings and the tickler was about an ohm. Band switch has been cleaned, 6A8 tube socket was replaced due to loose pins,
While I had the tuning condenser out, I pulled the oscillator coil out for inspection and measurements. There were a few dark spots on the coil but it did not look like burn marks, but I could be mistaken, and there was continuity in all coils. I stuck in a generic 455kHz oscillator coil, still no luck and tube voltages had not changed much.
I used a signal generator to inject a signal that was 455kHz above the stations that I can tune during the day and only the station at 920 lHz could be tuned. Measured the oscillator resonance and found that the oscillator peaked out at 1435 kHz which means the radio cannot tune above 980.
Does anybody have any other ideas I can try before using a substitute tube or possibly rewinding the oscillator coil?
Sometimes I wonder whether the world is being run by smart people who are putting us on or by imbeciles who really mean it.
