12-13-2018, 12:56 PM
Hi Rich
To continue the troubleshoot on the FM osc section I would use a counter to read the oscillator frequency. That would give you the definitive answer.
Trying to measure the dc voltage on the control grid of the osc tube is going to be marginal at best. The voltmeter more than likely is going to cause the osc to shutdown when connected. It's a fairly critical circuit @ 100mc or so @ 1mc not so much. Wouldn't necessarily believe the tube tester as the tube might work ok at bc and sw frequencies but not fm frequencies. I usually try a few spares just to be sure it's not an anomaly w/the tube in question
If you have a 100mc scope w/a low capacitance probe you can have a look at pin 5 of the 7F8. this is where the osc signal is coupled thur C-414 to the rf side of the mixer.
Do check the dc voltages @ the plate and cathode. Should be within 5 or 10% of the list voltage. Low plate voltage more than likely is caused by a resistor in the hv leg have gone high in value or a leaky cap. Too high of a reading on the cathode would indicate an open osc coil or a switching issue, dirty contacts or miss wired.
Happy Hunting!
To continue the troubleshoot on the FM osc section I would use a counter to read the oscillator frequency. That would give you the definitive answer.
Trying to measure the dc voltage on the control grid of the osc tube is going to be marginal at best. The voltmeter more than likely is going to cause the osc to shutdown when connected. It's a fairly critical circuit @ 100mc or so @ 1mc not so much. Wouldn't necessarily believe the tube tester as the tube might work ok at bc and sw frequencies but not fm frequencies. I usually try a few spares just to be sure it's not an anomaly w/the tube in question
If you have a 100mc scope w/a low capacitance probe you can have a look at pin 5 of the 7F8. this is where the osc signal is coupled thur C-414 to the rf side of the mixer.
Do check the dc voltages @ the plate and cathode. Should be within 5 or 10% of the list voltage. Low plate voltage more than likely is caused by a resistor in the hv leg have gone high in value or a leaky cap. Too high of a reading on the cathode would indicate an open osc coil or a switching issue, dirty contacts or miss wired.
Happy Hunting!
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry