08-27-2008, 02:09 PM
I think I've made some progress in the power supply, but I'm not listening to the radio yet.
Basically, after I replaced the bad rectifier and oscillator tubes that had internal shorts, I still had too much current through the 1800 ohm resistor I was using to substitute for the field coil. I suspected I had wired the speaker cable wrong, but I found it confusing to trace it down because the tracers were so faded, so I finally decided to replace with new colored wires.
Once that was done, I set the variac at 117 VAC. The DC voltage at the rectifier and B+ at the output tube plate seemed right, and the current was a steady 43 milliamps through the resistor, so I decided to reconnect the speaker. Now I have some AC hum and static noises in the speaker, but no signal. Tuning through the dial makes no difference, but I get a little pop in the speaker when I touch each plate in the signal path with the meter, but not the oscillator.
B+ readings at each plate were all close to right (5 to 10 volts low or high) EXCEPT the oscillator, which was 51 volts instead of 75. Oddly, the high side of resistor #41 (33K ohm) is about 170 volts, which is close to what I think it should be, but the drop across that resistor is much higher than seems right. That makes me think the oscillator plate current is too high.
I ran out of time last night, but when I get back to it I will look at voltage on the high side of resistor #42 (2200 ohm) and replace the oscillator and see what I can see.
Any other suggestions would be welcome.
Basically, after I replaced the bad rectifier and oscillator tubes that had internal shorts, I still had too much current through the 1800 ohm resistor I was using to substitute for the field coil. I suspected I had wired the speaker cable wrong, but I found it confusing to trace it down because the tracers were so faded, so I finally decided to replace with new colored wires.
Once that was done, I set the variac at 117 VAC. The DC voltage at the rectifier and B+ at the output tube plate seemed right, and the current was a steady 43 milliamps through the resistor, so I decided to reconnect the speaker. Now I have some AC hum and static noises in the speaker, but no signal. Tuning through the dial makes no difference, but I get a little pop in the speaker when I touch each plate in the signal path with the meter, but not the oscillator.
B+ readings at each plate were all close to right (5 to 10 volts low or high) EXCEPT the oscillator, which was 51 volts instead of 75. Oddly, the high side of resistor #41 (33K ohm) is about 170 volts, which is close to what I think it should be, but the drop across that resistor is much higher than seems right. That makes me think the oscillator plate current is too high.
I ran out of time last night, but when I get back to it I will look at voltage on the high side of resistor #42 (2200 ohm) and replace the oscillator and see what I can see.
Any other suggestions would be welcome.
John Honeycutt