07-26-2022, 06:49 PM
Hi Rob,
This is an interesting issue, as the 1st IF Primary connects directly between the 6A8 plate and the B+ Bus (that supplies B+ to Audio and through resistors, to the plates and screens of the other tubes) and the B+ Bus is working. I would look for a cold solder joint. One technique:
Confirm the resistance of the 1st IF Primary again. At the exact same points that you are measuring the resistance, without moving your leads (Test clips help here), switch the DVM to DC Volts and fire up the radio. If you see a voltage in the 200V Range, then a bad connection exists between the 2 points that you are measuring. This can be a cold or a poorly soldered joint inside or outside the 1st IF transformer. The resistance reading is good due to the very low voltage and low current put out by the DVM.
However, if the voltage across this point is close to 0V, then look elsewhere, but you are close. I do not suspect that the plate voltage is 0V due to a short in the tube or at the socket, as this would put the full 250V B+ across the 1st IF Primary winding and burn it out!
After resolving this, I would worry about the 6A8. With no plate voltage (and no plate current), then the various grids connected to B+ will handle all the current and possibly burn up, shorting against adjacent grids. This is very common in the output stage of radios if operated without the output transformer connected. This is why most radios of this vintage that have the transformer mounted on the speaker and connected to the Output tube via a plug also interrupt B+ to the rest of the chassis when the speaker plug is disconnected.
Hope this helps. Good Luck.
Best Regards,
John, MrFixr55
This is an interesting issue, as the 1st IF Primary connects directly between the 6A8 plate and the B+ Bus (that supplies B+ to Audio and through resistors, to the plates and screens of the other tubes) and the B+ Bus is working. I would look for a cold solder joint. One technique:
Confirm the resistance of the 1st IF Primary again. At the exact same points that you are measuring the resistance, without moving your leads (Test clips help here), switch the DVM to DC Volts and fire up the radio. If you see a voltage in the 200V Range, then a bad connection exists between the 2 points that you are measuring. This can be a cold or a poorly soldered joint inside or outside the 1st IF transformer. The resistance reading is good due to the very low voltage and low current put out by the DVM.
However, if the voltage across this point is close to 0V, then look elsewhere, but you are close. I do not suspect that the plate voltage is 0V due to a short in the tube or at the socket, as this would put the full 250V B+ across the 1st IF Primary winding and burn it out!
After resolving this, I would worry about the 6A8. With no plate voltage (and no plate current), then the various grids connected to B+ will handle all the current and possibly burn up, shorting against adjacent grids. This is very common in the output stage of radios if operated without the output transformer connected. This is why most radios of this vintage that have the transformer mounted on the speaker and connected to the Output tube via a plug also interrupt B+ to the rest of the chassis when the speaker plug is disconnected.
Hope this helps. Good Luck.
Best Regards,
John, MrFixr55
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
Best Regards,
MrFixr55
