11-09-2024, 03:42 PM
Hello! MrFixr55: thanks for your detailed troubleshooting tips. My results are as follows...
By R38, do you mean C38?
Yes, I meant C38.
Ohm between the green /white wire (junction between cathode of rectifier and R37) and ground. The reading should be infinite (after charging of the Cap R37. Repeat the test measuring between the green / white wire and the HV secondary center tap. Again, the reading should be infinite after the cap charging. A low resistance or short indicates a short in the wiring, C37 or a grounded Rectifier filament winding. Look for frayed wires, solder splashes, etc.
Cap continued to slowly charge but after a minute or so it was at 2.8MΩ and I figured that was close enough to infinity. Could this be the issue?
Leaving speaker disconnected, measure between white wire (junction of C38, the field, output transformer B+ Radio B+ circuit, etc. the reading should be the sum of resistors 30, 31, 32 and 33, approximately 90K Ohm after C38 charges. A low resistance indicates a short in the power supply or these resistors. (Short in R32 or r33 would contribute negligibly to the issue but would cause bias problems in operation.
89.3KΩ.
Again, with speaker disconnected, measure between plate pin of the 43 (output) tube and ground. Reading should be infinite after charging of bypass and tone caps. Low reading or O Ohme indicatres a short in wiring or the tone control cap C47) or switch or HF bypass cap (C48)
Infinity.
Leave speaker disconnected. Run current / dim bulb test. Current should be negligible but monitor voltage between green / white wire and chassis. Do not allow the voltage to increase to a level higher than the "WV (working voltage) of the capacitors.
0V.
Leave the speaker disconnected, install all tubes other than the 80. Repeat the dim bulb test. Current should be substantial, I would think at least half or more of the rated value, as the filament of each tube draws 0.6A except the 42 which draws 0.7A, so the total load on the secondary is 3.1A at 6V. so 0.3A transformer current draw is not out of the question.
No glow on the dim bulb tester, all filaments glowing and pulling .125A.
With the speaker disconnected, remove all tubes and reinstall the 80. The 80 filament draws 2A, therefore you will draw 0.2A just from the filament. Measure between chassis ground and the junction of the 80 Filament, C37 and the green / white wire (goes to the field coil). You should get about 350-450V.
Bringing power up slowly the dim bulb begins to glow at 80V and glows brightly as I increase the voltage. At 110V I'm only getting about 16V at this junction. Tried another 80 tube and got the same results...
This tells me that 1) the speaker isn't the issue and 2) there's still something shorting out or overloading in the 80 circuit. All the wires are in pretty good shape (especially considering their age) and ohm out fine. The transformer leads to the filaments of the 80 tube measure about .2Ω when the schematic calls for .1Ω, close enough.
37 filter cap is reading 14µf, though their may be something in parallel that's increasing that value from the 8µf I put in there. I guess I keep looking at those caps, hoping for operator error! Not sure what else to try at this point...
By R38, do you mean C38?
Yes, I meant C38.
Ohm between the green /white wire (junction between cathode of rectifier and R37) and ground. The reading should be infinite (after charging of the Cap R37. Repeat the test measuring between the green / white wire and the HV secondary center tap. Again, the reading should be infinite after the cap charging. A low resistance or short indicates a short in the wiring, C37 or a grounded Rectifier filament winding. Look for frayed wires, solder splashes, etc.
Cap continued to slowly charge but after a minute or so it was at 2.8MΩ and I figured that was close enough to infinity. Could this be the issue?
Leaving speaker disconnected, measure between white wire (junction of C38, the field, output transformer B+ Radio B+ circuit, etc. the reading should be the sum of resistors 30, 31, 32 and 33, approximately 90K Ohm after C38 charges. A low resistance indicates a short in the power supply or these resistors. (Short in R32 or r33 would contribute negligibly to the issue but would cause bias problems in operation.
89.3KΩ.
Again, with speaker disconnected, measure between plate pin of the 43 (output) tube and ground. Reading should be infinite after charging of bypass and tone caps. Low reading or O Ohme indicatres a short in wiring or the tone control cap C47) or switch or HF bypass cap (C48)
Infinity.
Leave speaker disconnected. Run current / dim bulb test. Current should be negligible but monitor voltage between green / white wire and chassis. Do not allow the voltage to increase to a level higher than the "WV (working voltage) of the capacitors.
0V.
Leave the speaker disconnected, install all tubes other than the 80. Repeat the dim bulb test. Current should be substantial, I would think at least half or more of the rated value, as the filament of each tube draws 0.6A except the 42 which draws 0.7A, so the total load on the secondary is 3.1A at 6V. so 0.3A transformer current draw is not out of the question.
No glow on the dim bulb tester, all filaments glowing and pulling .125A.
With the speaker disconnected, remove all tubes and reinstall the 80. The 80 filament draws 2A, therefore you will draw 0.2A just from the filament. Measure between chassis ground and the junction of the 80 Filament, C37 and the green / white wire (goes to the field coil). You should get about 350-450V.
Bringing power up slowly the dim bulb begins to glow at 80V and glows brightly as I increase the voltage. At 110V I'm only getting about 16V at this junction. Tried another 80 tube and got the same results...
This tells me that 1) the speaker isn't the issue and 2) there's still something shorting out or overloading in the 80 circuit. All the wires are in pretty good shape (especially considering their age) and ohm out fine. The transformer leads to the filaments of the 80 tube measure about .2Ω when the schematic calls for .1Ω, close enough.
37 filter cap is reading 14µf, though their may be something in parallel that's increasing that value from the 8µf I put in there. I guess I keep looking at those caps, hoping for operator error! Not sure what else to try at this point...
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