02-09-2023, 02:14 PM
Is it a 60Hz hum or 120Hz hum?
A few things can be going on here.
First, check wiring, particularly Caps c48 and C51. Note that C48 and C51 do not go to ground, they go to to the center tap of the B Supply. This grounds through a resistor for the purpose of developing the C- Bias supply required for the Output stage, among others.
Second, with the power off, remove the 27 tube that is labeled 1st AF Amp on the schematic (Actually, this is the driver for the output stage). Do you get the hum at the same volume or is it greatly reduced. If greatly reduced, the power supply may be OK and there may be a filament to cathode short in a 27. I don't know how common that is. If not greatly reduced, then there is likely a big power supply issue.
Third, Power down, pull the 27 that serves as the "Det - Amp", reinstall the 27 in the 1st Audio Amp and power up. Did the hum return or remain the same as when the 1st AF tube was out. If same as when the 1st AF tube was out, Power down and remove the Det Rect tube and install it in the Det Amp position and power up. Still no hum, then either the 27 tube left out is bad with a short, or there is an issue before the detector.
If Unit hums as before with the 1st amp in and the Det Amp out, power down, remove the 1st AF tube and install another 27 in its place, leave the Det Amp out and power up. If no change, then there is a hum in the power supply that is being amplified. If removing any of the 27sreduces the hum to an acceptable level, then work the 24s in the same manner.
Of course, if you have an o-scope, this can be useful.
Other considerations:
How loud is the hum compared to other radios using a 71A, 45 or 47 tube? These tubes are heating a direct filament heater with AC. There is no "hum balance. Instead, the transformer is center tapped. If there is a short in the filament winding "B". Check by:
Removing the rectifier tube. carefully measure between one filament leg on secondary b and its center tap. compare to the other. the difference should be very low, I would guess < 5%
A few things can be going on here.
First, check wiring, particularly Caps c48 and C51. Note that C48 and C51 do not go to ground, they go to to the center tap of the B Supply. This grounds through a resistor for the purpose of developing the C- Bias supply required for the Output stage, among others.
Second, with the power off, remove the 27 tube that is labeled 1st AF Amp on the schematic (Actually, this is the driver for the output stage). Do you get the hum at the same volume or is it greatly reduced. If greatly reduced, the power supply may be OK and there may be a filament to cathode short in a 27. I don't know how common that is. If not greatly reduced, then there is likely a big power supply issue.
Third, Power down, pull the 27 that serves as the "Det - Amp", reinstall the 27 in the 1st Audio Amp and power up. Did the hum return or remain the same as when the 1st AF tube was out. If same as when the 1st AF tube was out, Power down and remove the Det Rect tube and install it in the Det Amp position and power up. Still no hum, then either the 27 tube left out is bad with a short, or there is an issue before the detector.
If Unit hums as before with the 1st amp in and the Det Amp out, power down, remove the 1st AF tube and install another 27 in its place, leave the Det Amp out and power up. If no change, then there is a hum in the power supply that is being amplified. If removing any of the 27sreduces the hum to an acceptable level, then work the 24s in the same manner.
Of course, if you have an o-scope, this can be useful.
Other considerations:
How loud is the hum compared to other radios using a 71A, 45 or 47 tube? These tubes are heating a direct filament heater with AC. There is no "hum balance. Instead, the transformer is center tapped. If there is a short in the filament winding "B". Check by:
Removing the rectifier tube. carefully measure between one filament leg on secondary b and its center tap. compare to the other. the difference should be very low, I would guess < 5%
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
Best Regards,
MrFixr55
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