I have three similar radios, all are Hallicrafters. One is a S-38, one is a WR-600, the last is a SW-500 (the last two are the same as the S-120). All of them are transformerless radios. They all seem to suffer from a similar issue, hum when the volume control is at a minimum. The hum starts when the tubes warm up. The hum is not as loud as bad electrolytic capacitor hum, but it’s loud enough that it makes listening to the radio objectionable.
I’ve done everything that is recommended to do. Replace the old electrolytic capacitors, check. Replace all the paper capacitors, check. Replace out of tolerance resistors, check. Replace bad tubes, check. Check for bad solder joints, check. Check wiring for AC wires running to close to audio, check. I’ve moved them to other areas of the house, and the hum remains. I don’t know what else to check. I’m hoping that someone on this forum has some ideas. Thanks.
Kelley
Welcome, I would start with grounding the grid of the audio output tube. If the noise goes away, you know it is further down the line. Also, don’t rule out a bad tube (heater to cathode short). I had a radio recently (transformer set) that was making all kinds of hum after a rebuild. First I found a poor ground on the driver tube (a tube that drove the output tube). This fixed a volume down hum but it still had one with the volume up. I traced it to the 6H6 detector tube, swapped the tube and it was perfectly quiet. Swap them out if you have test tubes available. Good luck
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