Marshall 9200 60Hz Hum & Noise

[AmpFix]

Could a PT cause 60Hz noise?

[Enzo]

I can't see your sweep rate, but are you sure it is 60Hz and not 120Hz? Leave the scope settings along, and pull the probe off teh circuit and touch it with your finger. The resulting ugly waveform will be 60Hz. it it has the same number of cycles across the screen then your signal is 60Hz, but if the finger yields half as many cyclces, then you had 120Hz.

Lots of swapping is not troubleshooting, it is just guessing. Plus, it isn't always a bad part causing issues.

Isolate the problem. First verify voltages. Pull the power tubes in the noisy channel. is there good B+ on pins 3 AND 4 of all four tube sockets? And is the bias getting to all four on pins 5? And verify ground is at pin 8 of each. Missing screen voltage on any tube will unbalance the output thus killing the normal hum cancellation there. And bias? If the DC bias voltage is clean, then it is clean. It won;t be the AC's fault if the DC isn;t clean.

return the tubes after fixing anything wrong there. SO assuming nothing was wrong in the sockets, the hum is still there? SO the volume control at zero does not kill the noise? You have an input buffer tube, a paralleled dual triode 12AX7, then a phase inverter stage using two sides of a 12AT7. Pull the 12AX7, hum stay or stop? If it stops, the first stage is gathering the hum, if the hum stays we move on and pull the phase inverter. Hum stay or go? Check the voltages on the phase inverter. Are the two plate pins, 1 and 6, both at about the same voltage? Looking for a gross difference, not a couple volts off. Note there is a balance trim control on the board. I am concerned maybe ther is an open plate resistor or some other reason the stage is way off balance.

If pulling the small tubes does not kill the hum, then the power tube stage is at fault. Pull all the power tubes, and pick one you know is good, one that sounded OK in the other channel perhaps. Plug that ONE tube into one of the four sockets and power up. Listen to the sound, now move that same tube to the next socket and listen, and so on, testing all four sockets one at a time with the same tube. It SHOULD sound the same in all four, if one sounds different, that is a clue.

We need to find WHERE the problem is, then it is much easier to find out what is causing it.