Solo 50 troubleshooting

I'm relatively confident with point to point amps, but now I've got a Mesa Single Rectifier (series 1 (rev e, I think), solo 50) on the bench. I've worked little with PCB-base amps before, but this seems fairly okay because all pots and connectors are wired off board. Now to the problem - theres some irritating hum in the amp. I've replaced all tubes (with new ones) and the noise got better but the hum is still there. It's specially noticeable in the clean channel and really bad when the clean channel is turned to "pushed" mode. But even if I turn down the master volumes the hum is still present, way too loud to be okay. So I've chopsticked a bit around in the amp an found that poking the wires comming off the clean channel gain pot is awfully microphonic, almost like tapping a microphone. This does not happen on the second (overdrive) channel, which is kind of weird because theres a whole lot more gain going on there.

I must add that when I got the amp the fuse was blown and the output tubes was bad. Can this be the output transformer causing the noise? Is there a way to measure the OT without replacing it? Also, there's some burned PCB around the rectifier-diodes for V1-heater which seems to be fixed with new diodes way back. Is that where I should start replacing parts? Or should I start in the other end since the hum is there when the master is down and nothing is plugged in?

If the hum is present with no output volume, it could very well be the OT. Another person here isolated their OT with rubber washers and it killed their hum. Maybe, take a look where it is soldered on the board. My OT looked like a monkey soldered it.

Since the tone stack for Ch1 immediately follows V1a and leads to V1b, it would be more sensitive to noise issues there. Maybe swap a couple of other tubes in just to see if it changes.

If V1 is noisy, putting it on an oscilloscope with a test signal and no signal will show if there are spikes from heater noise. However, if the PCB had a failure around those diodes, the relays would not work and V1 would not pass signals. A cold joint or oxidation/burn on the track could be a problem, but the hum is still present when the preamp is silenced. I guess if the heater mains have an issue, it could affect the output tubes' heaters, along with all the other heaters. Check the grounding.

Thanks afu, I'll dig into that. I cleaned up the diode area - I replaced the diodes and resoldered from the back, it looks like it was done from the front the last time. I also ordered new filter caps, both power and for the 6.3V stuff (6800uf, but I ordered some 10000uf as well as I recall the earlier rectos used that, right?). I also ordered some more preamp tubes just in case. I have another OT (for a Marshall-ish build though) so I could try to replace the OT if isolating doesn't cut it. I also hear that the PT is humming/resonating quite a bit in itself when it gets power applied to it... I'll have to wait for the new parts before I can troubleshoot some more because I removed all filter caps while waiting for my order.

I've replaced all the large filtercaps (220uf, 30uf and 10uf), filter for the 6.3v DC (6800uf, replaced with 10000uf) and the bias caps 220uf/63V. I also , as previously noted, cleaned up the diode area. The hum is gone and the amp is as good as new.

Awesome. I didn't think of that. I had a hum in an old Harmony amp and had to replace the filter caps for the same reason. The Solo 50 might be old enough to need a cap job if it was used a lot. I don't know why I didn't think of it.

The PT would have noise, wouldn't it? It's 120 Hz AC being converted to a higher DC voltage. Hence the choke and RC filters in the supply and the additional diodes for V1's heaters.