Fender Bassbreaker 15 - 120Hz Hum When Muted

[Meerkat]

I just purchased a used Bassbreaker 15. As soon as I got it home and plugged it in, I noticed it had a hum that I hadn't noticed when I tried it out before I bought it.

This hum starts as soon as I flipped the power switch, even with the mute on. Listening and comparing to different tones, it seems a dead ringer for 120Hz.

First question:
1. What would be the most likely cause of this?

From what I could find online, 120Hz hum seems to be most associated with the filter caps. From looking at the schematic in the service manual (linked here), this seems to be C42,C43,C44,C49, C80, and possibly C32 and C50 as well.

Second question:
2. Are these (C32, C42, C43, C44, C49, C50, C80) all of the filter caps that people would refer to when they talk about "replacing the filter caps"? Or are others in the circuit included power section (like C30, C31, C36, and C41, or C45, and C46)?

Third question:
3. How would I find out how to order replacement capacitors; the parts numbers in the service manual seem to be internal to fender, rather than manufacturer part numbers.

Sorry if any of these questions seem to have obvious answers. I have a reasonable amount of experience working as an electronics debug technician, but this is my first time trying to fix a guitar amp.

[glebert]

On this amp there is an op amp after the muting FET. Assuming it is power supply ripple it could be on a low voltage supplies for the op amp or something related to the tube section. With the usual warnings about electrical safety, I would start by measuring the voltages for ripple either with a DMM or an oscilloscope.

Also, it wouldn't hurt to try a different 12AX7 in V3. You can also pull V3 and see if the hum is still there. I would also check the voltage across the power tube cathode resistors (R134 and R70) to see if they are conducting similar current at idle. If the power tubes are conducting equally I would expect them to cancel out most ripple on B+1 or B+2 since this is a push-pull amp (although I could be wrong on this).

As far as replacement capacitors, usually you need to look at the size of the caps on the PCB and the types of the leads and spacing. I have had amps that had very constrained space for caps and it really limited what I could use for replacement parts.

[J M Fahey]

Canīt find an useful purpose for that stupid "mute" switch, maybe designer was weaned on chipamps and misses it dearly?
Nonsense in a Tube amp.
Whatīs wrong with a real Standby switch, even a light version which just cuts screen voltage off?
At least it would reduce tube dissipation and wear when not playing.

As-is, it does nothing of that, and allows any junk on power amp to fully appear at speaker out.

I would :

1) measure each EL84 cathode voltage, one of those silly Zeners D12/D13 might have shorted leaving matching power tube in full self nuke mode, grossly unbalancing output stage and heavily loading power supply.

2) measure +V and its ripple.
Mind you, some cheap meters cannot separate DC from AC on AC scale, typically those with just 2 AC scales: 200VAC and something about 700VAC