Mesa Boogie Fillmore 25 (schematic request + repair)

[Steelwitch]

Hey! I have a boogie Fillmore 25 up on the bench right now. I’m posting here because I don’t have access to a schematic or else I’d be further along. If anyone has resources to provide that or experience with this amp please chime in!

The main issue is that there’s crackling coming from the output section (present with all knobs turned down) I traced it back to V3 which looks like it’s responsible for channel switching / phase inverter? When I probe the grid of V3 with my scope the noise gets shunted to ground so I have to assume it’s somewhere before that. Is there common issues with the relays? When switching channels there’s an extremely noticeable “thump”. I’m considering whether it’s a possible ground issue. I’ve ruled out tubes / sockets / anything glaringly obvious. All voltages on tube pins look normal. If anyone can help with how to proceed I’d super appreciate it! I can provide more info on request if there’s anything relevant that would help move this along. A schematic, if anything would be so awesome! Thanks y’all.

[glebert]

If the noise goes away when you probe V3 I would be guessing oscillation, but that's a guess. I do some repair work for a Mesa authorized shop, and frankly for at least half the Mesas we have to have the customer send it to Mesa for repair. They are not, at all, designed to be fixed by anyone other than Mesa (at least the more complicated models). I don't even know if Mesa is good at fixing them or if they just swap boards. I haven't done anything with that model, though.

[Jon Snell]

Its only an amplifier
If you shunt V3 then it is before V3. Follow the signal backwards and you will soon find it.
Is the signal at the effects sockets clean?

[Pedro Vecino]

There are many types of crackling. If you put an audio in, it could be interpreted to associate it with a possible origin.
Without further data, it could be a faulty anode resistor in a triode. In the loop, for example, problems in the second triode are relatively frequent in some models.
As for the noise when switching channels, try lowering the reverb controls to zero to see if they continue. It is common in some models to use J175 FETs associated with it to create a temporary mute in the effect when changing channels. Sometimes they fail.

[Arbutt]

Hey! I have a boogie Fillmore 25 up on the bench right now. I’m posting here because I don’t have access to a schematic or else I’d be further along. If anyone has resources to provide that or experience with this amp please chime in!

The main issue is that there’s crackling coming from the output section (present with all knobs turned down) I traced it back to V3 which looks like it’s responsible for channel switching / phase inverter? When I probe the grid of V3 with my scope the noise gets shunted to ground so I have to assume it’s somewhere before that. Is there common issues with the relays? When switching channels there’s an extremely noticeable “thump”. I’m considering whether it’s a possible ground issue. I’ve ruled out tubes / sockets / anything glaringly obvious. All voltages on tube pins look normal. If anyone can help with how to proceed I’d super appreciate it! I can provide more info on request if there’s anything relevant that would help move this along. A schematic, if anything would be so awesome! Thanks y’all.

here it is
Fillmore25_200608.pdf