Looking for ideas on a troubled Marshall Valvestate

[Rockhorst]

*note: schematic attached at the bottom*

A friend of mine came by with his Marshall 8280 Valvestate Bi-Chorus. It has a clean/crunch and a OD1/OD2 channel. When OD2 is selected, playing chunky riffs through high output pickups (passive or active) on the lowest string, especially with palm mutes in there, the amp clamps down the signal. Trying to describe it in words I would say it sounds like your signal is 'sucked' away. It's like a really aggressive compressor that sucks away tone, volume, crunch and only releases when you let the guitar ring out again. The amp was in storage for a long time but my friend does believe that it didn't do this in the past. What I've tried so far:

- replace 12AX7
- replace power filtering caps
- replace the caps around the 12AX7

I then audio probed through the preamp circuit and found that the problem is really early on in the circuit, before the tone controls and also before the tube, although it gets more pronounced/amplified by the tube stage. Something around the dual opamp gain stage is causing problems. My friend also has the 40W version of this amp and he claims that it slays the 2x100W version. I've put the partial schematics of the input stages side by side and attached it. Note that the dual opamp has an internal switch: only one half is active at every time (neat trick to switch channels by the way, could be useful for projects!). The 8280 has a discrete mosfet input stage before the opamp. The 40W version uses a TL072 instead and according to the schematic has a 720Hz roll off.

My working hypothesis right now is that there's too much bass going into the amp and that I may be able to tame that down by lowering C2.
That said, I'd like some alternative suggestions if there's anything on offer.

[aiaosu]

I've heard that the high power resistors on the back side of the PCB can become desoldered. I would look over the board with a magnifying glass and look for cracked or damaged solder connections.

FYI, I've heard of this but never worked on a Marshall Valvestate. I have found solder issues on many boards though, especially where heat was involved (i.e. pre amp tubes, power amp tubes especially EL84's, etc).

Just my initial 2 cents. Someone else will probably be more knowledgeable than me.

Sent from my LM-V405 using Tapatalk

[lars]

My working hypothesis right now is that there's too much bass going into the amp and that I may be able to tame that down by lowering C2.

That probably won't help since at one point the amp was working fine with the original value for C2 in there.
Your best bet sounds like to focus on those zener diodes (ZD3, 4, 5) that are present in the 8280 circuit, but not in the 8240. Since diodes are often used to clamp a signal, the fact that you're saying the amp sounds "compressed" and "clamped" when you play heavier notes, gives me the impression that one or more of those diodes aren't functioning correctly.

[Rockhorst]

I've heard that the high power resistors on the back side of the PCB can become desoldered. I would look over the board with a magnifying glass and look for cracked or damaged solder connections.

I used an audio probe to tap off the signal just before and after the tube and feed that through an external power amp. So the Valvestate's power amp is out of the equation.