Markbass MoMark almost no output

[glebert]

I've got this modular Markbass amp with just a whiff of output signal. I think the problem is that the signal isn't getting from the preamp to the power amp. When I listen to the white noise at the speaker with no input it sounds about right for a big bass amp, and when I turn the knobs on the master volume module (MVVL) I will sometimes get a really good pop from a dirty pot. When I put a sine wave into the input I can trace the signal through the EQ module and over to the op amp on the backplane behind the MVVL module. It seems like I have signals going to the MV module but they don't change with EQ, which is weird. Because of the multilayer boards and backplane topology it is a bit of a challenge to probe some points to trace the signal.

I also looked on the line out and don't have any signal there.

I know Markbass doesn't give out technical docs. Anyone have any info about where the signal should be coming into and out of the MVVL module and through the backplane?

TIA,
Greg

[Enzo]

Did you plug a cord from FX send to FX return? (Or whatever those two jacks are labelled)

[glebert]

Did you plug a cord from FX send to FX return? (Or whatever those two jacks are labelled?

Yes, output was almost inaudible, but it is not a simple FX loop, it is parallel with a return pot on the MVVL (although there are jumpers on the board that supposedly change from parallel to serial. ( I tried both, no change).

Speaking of jumpers, there are a couple of 3 pin header selectors on the MVVL module that are labeled as "mono" or "BIB". The jumpers were set for BIB, I have not tried mono, as I can't find any info on it.

[TomCarlos]

Is this the small cube type unit that fits in a horizontal frame (where you can have other units)? Can you send us a link to this or post a front and rear photo? I am curious to see what you have.

Thanks.

[Jon Snell]

Have you output with a signal inserted into Effects Return ... that would demonstrate which way to fault find.

[glebert]

Is this the small cube type unit that fits in a horizontal frame (where you can have other units)? Can you send us a link to this or post a front and rear photo? I am curious to see what you have.

Thanks.

Yes. After digging in it seems like the basic architecture is the same as the Little Mark III with the preamp split into three modules. The modules are normally housed in plastic boxes like the preamp (left module) is in the picture.

[glebert]

TV

[glebert]

Luckily the service manual for the LM3 is on elektrotanya. The part designators are obviously different for the preamp but it makes it possible to follow the signal flow.

And yes, I have plugged into the effects return, sound is almost inaudible.

https://elektrotanya.com/parsek_mark.../download.html

I can trace the signal going into the op amp IC4b (service manual designation) but nothing coming out. This is consistent with the fact that I get loud pops from the dirty VPF pot, which is the next stage in the chain and pretty much right before the master volume.