FBT M12X4 preamp Problem - service manual or schematic needed

[tedsorvino1]

Hi everyone. I recently bought a used FBT M12X4 preamp unit and apparently there is a problem with one of the 2 channels (tested in every input).
I have experience on repairing stuff but i can’t find any schematic or service manual online.
Does anyone have the schematic for that unit or any idea of any similar fbt device.
The whole defice is based around tl072 op amps mostly (and a few ne 5532). Could also be something with cleaning but it would be so much better if i could follow a valid schematic

Thanks in advance

 

[turk 182]

make certain the priority switching isn't affecting the operation.

there's not a whole lot to these so do you really need the schematic?

looking over what info i could find online about them i'm not sure how they can call it a 12 input 4 output mixer...

 

[tedsorvino1]

Thanks for the reply Turk.
I think i may have found the area where things go wrong. But it’s still a guess. The board is messy. A lot of op amps.

I found that things don’t look good on the input and output pins of the only LM 339 ic on the board.
For switching there are some vactrols involved. Hopefully things are not wrong there since i can’t really test them since they are expensive components.
There are also a few pnp npn transistors, i guess they are used for switching too.

I do agree that calling a 6 simultaneous in preamp a 12 one is not correct (but with switching it has 4 stereo + 4 mono ins). It is a 4 out one (2 normal and 2 rec).

But the major disadvantageis that there is no input panning and no way to direct 4 ins to 4 individual outs.

The schematic might give me a clue how to achieve such a mod.But still no luck.

 

[turk 182]

any small TO92 style transistors are likely to be JFet's for muting.

the LM339 is likely used to trigger the muting transistors so if that's looking funky then yeah signal weirdness is likely.

 

[conanski]

But the major disadvantageis that there is no input panning and no way to direct 4 ins to 4 individual outs.

The schematic might give me a clue how to achieve such a mod.But still no luck.

That is a totally different type of device... called a matrix or zone controller, it won't be possible to modify this unit to achieve that kind of routing flexability.

 

[tedsorvino1]

any small TO92 style transistors are likely to be JFet's for muting.

the LM339 is likely used to trigger the muting transistors so if that's looking funky then yeah signal weirdness is likely.

Since i have no experience with muting circuits what other components could be involved ( because according to the block diagram there are plenty of muting circuits in there)?
Could for example a side of a dual op amp being used for muting?