Marshall AVT150 Switching Woes

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Marshall AVT150 Switching Woes

Post by Bacchus »

In my bid to have a stupid yet stupider still living room setup, I recently brought my Marshall AVT150H head back from Ireland.

It's been in storage for probably a decade at this point. In various places, including an outside shed. It's much cleaner inside than I expected (could have had actual rats or hedgehogs in there, for all I knew). Before that, the amp had a hard life, being used for lots of teenage punk gigs and as the centre piece of various student digs I lived in. At some point, I will have definitely tried pouring beer on it.

Some scratchy pots, which is to be expected, but the biggest issue is that I can't get it to switch to the clean channel. I don't know what I'm looking at, but I suspect that (despite being crusty looking) the switch is sound. I think this because whenever I press the switch, the Acoustic Simulation channel kicks in instead (see video below of front PCB ). So I suspect this might have something to do with however it handles channel switching on the back PCB.

The switch on the physical board is labelled SW7. On the schematic, this is right beside (and sort of part of) the ribbon cable that goes to the back PCB to a a part of the board where the footswitch connects. This was gunky as fuck (it looks like something sticky has spilled through the vents on the top). I've given it a go over with a paintbrush to dust it, and might have a go at removing the board to clean more thoroughly with alcohol.

Anyone any ideas? I'm not sure how fixable these amps were ever intended to be...

Service Manual (this labels the boards as AVT-150-60-02. Mine are labelled AVT-150-60-00. I'm assuming/hoping these are minorish revisions and that the manual is still useful): ... nload.html
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Re: Marshall AVT150 Switching Woes

Post by NickS »

I hate this kind of circuit diagrams (split over multiple pages) and trying to make sense of the connections on a phone is not ideal... I can probably make more sense of it when I can print it out next week. You didn't come across a block diagram as well, did you?
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