Solid State Guitar Amp Forum | DIY Guitar Amplifiers

I have a Marshall AS100D acoustic amp that, for a long time, has had problems with certain channels cutting out at loud volumes and now has several that don't work at all.
Can anyone give me any information on what might be causing this and/or how to fix it?
Thanks.

Or even just advice on how open the amp properly.

Wow. A very complex amp. It won't be easy.
Start by asking Marshall for its schematic and post it here.
They *should* provide it if you send them the serial number as proof of ownership.
Meanwhile, probably what you have is what I call "mechanical" problems, meaning dirty pots, jacks and switches, unseated ribbom connectors, etc.
Anyway I'm surprised at having "many" channels off, in a new or very young amp.
Either it was very much abused or it's of quite poor quality.
*If* it's still within the warranty period, rush it to the shop where you bought it and use it.
If not, maybe you lost it because now most Companies treat their produce as disposable, and not even them, not even paying, repair them but anyway ask for the schematic.
Buy a can of contact cleaner such s DeoxIt or similar.
Most combo amps have the chassis held by 4 top screws (they might be on the top sides also); you pull them and slide tha chassis backwards as a drawer.
Be careful because it sure has a couple wires connecting them, at least to the speakers and maybe a reverb tank.
Such wires *should* have some kind of connector which aloows their disconnection for convenience, but label them, at least mark them with a Sharpie and the way they are plugged, so later on you can connect thm the right way.
Take a couple sharp well illuminated pictures of the chassis inside.
Good luck.