A lot of the newer Marshalls have circuit boards with bad solder joints. If your amp is one of these and heated up enough sitting there that could be your problem. In any case I would recommend having it serviced. There are sources on the internet for the schematic but unless you have electronics troubleshooting experience and the proper tools you are better off taking it to be serviced. If it is still under warrantee it shouldn't cost you very much.
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A lot of the newer Marshalls have circuit boards with bad solder joints. If your amp is one of these and heated up enough sitting there that could be your problem. In any case I would recommend having it serviced. There are sources on the internet for the schematic but unless you have electronics troubleshooting experience and the proper tools you are better off taking it to be serviced. If it is still under warrantee it shouldn't cost you very much.