How Can I Fix My Aplifier?

Hey, I own a Marshall 250dfx, and having some trouble with volume! I have looked all over the web to no avail so here is the problem...

With all gain/volume dials on full, it produces very little sound, so quiet, it could be overpowered with a sneeze. At these levels it should be smashing windows ! Both channels are the same, clean and overdrive, althought its only the overdrive that has done this recently (I rarely use clean) I have opened the amp up to check for loose connections or components on the pcb but everything seems fine. Does anyone have any ideas ? Thanks, Ric