BACKGROUND:  I have a Marshall Park series 30w amp that has recently stopped putting out sound, and its got me banging my head on the table.  When I turn it on, and turn the volume up to 10, it still puts out very loud white noise like it always did before any signal comes in from the guitar.  It sounds like it's ready to split your eardrums if you pluck a string, but when you do no sound comes out at all on the clean channel.  When I switch to overdrive and turn the gain all the way up, almost the same thing happens, but the guitar sound does come through very, very faintly and clean, as if every knob on the amp was at 0.1.  You can barely hear it when it should be full blast.  I thought it might be the input jack, because once I jiggled it and it rocked to life for a split second, but it only did that once, I could never make it happen again.  I replaced the jack anyways, and it didn't fix anything. 
HELP NEEDED: I think that the preamp must be bad, but I cannot find anything on how to repair it myself.  I am a decently competent amateur with electronics.  I feel like I can fix it if I can just find a how-to or some guidance... can anyone out there help?  Its become sort of a quest for me know, gotta prove that I can fix this thing haha.

Thanks for any help.