Wierd hum in Peavey Transformer

[crunchee]

Hi all

I have a Transformer 1x12 and it exhibits a hum thats independant of volume and guitar plugged in or not.Sometimes it does this when first turned on, other times after its been on for bit.Now I can rid of this by pounding on top of the cabinet.Goes away and comes back.Also, the amp just doesn't sound like it used to.Distortion sounds fuzzie and notes die off early.
Ive looked at the chassis and nothing looks obviously bad,connections all tight.
The only thing I noticed was the fuse for the ac plug input looked cooked.A dark brown spot on part of the fuse filament.Not sure if this means much since the amp turns on.
Any ideas?
Thanks

[Amp Kat]

Sounds like a ground problem or a cold solder joint on a ground spot. Try touching up the input jack underneath the pcb with some solder. Sometimes they pull loose and you can't see it. If the amp plays ok that rules out the output drivers but definitely sounds like a connection problem. Touch up the filter caps also. They will be the biggest capacitors in the amp.