Hey Ive had this mark iv for about 4 months now. bought it from a friend who had no problems with it. all the sudden after a couple weeks of playing ive started to get horendous feed back and noise/loud buzzing on the lead channel and faint buzzing on the rhythm 1 and two channels that gets worse as you turn up the preamp volume and or poweramp volume. please help?
I'd start with replacing the preamp tubes.
Take it to a tech, mine had a similar problem that ended up being shoddy soldering.