Marshall JMP 1 preamp making weird clipping sounds

[marktokach]

I have had this since they came out in late 80's early 90's. I never had any problems B4. When i set the volume above 15 or so and play hard I get these loud pops like it's clipping. If i keep it lower than 15 it doesn't. Never did ths before even with volume on 20? TIA, Mark.

here is a link to it;

http://www.blamepro.com/mar1730.htm