Deep Switch Won't Work

I own a 112RP-65, dated 4-2-79. When I flip on the deep switch it kills all sound. I've tried another switch, replaced the capacitor, no change. It sounds like the signal goes to ground. Is there something I'm overlooking? }}

THere is a 1 meg resistor across the legs of IC1 that MAY be open. That would give you a much shorter path if it was open. OR IC1 can't take the strain. I'd look at the 1 meg first but I'm betting on a junk IC.

Bad IC is the problem. My amp has 7 IC's. 2 are LM 1458N, 5 are MC 1458P. Are these the same or is there a difference? Should I look for an exact replacement? Or is there another IC that I can try for a different sound? Where is the best place to buy a new IC?