Looking at pcb board side it appears a njm4558l was snipped out and there was tone on one of the pins but no others. so being a Operational Amplifiers - Op Amps Dual High Gain. I think I can stop here until I can order one in.
This pa was given to me in an unknown state so anything done to it is unknown.
I now have signal passing through the preamp channels 1-5 now, very low output though ( like putting my ear to the speaker low). Master volume makes no difference when rotated. So I used a pair of headphones and probed the master volume with 1k sine going in ch 5. I get sine on one leg, then move to the other leg of pot hear no sound until I get to like 9 or 10 on switch. part number is 503A 50k pot. Going to sub another in and see how it goes.