Strange behavior in the RICM PDP-8/L

Hm.... Have you found and fixed the problem in the meantime? Here a few stupid ideas. Did you swap the core as well? I could imagine a timing issue. It *might* be possible that some timing is slightly wrong globally and that little (and normal) deviations of the core stack's properties begin to show. Did the CPU correcly execute the IAC? Is only IAC affected? Or any other instruction at xx00? Try something like 0075: ISZ 0050 ; just to have the CPU running "a while" before doint the test 0076: JMP .-1 0077: CLA 0100: IAC 0100: HLT and look for the AC value. Try different instructions if you haven't already done. Isn't there a "manual timing generator" involved? I just don't have the schematics handy, but isn't manual cycle timing generated differently than running cycle timing? My first bet is still timing problem. Kind regards, Philipp