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Quick update. A visual examination of the 540 board found one ceramic capacitor (C40) with a lead that has minutely separated from one of the contact pads. C40 is situated between U1F (2114) and U1E (74175). The suspect lead looks like it is barely touching the pad. The only other visual item of interest was U6K (7420) which has pins with lots of carbon dirt but otherwise seems fine.

I've replaced the 2114s at U1F, U2F and U2C. I need to fix the broken contact point for C40 and then I'll reinstall the board and see what happens next.

I replaced U1F, U2F and U2C and soldered back down C40, reinstalled the board and all is back to normal. Thank you, Mark. You really nailed it. Tom

U1F is probably fine. One of the other two is bad for sure, maybe both. I'm glad you have it working again! Good job!