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[ried]

Started up my Lisa 2/5 for the first time in several weeks and heard two beeps seconds after pressing the softswitch. Waited for the CRT to illuminate and was greeted with a POST error, showing the CPU card and error 40. First time I've ever seen that on any machine.

Pressed the softswitch to restart and same thing. Huh.

Re-seated the CPU board, no change.

So I moved the CPU board to a Lisa 2/10 and saw the same CPU error 40 during POST.

The 2/10's CPU board worked fine in the 2/5, so I swapped their 68000 CPUs. No change to either board's behavior: It's not an issue with the CPU itself.

The only socketed chips on the board generating error 40 are the CPU, boot ROMs, and video state ROM. Everything else is soldered to the board.

Any idea where to start looking for the cause?

[sigma7]

The only socketed chips on the board generating error 40 are the CPU, boot ROMs, and video state ROM. Everything else is soldered to the board.

Most of the DIP parts in a Lisa are TTL and fairly rugged.

A few are NMOS/CMOS and those are more delicate/sensitive/vulnerable. On the CPU board, the three MMU RAM chips U8A, U9A, U10A are among the parts in this category. I think a failure in one of these is likely to cause error 40.

On some CPU boards U8A, U9A, U10A are in sockets (possibly because they were found to need field-replacement, or maybe lack of parts when the boards were wave soldered).

It sounds like the board in question has these parts soldered in. My first step would be to desolder them and install sockets. That would allow you to swap those parts with another CPU board to see if any are defective.

I suppose prior to that I would wonder why it failed without provocation, and inspect it for some kind of mechanical cause, such as conductive foreign matter stuck to board or something else that doesn't belong or isn't normal.

Good luck, let us know what you find!