[sdiy] [OT] puzzling EPROM problem

[Byron G. Jacquot]

>Any thoughts? It's bad enough not being able to repair something, but it's
>worse having got something working and not knowing why...

If you can track down any release notes covering what changed between v3 and
v5, that might tell you what changed.

Does the system have non-volatile memory? (of course, with highly integrated
micros, you might have a small patch of NV RAM or EPROM and not know without
finding the specs of the processor)  One guess would be that v3 detects some
kind of error, writes the error details to that NV memory, then closes up
shop.  The next time it boots, it checks that memory, learns about the
error, and refises to do anything.  Until the error mark is cleared, you're
stuck.

When it hangs, it might be waiting for intervention from a diagnostic tool
to ask for the code...possibly for field failure tracking.  There might also
be a bug in the error correction routine, which isn't an impossible
situation.  Some types of errors can't be simulated in the lab, so the code
can't be tested until the device is delivered.

The new version might change any of the above.  It could fix a bug, or
ignore the failure, or be looking in a different memory location for the
error code...software is easy enough to change, that without knowing what's
different, it can be very hard to guess.  Would it be worth your effort to
disassemble those image files?

Of course, if the thing has been subjected to lots of vibration (in the form
of loud music) and/or cigarette smoke, I'd also begin to suspect the socket.
Put v3 back in and see what happens.

Byron