Restoring a HP9830B calculator.

[Tony Duell]

It is very unlikely that this is caused by a problem with either the A or T registers. I think that would stop the machine from working at all

An ALU problem -- the second ROM that handles the 3 higher bits of the BCD adition -- is also not very likely, but possible. Check the BCD carry flag flip-flop too, although that is also not too likely to be the problem.

Alas my first guess is a main ROM problem. Or maybe RAM -- perhaps a few locations in the system area are malfunctioning.