Restoring a HP9830B calculator.

[Tony Duell]

I am (slightly) surprised, ALU problems normally stop these machines from working at all, but I guess BCD operations are only used for expression evaluation on the 9830

Yes, input A7 to the first ALU ROM is signal X2 on my schematic. X2 is the active-high version of output 2 of the decoder for the microcode word X field, it is asserted for BCD operations. You were, I guess, lucky. If anything else had failed in this ROM, nothing would have worked.

Let's hope that's the only problem.