Advice for a recalcitrant 29c

[[kby]]

I have a 29c that I don't seem to be able to make further progress on (although it's "better" than when it started, at least).

This 29c arrived with no display on power-on The power supply voltages looked OK (powering from charged NiCd batteries; no charger), but there was no Phi 1 or Phi 2, so I replaced the ACT with one from a -67 logic board (same chip number).

At that point, the calculator does show signs of life. It powers up to "Error" (which now blinks once and still says "Error"; can't remember at the moment whether it was the same just after replacing the ACT, but I would guess it was). I was maybe 90% sure the ACT from the 67 was OK as I think the only thing wrong with the logic board was I'd removed the CRC (can't remember why; might have thought it was the ACT long long ago), and it had the buggy -67 ROMs.

A more detailed note of the behaviour:
1. Always powers on to "Error" that blinks once, even if batteries are not removed.
2. Pressing any key clears "Error" and results in "0." in the display.
3. Key presses seem to register (display blinks when key is pressed), but many don't work as expected. Digits, if entered by themselves, change the display to "0.00000…0 00" (e. g. full scientific notation of 0). PRGM/Run switch works properly. SST/BST work properly in both Run and PRGM modes. Function keys sequence properly although the display still winds up being 0. at the end: for example, if you give "f ln" in program mode, the program counter does not advance until you hit the ln key (which is the 7 key). In Run mode, f ln yields "Error" after hitting ln, as it should. Since I can't get any other numbers in, it's hard to test much else functionally. All prefixed operations appear to execute in the sense that the sequence of keys is processed as expected (e. g. f ln requires two keystrokes; sto + 3 takes 3), but every thing results in "0." at the end (unless it's an error as in f ln). In short, keystrokes do seem to register sort of correctly, but no useful results can be obtained. Operations don't get entered into program memory in PRGM mode, even though the step counter advances; program memory remains all 74s (R/S).

I obtained a copy of the service manual and with its guidance have tried the following, using chips from a 19c that had mad printing disease originally, and now parked the head but still seems to have the motor energized. I understand that its chips may thus be suspect, although the symptoms above have not changed regardless of what I've done, so unless the failure mode is the same, it seems unlikely.

1. Replaced ROM 3
2. Replaced ROM 1/DS 1
3. Replaced DS 2
4. Replaced the ACT again with the one from the 19c

Reading through the failure chart, ROM 0 seems unlikely as CLEAR PRGM, CLEAR Prefix, SST, and BST are known to work. ROM 2 seems to be dedicated to trig functions, so, although it may or may not work, it shouldn't cause this problem. DS 0 are the primary and statistics registers; again, it could be bad but it seems I haven't been able to get far enough to have this be testable. The LLD chip seems like it might affect the "Error" message on startup, but nothing else.

I think I've vinegared the board, but it looks clean; no obvious corrosion or missing traces, although many of the traces go under chips. It's a little hard to test the continuity without a board picture or pulling off all of the chips at once.

Any thoughts? Thanks.