Advice needed: 29C Problem


I have an HP-29C which does not show any display at all. The previous owner stated that it did work the last time he used it, but the battery pack was dead. When I replaced it with a good pack, I still get no display at all. The original batteries were badly corroded, and I fear he may have plugged in the AC adaptor and damaged it.

Anyway, can anyone comment on the failure mode? I know that there are particular chips in this beast which are likely to be fried if the voltage is allowed to spike at all. In this case, is the failure mode no display at all, or is it something else?

Things I have tried:
1. Connected a 3V supply directly to the terminals.
2. Opened the case and verified 3V on the PCB when the power switch is on.
No result from either.

Any advice is appreciated. I would sure love to bring this one back to life.



Most likely is a bad connection to the battery contact becuase of the corrosion. Clean very well and perhaps wrap the calculator contacts in aluminium foil. Also place small wads of foil on top of each battery terminal. Make sure your battery is good.

If the green gunk corrosion got into the machine itself, you might try standing the circuit board up in white vinegar up to the level of just past the battery contacts for half and hour or so. Rinse very well. Shake dry. Then let dry overnight in an oven on LOW (like 120 F).

Next most likely is a bad ACT chip. It is the big chip bear the battery contact area of the circuit board. If you know what you are doing and have the proper delsodering equipment you can replace this chip with one from an HP21/22/25/25C/29C/67/97 (but not a 27).

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