HP-41C malfunction during startup

[mfleming]

Hi prwktokali, welcome!

My guess as to the most likely cause for your symptoms is a poor connection between the CPU board and the main PCB. This can manifest itself in power fluctuations and signal problems to the display or plug-in modules. By the way, congrats on the post fix, that's usually the first mechanical failure in this model.

I'd start by removing and cleaning the CPU connector, located under the CPU board and inserted over the two lower posts. It should have two parallel conductive strips that carry signals from two rows of contacts on the bottom of the CPU board to corresponding contacts on the main board. The best cleaning fluid to use is called DeoxIT Red. If not easily available use clean water or isopropyl alcohol. Use a Q-tip style applicator.

Start by making sure the gold contacts have no debris on them (they're usually OK). The connector conductive material itself is either a rod with parallel gold rings or a conductive elastomeric strip. Either sort may have debris that should be cleaned off. Both of these styles of conductive connectors are compressible so they will make good electrical contact between boards when you tighten the lower screws. With age they remain pressed down when the CPU board is removed and replaced. Try squeezing or rolling them a bit to restore their original shape. You may need to play a bit with repositioning the connector, turning or flipping it to get a new connection.

Look for and clean up any green oxidation you might see. If the oxide is the result of a battery leak then dilute vinegar will neutralize and help remove it. That's a big cause of problems with calculators that have been stored for a while. I've found that most every HP-41 can be restored to full operation, and yours sounds like it's almost there.

Happy restoration!
~Mark