HP41.org

I have a HP41cx that just decided to start displaying strange characters one day. I tried all the usual (master clear, new batteries, clean the contacts between the CPU and the keyboard) with no luck. I then pulled the CPU out and swapped it with my other HP41cx (lucky I had a spare) and found that the CPU was OK and that the swapped out CPU did not work with that keyboard assembly, same strange characters. My question is, is the problem most likely a bad keyboard circuit board or the display board connected to it?

I would suspect the display board, but if it were mine, I'd send it to fixthatcalc.com which has an excellent reputation for fixing these things. Hmmm... Now that I think about it, last time I heard, they did have quite a backlog, so turnaround time isn't the best. For anyone here who could offer more help, it will probably help to tell them if it's a coconut or a halfnut. (There never was a "fullnut," and it sure bugs me when people call it that!) Fortunately I've never had any trouble so far with my 41cx halfnut or 71B's.

It is a coconut.....not a 1/2 nut.

Good news! The problem was with the connection to the display board. I was able to locate a bad solder joint between the keyboard printed circuit board and the display board. Five minutes and a little solder latter, my HP-41cx is working like new! The important thing to note here is that I could not see the bad connection even under 10x magnification. I needed to use a continuity tester to locate the bad connection.

Nice that you fixed it.

In the early day while I did work at HP, I sometimes resoldered all contact to the display to make sure it was ok.