HP41C : Problem with Modules


For more than 15 years I used to work with an HP41C that I had bought when I was a student. Alas, it got stolen 3 years ago and I could never really get used to the HP48GX I had gotten as a substitute, I missed a lot the simplicity of my old friend. I recently managed to buy a used HP41C as a replacement (S/N 2232S40318). As an extra, it came with 3 pug-in modules : a Quad-memory, a Math-Pac- and a combined Math+Stat-Pac that I certainly would like to use (especially the Quad-mem and the Stat-Pac). The bare calculator is working allright on N-size batteries, (the rechargeable NiCd batteries are dead of course) but whenever I try to plug in any of the three modules I get the same odd phenomenon : when I switch the calc ON after module insertion, the display looks perfect and it seems that everything is OK, but as soon as I punch a key (any key in fact), the calc starts looping and displays a blinking row of 12 @s (frequency about 1 Hz). I do not get this all the times though : the Math-Pac modules gives me this approx. 8 out of 10 times, the Math+Stat module in one out of 5 attempts only. For the Quad-memory, I only once managed to get the HP41C to accept a few key press and to display the expected 319 regs (out of may be 20 attempts). I initially thought it was bad contacts. I cleaned the HP41 ports an the modules, I also lightly bent the modules springs, but this did not improve things.
Some other details may be worth mentioning : (a) I get the same phenomenon whatever port I use and whichever combination of modules I try to plug in. I of course put the Quad-mem in the lowest’s number port (b) When the HP41 « accepts » the Math+Stat module, I have full access to all of the functions, I could never properly test the Quad and the Math-Pac. This makes me think the calc is the problem, (c) this HP41C might have been left unused for several years but was properly stored, NiCd batteries out of the calc, (c) when I first put new batteries into the calc and switched it ON, it took it about 5-10 secs before lighting up and displaying the expected « MEMORY LOST »), (d) as my previous unit, the 4 leftmost digits are « weak », they tend to lit up more slowly than the oter ones and the LCD is more sensitive to finger pressure ?
Has anybody got any idea of what prevents me from using the modules ?
Many thanks in advance for your help.


I think it _is_ bad contacts. Firstly, try cleaning the contacts in the modules _and the contacts in the HP41_ with
isopropyl alcohol (propan-2-ol, isopropanol, etc). If that doesn't do it, and you are happy working on CMOS circuitry (the HP41 is fairly static-sensitive and you can damage chips by touching the pins if you are not careful!), then
take the HP41 apart (the screws are under the stick-on feet), and clean the contacts on the flexible PCB that's wrapped around the plastic
holder in the lower case (this connects to the battery back and the modules) and the corresonding contacts on the
keyboard PCB in the top case. Gently squeeze the foam under the flexible PCB to improve the contacts.
Put it back togeter, tighten the screws firmly (but don't crack the plastic) and
see if it's any better.


Dear Tony,

I just opened the calc as you indicated. There was a noticeable blue-green deposit (copper oxide ?) between the keyboard PCB and the flexible connector, as well as between the keybord PCB and the IC's PCB connector. I cleaned the PCBs and connectors as you recommended, and "voilà". It is now working 100% and has done so for the last three days, the Quad as well as the Math and Stats modules are fully accessible.
Many thanks for your help.



A friend of mine bought long ago a 41CV. This machine always displayed a string of overbars (like the "null" character in Synthetic Programming) whenever you turned it on. It was just a flicker, and then everything was OK.

Just a note to let you know that the problem may be deeper than just dirty contacts, although that IS the most likely explanation...


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