HP-41C malfunction during startup

[prwktokali]

Excuse me if this is not the right place to post repair related issues, this is my first post on the forum.

So i recently found an HP-41C calculator while searching my fathers old stuff. It came with an extra memory module, a magnetic card reader (although i couldn't find any cards) and a small printer with some rolls. All the manuals were completely destroyed by mold, but i managed to find almost everything in pdf form with a quick search.
Since i really like vintage tech stuff i decided to try and make it operational again. After buying batteries (which were quite expensive!) i found out that it wouldn't turn on at all. After opening it up and searching for all the common troubleshooting cases, it became obvious that my fathers calculator suffered from a broken lower screw post. It was an easy fix, just using a longer screw. After inserting batteries the calculator seemed to work just fine. The display was clear and the operations got carried out normally. However after i put it aside and reached for it again after a few hours, it became clear that there were still some issues. When pressing the ON button, the display would light up on random places (mostly on the upper part) for a brief period (half a second or less is my best guess) and then completely die out. After repeatedly pressing ON or other keys such as ALPHA and USER (or removing and placing back the batteries in extreme cases) it would eventually turn on and operate flawlessly. Turning it off and then quickly on again was also fine, but if left off for more than 2-3 minutes it would be again very hard to start up. It always behaves like that and sometimes it also completely locks up and makes weird sounds. I get the general feel that it "struggles" to turn on, but if it does it works perfectly after that. No MEMORY LOST seems to appear in any case. It's also interesting that when i connect any peripheral, it will never turn on no matter what, which makes me believe even more that it has some issue with power supply during startup.
After searching for quite a while i couldn't find any similar problem mentioned here or other forums/groups, so i decided to take the time and post here for help, since this calculator apart from being pretty cool also has sentimental value to my father who was pretty happy to see it running again, at least with that culprit.
I'm really sorry for the long post but i tried to be as detailed as possible. I'd like to thank in advance anyone reading this for the time he invested and hope that you have some info on the problem described above.

[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

[prwktokali]

Thanks a lot for the detailed instructions. During the last open up of the device i did notice that the sponge like material connecting the two boards had formed permanent cavities where the contacts are, so i am going to work on it tomorrow and keep you up to date on the progress of the repair!

[damaltor]

Not related to your repair, but please do not use the card reader without disassembling it and replacing the rubber wheel beforehand. It has probably turned into goo over the years and would then stick to everything.

[prwktokali]

Thanks for the warning damaltor! Either way i have to find a magnetic card in order to use the reader, which i haven't yet.

As for the repair, after opening up and tinkering with the cpu connector a few times i finally managed to get the calculator to power up without issues. The new issue that came up is that the display will now turn off by itself after 15-20 seconds of inactivity, which is not a huge problem thanks to the constant memory. Just press on and continue from where you left off. I found a post regarding the same problem and offering possible solutions but i am hesitant to open up the calculator again since i can already feel the screw posts getting weaker and i am afraid that the repairs are not gonna last for long.
I suppose my next step is to learn how to set the constant ON flag, but until then there's lots of reading that needs to be done.
I'd like to thank you all for your help and for maintaining this community. Hope that as time goes by i will get to be an HP expert myself and help others in return

[rprosperi]

(06-30-2018 12:35 AM)prwktokali Wrote:  I suppose my next step is to learn how to set the constant ON flag, but until then there's lots of reading that needs to be done.

The "ON" ([XEQ] [Alpha] [O] [N] [Alpha]) command disables auto-shutoff, or at least it does for a 41 that is behaving right.

[g8241CV]

+1 on mflemming's methods and thinking. My beloved 41CV took a fall during an exam and yet worked perfectly for another 10 years, then the display would time out quickly as you described. It limped along with the EXQ on flag trick for few more years before the display got dim and went flaky and died. Downside to partial repair: if you write/store any programs you may lose them during flaky power on etc. BTDT. Finally got the guts to open, find and repair a crack in the backshell (post screw boss), and cleaned the interconnects. Those steps stopped the short time-outs. Later found a cold solder joint on the display connection. Now the calculator works perfectly. Don't use IPA on the silkscreened HP log strip and the tab between the USER and PRGM keys. Have fun.