HP 41C - connection missing?


Hi all,

I have a very nice looking HP 41C (squared corners display) that stopped working right in the middle of something. So I disassembled it and applied some WD-40 to several points as a solvent and as a contact enhancement. I must have put it in the wrong places. My 41C didn't wake up after reassembly. I took it apart a second time and was able to get the keyboard top utrasonically cleaned at a local optician.

When reassembling the thing I found that the row of connectors between the keyboard PC and the LCD looked like this in the top left corner:

There are three soldered connectors but it looks as if a fourth is missing, there's even solder on the two contacts on both PCs, though there is no conductive path leading away from the contact on the keyboard PC.
All pictures that I could find on the i-net show only three connectors in that place. Is mine missing a connector or is it plain normal?

Edited: 18 July 2011, 9:59 a.m.



three connections are correct. maybe, somebody tried to "hack" it and connected a fourth wire just to find out, whether there were some extra features to discover.



Besides from the case not looking pre-opened before I opened it, why would anybody solder in a connection going nowhere?


there were a few revisions on the LCD panel. You have a later revision due to the appearance of the epoxy caps.

I have seen four and three connections at this point including HP swaps for service. Also, the connections may be factory made with 4 contacts, and when the revision came out, three contacts were required so the forth was removed.

WD 40 is a lousy contact cleaner. It actually coats the contacts. Clean them with a lint free cloth and rubbing alcohol (enthanol) or contact cleaner. I use a fibre glass pen to clean up dirty contacts.

Clean the contacts on the logic pca.  
Clean the contacts on the battery module bloc.
Clean the contacts on the keyboard pca.
Check the screw posts for cracks.
Check the screw post threads, ie does the screw grip the screw post or is it stripped.

No conspiracy on the missing contact ;-)



Thanks to both of you for your kind replies.

WD 40 is a lousy contact cleaner. It actually coats the contacts.

Yeah, I found that out the hard way...
I cleaned all the contacts I could reach with isopropyl alcohol afterwards and dipped the keyboard part in an ultrasonic bath. The latter did only half the job and the machine was still acting erratically, so I took a deep breath and soaked it in warm water with dishwashing detergent for an hour and dried it over night. Tadahh! Now it's working, but still doesn't always react to the ON key at the first press. I haven't decided if I will take a deeper look into that flaw or if I will just be happy with what I've got.

No conspiracy on the missing contact ;-)

Was there even the word conspiracy back in the 80's??? ;-)

Edited: 19 July 2011, 12:40 a.m.


Time to invest in some de-oxit:

deoxit site

Your local electronics shop may have it. Look for deoxit gold in a tube.

Before you do that, take a strand of copper wire from a standard house hold cord. Locate the pinhole on the reverse side of the keyboard that corresponds to the ON key bubble contact. Push it into the pinhole and move it around. Introduce some isopropanol to the hole and swish the wire around. With compressed air blow air into the pinhole. let dry.

See if that helps.

next take the tube of deoxit and press the nozzle against the hole. Squeeze a tiny amount into the hole and work the key. I have fixed many intermittant keys including the on key. As you use the key the response gets better over time.



Hi Geoff,

thanks for the deoxit link. As I'm located in Germany the nearest source for it seems to be UK ebay, where I found some items.

Unfortunately the ON key is not my only problem anymore. After working well for a while, the display constantly showed NULL after pressing the SUM+ key and the 41 stopped working altogether. Now I get only a row of @ in the display when I reinsert the batteries. Or parts of @ characters or some strange signs scattered all over the place.

I performed ON <- resets and ON ENTER <- resets repeatedly, taking out and reinserting batteries with no avail. I cleaned (with isopropyl alcohol, not WD-40!) and checked the pressurized connector of the battery compartment as well as the foam connectors to the CPU board. They all seem to look ok. I could even measure connectivity through the foam connector to the printed leads underneath.

I think I will let the 41 rest for a while, maybe there is still some humidity inside the keyboard causing it to act up. The funny thing about humidity is that I found this document from HP

Reset the HP41

which is about possible static electricity affecting the calculator, that says one should increase humidity to solve problems... ;-) So I just soak it again and try to operate it under water! ;-)

EDIT: I didn't keep my promise to let both of us rest - me and the 41. I 'screwed' with it one more time and assembled it in a way I hadn't before. When I took it apart the first time, a brown plastic rail came off and I didn't see where from. As it had the exact length of the CPU board, on reassembly I put it under it on the side of the battery compartment. This thin plastic rail also has the exact length of the LCD display, which is where I put it now: in the gap between the top of the calculator casing and the LCD. And voilà, it's working. That rail probably screwed up the connection of the CPU board and the keyboard PCB, because it wasn't meant to be there. Can anybody confirm this? TIA

Edited: 19 July 2011, 7:10 a.m.


The rail is a dust protector and probably something to with shock protection for the LCD. Yep it belongs at the top of the LCD panel.

This sounds like a connection problem a the CPU (logic pca) to keyboard pca. You say 41C, is the logic pca, is it attached by hex nuts or was it pressed into place by the back shell cover?

Focus on the the keyboard, logic connection. Open it up and place it in front of a fan for a couple of days. It sounds like you gave it a real soak. Did you rinse with water? If not, rinse with water to remove the soap residue, (avoid getting the LCD wet). Then place it vertically LCD panel up in front of a fan. Leave it for a couple of days just to be sure. DO NOT use a hair dryer on high (LCD damage) be patient.

If you have hex nut board attachments then the screw post that the hex nut attaches to may be stripped. If so try this:

You can substitute stryene tubing cut the same length as the screw post, that is, when added the tubing is flush or slightly proud of the screw post when the board is added. The photo is silicon tubing for use with card reader restorations.

Do you have an email? I have just finished a design for reinforcing screw posts, repairing stripped posts and cracked posts. I have to clean it up for a posting here.


PS that is the 41CL board in place in a tall keys 1933Axxxxx serial numbered case! incase you oldtimers were wondering!!!!

Edited: 19 July 2011, 2:16 p.m.


Thanks a lot, again, Geoff!

It's the pressed-by-back-cover-version and my problem was entirely solved by taking away the plastic rail from underneath the CPU PCB and putting it in the right place.
The screw post are ok, too.
I really gave it a good soaking: 30 minutes completely under warm water, after the ultra sonic bath at the optician! And I dried it with a cold (!) blow drier and by putting it in my closed convertible in the summer sun. I'm not the patient kind of guy, you see...!!!

Added bonus: after the ultrasonic cleansing, the thing looks like new.

Edited: 20 July 2011, 6:10 a.m.

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