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HP41C On Button Oddity / Help please
03-20-2020, 07:45 PM (This post was last modified: 03-21-2020 02:00 PM by ksagis.)
Post: #1
HP41C On Button Oddity / Help please
Folks, I'm having a bit of an odd problem with a HP41C (sn 2141S10799) which I bought off ebay in a non-working condition.

The battery contacts were in quite good shape and I took it apart to clean the various PCB contacts inside (zebra, battery PCB, etc. When I had it open, I also needed to replace the elastomeric zebra connectors (the original ones didn't seem to have continuity) with new ones from thecalculatorstore. The logic board is the final HP-41C version with the two capacitors in the top, left corner. I'm not an expert, but it seems to have the later version of the display driver with the two "blobs" present.

The 41C works again now, but it intermittently turns on when pressing the On button. If it has been sitting for a while (10 minutes or more) and is off, I need to hold the on button for about 10 seconds or so to get it to turn on. Once it turns on, I can give the On button a quick press and it turns off/on just fine.

The case is tight and clamped well.

I've read most of the archived threads and can't seem to find something the same as my issue, all advice appreciated.
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03-21-2020, 05:01 PM (This post was last modified: 03-21-2020 05:03 PM by BobVA.)
Post: #2
RE: HP41C On Button Oddity / Help please
I had pretty much the same problem - my HP41c would turn on/off just fine until it sat for a day or so, then it wouldn't respond to the power switch.

I tried a bunch of things but it was eventually resolved by touching up all the solder joints on interconnect wires on the LCD/switch board (i.e. the wires that connect it to the keyboard PCB).
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03-24-2020, 02:26 PM
Post: #3
RE: HP41C On Button Oddity / Help please
No joy after resoldering both sides of the jumpers between the LCD and keyboard PCBs. Seeing the same behavior as before.
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03-27-2020, 02:17 PM
Post: #4
RE: HP41C On Button Oddity / Help please
All, turns out the original elastomeric zebra and the replacement zebra connector were my problem. I installed one of these nifty replacements and the HP41 has come back to life:

https://www.hpmuseum.org/forum/thread-8388.html

Many thanks to all.
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03-27-2020, 04:13 PM
Post: #5
RE: HP41C On Button Oddity / Help please
If these are still available, I would love to order some.
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03-27-2020, 06:11 PM
Post: #6
RE: HP41C On Button Oddity / Help please
There is also the replacement zebra connector sold by The Caculator Store.
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03-27-2020, 10:29 PM
Post: #7
RE: HP41C On Button Oddity / Help please
(03-27-2020 06:11 PM)Sylvain Cote Wrote:  There is also the replacement zebra connector sold by The Caculator Store.

Those strips have a resistance of about 50 ohms per contact, so they are not suitable if you are going to use a card reader. They will work with a CL, but only if you don't want to update the Flash memory. 50 ohms times 50mA gives a 2.5V drop in voltage. Jose is looking into alternatives.
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03-27-2020, 10:55 PM
Post: #8
RE: HP41C On Button Oddity / Help please
I bought some of the strips from the calculator store and could not get them to work. They were not thick enough and did not stick up higher than the plastic holder so they did not get compressed when screwing the two halves back together.
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03-28-2020, 12:24 AM
Post: #9
RE: HP41C On Button Oddity / Help please
Per Monte's comment, the flex circuit (my avatar, btw) has a contact resistance on the order of one or two ohms max. Pretty close to the original wire wrapped zebra contact I would guess. The Buna-N cord that separates the flex halves does have a similar failing to the Pioneer and later model keyboard sponge that compress over time. I found if you open an HP-41 with a flex connector, you need to roll the cord between your fingers to return it to its circular shape before closing the case back up. You often need to do the same with the elastomeric pads anyway, plumping them up when the connection gets dicey.

Sorry to say twoweims, the last of the flex have been sold. I was surprised to get the query about them, given the last sale was about this time in 2017. If others are interested, I could post the Gerber files for anyone who'd like to replicate the flex circuits. Keep in mind though the minimum order amount would total around $100, so it's not practical for just one person to make some. There would be enough produced though (20~25 in a min order) that you could arrange a group buy to spread the cost around.

One other thing has changed in the intervening three years; national post offices have become very picky about what constitutes a letter versus a package. That has increased U.S. and International First Class postage by about a factor of three if the Buna-N cord is included! The postal clerk now checks any envelope I bring in for PCBs or other non-paper content when she sees me come in Smile

~Mark

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03-28-2020, 12:25 PM
Post: #10
RE: HP41C On Button Oddity / Help please
(03-27-2020 02:17 PM)ksagis Wrote:  All, turns out the original elastomeric zebra and the replacement zebra connector were my problem. I installed one of these nifty replacements and the HP41 has come back to life:

Excellent! Glad you got it sorted and thanks for posting your results.
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03-28-2020, 12:38 PM
Post: #11
RE: HP41C On Button Oddity / Help please
Well, it might not be the same problem I had but yesterday I have used the 41 to register to this website and left it on.
I wanted to try again the card reader and the 41 wouldn't start at all.
Tried everything I possibly could, nothing.
Today I checked the batteries with a multimiter and they are well in the green area.
As I was already using my dremel for other contact cleaning jobs I passed the head on the contacts in the battery compartment and reinserted the batteries, the 41 fired up again, no uncertainties anymore :-)
Cheers
Marco
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