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HP-41CV
03-12-2019, 08:31 PM
Post: #1
HP-41CV
I'm new to this forum. I'm looking for some suggestions for fixing a problem with my keyboard that just started happening today. I was using my calculator and noticed that the "zero" key was not working. I had to switch to using the calculator on my smart phone to finish the task. Later, I started trying different things and discovered that by applying significantly greater pressure on the "zero" key, I was able to get it to work. Wondering if it was a problem with a specific area of my keyboard, I checked all other keys to test whether the problem was more pervasive. All of the other keys function properly with just a normal pressure tough to depress the key. Can anyone share what the problem might be and a possible repair or remedy other than just pressing harder. My fear is that the problem will become worse over time and require more and more pressure until I can not longer get a response from my "zero" key.

stevevoigt@yahoo.com

Steve Voigt, PE
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03-13-2019, 12:15 AM (This post was last modified: 03-13-2019 12:19 AM by burkhard.)
Post: #2
RE: HP-41CV
The 41 calculators tried a couple of pretty clever ways to simplify manufacturing. These innovations generally worked as planned, but sometimes caused unintended reliability effects. I think what is going on with your calculator stems from the "lower posts" being cracked.

The two boards in the calculator are not electrically coupled with a ribbon cable but rather a squishy multi-wire electrical connector that is kept pressed between them. If the compression is lost, some connections can be as well. Constant pressure is supposed to be supplied by self-tapping screws on the bottom of the calculator going into plastic posts on the top. After 30+ years of aging, embrittlement, and being dropped/jostled a few times, those posts can crack and the screws lose their bite. No compression results and then the loss of continuity between the keyboard and the rest of the calculator.

There are LOTS of people who have fixed these and different methods. If you search the forum for "posts" and "41", I think you will find some ideas. I have repaired some minor cracking on mine when I had it apart to put a 41CL board in. In my case, it was pretty easy and free, but they weren't too far gone. For ones completely wasted, there are replacement parts available as well.

It is fixable and will be perfectly operating again afterward, but you'll need to take it apart to do this. Read up on it. Geoff Quickfall among others has written a bunch on this topic I think.

Good luck and keep us "posted." Heh heh
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03-13-2019, 01:23 AM
Post: #3
RE: HP-41CV
A bad connection between keyboard and CPU would likely impact an entire row or column of keys. Another possibility is that debris or oxidation is affecting the single dome beneath the zero key. If debris, try using compressed air around the key. If that doesn't work, use a drop of cleaner like DeoxIT Red and then work the key to see if it improves. It would be best to open the calculator and apply the drop to the hole in the keyboard PCB beneath the zero key, but I've done a quick fix successfully by just putting a drop or two into the gap between key and calculator face.

~Mark

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03-13-2019, 06:40 AM (This post was last modified: 03-13-2019 06:42 AM by Massimo Gnerucci.)
Post: #4
RE: HP-41CV
(03-13-2019 01:23 AM)mfleming Wrote:  A bad connection between keyboard and CPU would likely impact an entire row or column of keys. Another possibility is that debris or oxidation is affecting the single dome beneath the zero key. If debris, try using compressed air around the key. If that doesn't work, use a drop of cleaner like DeoxIT Red and then work the key to see if it improves. It would be best to open the calculator and apply the drop to the hole in the keyboard PCB beneath the zero key, but I've done a quick fix successfully by just putting a drop or two into the gap between key and calculator face.

~Mark

Spot on, Mark.
Steve, you can refer to this thread to find where the holes are (halfnut vs fullnut).

Greetings,
    Massimo

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03-13-2019, 01:33 PM
Post: #5
RE: HP-41CV
(03-13-2019 06:40 AM)Massimo Gnerucci Wrote:  Spot on, Mark.
Steve, you can refer to this thread to find where the holes are (halfnut vs fullnut).

I agree with Mark and Massimo; try adding a drop of DeOxit Red into the gap around the zero key (try a small amount on all 4 sides) and work the key for at least a minute. Don't press excessively hard, but try to wiggle the key a bit when depressed - the idea is to use the DeOxit to wipe the contact clean under the key. Although DeOxit will generally not harm anything, don't add too much as it is conductive.

Try this technique before opening the case, as it is likely you can solve the zero key problem without risking making the problem worse by stressing and maybe damaging the posts.

Good luck and please report your results.

--Bob Prosperi
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