41CV Gone Wild!


I just bought a 41CV from the other side of the world (I like to take risks). Supposedly it worked great, but here's the rest of the story...

Usually, it works. Sometimes though:
1. The tan key (and less often, the sin and cos) has to be pushed several times for it to function. After it works one time though, it does fine for a good little while.
2. The display goes crazy, displaying weird symbols, lots of dim commas, this is especially true when I use the card reader (but sometimes it works just fine with the reader).
3. When the display goes crazy, I usually (but not always) can't CLx or turn it off. I have to remove the batteries. It maintains its short term memory, so that if I replace the batteries pretty quickly, it will once again display the weirdness.
4. Sometimes, when it goes crazy, I can hear a faint buzzing/tweeting sound.
5. There haven't been any crop circles in my back yard for over three months now, so I doubt that has anything to do with it.

There's no visible damage or corrosion.

Any help will be greatly appreciated.


X files apart (wich includes the classical "User error. Please replace user and press any key" (excuse me for the old joke), the 41 trouble-shooting is usually a question of:

a) Chemical (Corrosion -visible or not-)

b) Mechanical (Case tightness)

c) Electrical (Cap's, Transistors, IC's...)

a) and b) beeing the *winners* for a huge score. Symptoms you've described are more likely to be also on their count.

In the c) case Fullnuts still have some chances for minor repairments, while Halfnuts are mostly "unserviceable".

So my advice should be:

Take your 41 apart, clean thoroughly every contact area you see (Iso-Propyl Alcohol, diluted venegar, etc.) Make sure the plastic poles where screws fit are in good shape, without cracks and every hole have a good thread for the screws to thighten properly, fix them if needed.

Once you've cleaned and checked/fixed the above, chances are you get a nice working 41 after reassembling.

Hope this help in anyway.

P.S.: O.K. I'll tell my "coleagues" to stop "circling" in your backyard. ;-)



Hello, Gifron (I remember I saw your name in posts here sometime ago... Is it correct?);

I'd ask you where does your 41 come from, but I prefer this to be kept between you and the seller.

I agreee with Diego Diaz, but I want to ask you: is yours a fulnut or a halfnut?

This image is an original MoHPC.

If yours is a fullnut (original, earlier HP41 LCD) that you may also check for the zebra connectors (there are at least four types) between the mainboard and the keyboard.

The main difference from both types is not the LCD, but the LCD "look" is the best way to identify both types. For their guts, fullnuts have a separate mainboard with all components except LCD assy, while hafnuts (except for the CX) have only one PCB with keyboard, main "nut" CPU (not a nutz CPU... thanks to Doc. Mike, I could not resist; sorry) and LCD assy built in. This characteristic makes the fullnuts "serviceable", while halfnuts allow minor maintenance.

If you need extra advice, provided the type you have, let us know.


Luiz C. Vieria - Brazil


You've already gotten good advice from Diego and Luiz, but I wanted to bring up one more possibility. You said sometimes the problems are worse when you use the card reader. Do you leave the card reader plugged in all the time, even when you're not using it? If so, try removing the reader (and any other modules) and see if the problems still happen. Many of the symptoms you describe (especially the "faint buzzing/tweeting sound" and weird displays) can appear when a module is not making good contact in one of the four module ports. The port used by the card reader is especially susceptible to this kind of problem, because the reader only fits one port and people usually remove and reinsert the reader more often than other modules, so the contacts in this port may suffer more wear and tear than in the other ports. If the problems *only* happen when something is plugged into one of the ports, then try gently cleaning the contacts on both the module and the port and see if that makes a difference. (You may find that some modules work better in some ports than in others, so it might help to switch them around a bit. Unfortunately you don't have a choice with the card reader.)

Since testing the ports doesn't require opening the calculator, I suggest that you try my suggestions first beofre looking for more serious problems.


Hi Gifron, Wayne, folks;

Wayne is 100% correct, and I must admit I was lazy with this concerns. Nothing else to add, just my complete support.

Good reminds, Wayne; thanks.

Luiz C. Vieira - Brazil


I have to agree, Wayne's and Vieira's advices are really on the good track.

They've both pointed out what I simply express by: "every contact area", thus including I/O ports block as well as zebra strip connectors between processor board and keyboard PCB (I should have been more precise indeed on my first post, excuse me).

Anyhow, it's my opinion that *every* unknown, previosly owned, 41 (or whatever) MUST be taken apart, thorougly cheaned/checked, and reassambled, even if it works perfectly at first glance. Well I'm a "hardware nut" after all... ;-)

This is to be sure that no hidden corrosion is eating your precious 41's guts and to prevent minor cracks in the screw poles to become a bigger problem when they (sooner or later) fully break down.

Please confirm if our advice has been of any help.

Cheers from the Canaries.


Thank you all for your suggestions.

1. The module ports looked good as far as I could tell.

2. The screws were tight, but I took it apart and checked the posts for cracks anyway. No cracks found.

3. I cleaned all the contacts and reassembled to test. Screws held tight. Turned it on. Different garbage than before, but still garbage. I figured I needed to clean it a little better.

4. Disassembled, and recleaned. Reassembled. Different garbage #3. Hmmm...

5. Wondering about making good contact along the zebra strips, I pressed hard on the lower portion of the keyboard, and the garbage changed to a regular display! Kept up the pressure while running a card through the reader - No problem!

6. Disassembled. Sure enough, the left bottom post was cracked. I guess I didn't check it good enough. There's also a teensy tinsy crack at the top of the right bottom post.

7. Next step is to repair the posts. I've seen several suggestions for this repair, from longer screws to plastic weding solvent, to adhesive. Which is best? I know, it depends who you ask ;) Any STRONG opinions though?



My answer goes to the top of the forum...!? I can't really tell what has gone wrong!!

Anyhow you can follow your thread there.

Sorry about the "misplacement".



Do NOT use superglue. It will not hold for long. Use the plastic welding solvent.

Longer screws (if you can find them) are definitley a good idea. The screws are a #2-28 selft tapping screw. If they are long enough they can reach into the reinforced area of the keyboard frame below the posts. I have fixed several machines with totally sheared off posts with longer screws. The posts are now only bushings that keep the CPU card centered in the screws.



some fullnuts come with two plastic washers thicky enough to maintain pressure over the mainboard. They come between the mainboard and the back case, having the screw posts "inside" it. I built at least three sets with the same plastic used in current calcualtor "blisters", their thickness is enough. If you cannot find washers that precisely fit around the screw posts, you should anly take care when cutting the "custom" washers so they are not large enough so they will fit right under the back case screw holes and supports, as you'll easily see.

I can add some scanned images, if you need so.


Luiz C. Vieira - Brazil


First came HP41's where the CPU board was held down by two nuts. Problem was you could only undo the nuts once and still be able to have them hold the board down. These machines have two flat bottom screw bosses that touch the top of the bottom screw posts.

Next came the fat washers. They replaced the nuts. They are a teeny tiny bit taller than the screw posts. The flat screw bosses touch the top of the washers that hold the CPU board to the zebra strip.

Next came the no washer units. They had a modified screw boss with an indention in it that was as deep as the fat washers were thick. The CPU board was held down by the screw bosses.

Next the CPU boards were made of a slightly thinner material. Then appeared two very thin little shim washers to take up the slack.

Finally came the half-nut machines that had no CPU board to hold down agains those pesky little zebra strips.


... the nutless HP12C Platinum! cheers

Luiz C. Vieira - Brazil


The solvent recommended is methylene chloride, aka methylene dichloride (MDC), dichloromethane (DCM). I've found out that it is used in paint strippers, as a blowing agent in polyurethane foams, and as a decaffeinating agent, and that it causes cancer (depending upon whom you listen to). Nevertheless, I can't find any to use on my calculator. I found some paint strippers that contain it, along with a host of other unpronounceables, but not the straight stuff. As expected, any time I ask a store employee for help finding dichloromethane, they look just as lost as I do. Where can I find this stuff? Is there a brand name I can ask for?

Thank you in advance.


I bought some recently over the counter in a pharmacy. The smallest size they sold was 500ml, and they had to order it in from their supplier, so it took a day to get. No hassle, though.


As far as I've got to understand those crack's must not be too conspicuous, otherwise you'd have seen them at first glance.

So may be a single Superglue drop should be enough, let it fill the gap in the crack, then, wire-wrap the pole to turn it to its original shape and let it dry overnigth.

Longer screws are a good idea, but remember they are self-tapping, so you must screw them whith extreme care going back and forth in no more than 1/8 or 1/4 of a turn on each step as the new thread is forming, take your time.

Even the original shorter screws must be treated with the same care, due to possible Superglue filling of the old thread inside the plastic post.

This is my approach, hope it helps you.



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