How to fix an old HP41cx

[brmarcant]

Hi all,

I got an old HP41CX that is working partially. It’s difficult to power on, it powers on randomly pressing the on button. Keyboards works partially and also there randomly. LCD sometimes changes brightness an suddenly disappear. I think the calculator has been already opened and probably need a full verification, cleaning and tightening of all screws. But It m afraid to broke something in such a fragile calculator. Is there any tutorial/suggestions I can follow to make this work well done ?

Thanks

Bruno

[Klondor36]

Try my post for places.

https://www.hpmuseum.org/forum/thread-14904.html

A lot of people pitched in and commented. They were awesome!

BLUF: The Calculator Store has parts and repair "How-To's". Not on everything but most of what may be your problems.

[burkhard]

The 41 depends upon the lower two screws to clamp the top and bottom of the calculator together. But they play more than a structural role. That clamping force also establishes good electrical connection between the top and bottom via the "zebra connector". Google "HP41 zebra" without the quotes and you'll get a lot of hits including pictures to show you what I mean. Anyhow, if the screw posts (especially the lower ones, as the zebra connector sits right around these) are cracked or broken off, you won't be able to have sufficient clamping force and all sorts of gremlins ensue. The vast majority of the time, when a 41 is acting really flaky, it's because of that zebra connector and the main ultimate cause is broken/cracked screw posts which need to be repaired or replaced.

Let us know what you find.

[brmarcant]

(05-21-2020 03:42 PM)burkhard Wrote:  The 41 depends upon the lower two screws to clamp the top and bottom of the calculator together. But they play more than a structural role. That clamping force also establishes good electrical connection between the top and bottom via the "zebra connector". Google "HP41 zebra" without the quotes and you'll get a lot of hits including pictures to show you what I mean. Anyhow, if the screw posts (especially the lower ones, as the zebra connector sits right around these) are cracked or broken off, you won't be able to have sufficient clamping force and all sorts of gremlins ensue. The vast majority of the time, when a 41 is acting really flaky, it's because of that zebra connector and the main ultimate cause is broken/cracked screw posts which need to be repaired or replaced.

Let us know what you find.

Guys I need some help.

I opened the Hp41 and it's a real mess. The zebra that I find doesn't look like the proper one for an HP41CX.

As you can see in the picture there are 3 rows of contacts (2 long, under the connectors, and one at the center shorter) but only 2 connectors ... is it ok ? is something missing there ? (see figure 1)

On the top side there are 2 broken dents that looks like are needed to keep stable the battery connector of the battery bay. Are these dents necessary for the correct closure of the case or I can leave them ? (see figure 2)

I kindly ask for suggestions on how to proceed.

The calculator originally switched on . but I'm not sure it was full functioning.

[GreyUser]

That’s in decent shape, hardly a mess. The zebra connectors for the processor board are fine with nothing missing. The case bottom is a bit rough and it is missing the two upper battery connector locator/centering pins. That can be repaired with some styrene bar glued into place but it’s not the issue with intermittent behavior.

The bigger issue is your described problem. Since the bottom posts are sound, it sounds like a defective rom or ram chip on the processor board. The only way to troubleshoot is to substitute the processor board, this will tell you if the original board is the problem. To repair a bad rom/ram, you would need a service module and a donor processor board for the replacement ic’s.

[brmarcant]

(06-06-2020 02:22 PM)GreyUser Wrote:  That’s in decent shape, hardly a mess. The zebra connectors for the processor board are fine with nothing missing. The case bottom is a bit rough and it is missing the two upper battery connector locator/centering pins. That can be repaired with some styrene bar glued into place but it’s not the issue with intermittent behavior.

The bigger issue is your described problem. Since the bottom posts are sound, it sounds like a defective rom or ram chip on the processor board. The only way to troubleshoot is to substitute the processor board, this will tell you if the original board is the problem. To repair a bad rom/ram, you would need a service module and a donor processor board for the replacement ic’s.

Hi , thanks a lot for suggestions. But I want to be a bit more precise in the problem description. The calculator switched on several times. And I was also able to make some simple program. But moving it or continuously playing with it caused the system to switched off. Also keys doesn’t work all properly. The left line of keys looked to be a bit more unstable in working.
Do you still suggests that is a Rom problem rather than a contacts problem ?

Also got the styrene bar do you have any suggestion on how to repair it ?

Thanks a lot :-)

[GreyUser]

Any plastic case repairs should be performed with solvent based plastic model cement, often times called plastic solvent cement or plastic welding solvent. Using cyanoacrylates (super glues) will result in failure 100% of the time. Attached is a photo of a repaired case back, the spacers are 1mm square by 7-8mm long. The size is not critical, they only need to fit into the recess of the battery contact block.

The bottom screws that hold the processor board in place need to be the 3/8" version (9.5mm). The shorter screws will result in broken and cracked posts. I doubt your 41CX has the shorter screws as that change would have been incorporated into standard production by the time the CX was produced.

Another possible issue that popped to mind about your failure description could be contamination of some kind on the processor board. It could be on the top or bottom, anything conductive in any way will cause problems due to the extremely low operating currents of the 41.

Do you know the history of this CX or did you receive it as-is, partialy/intermittantly functioning?

[isanchez]

Hello,

I recommend the parts of http://www.thecalculatorstore.com

There are smart solutions for zebra connectors and many others. The new zebra design is incredibly simple and works very well.

For the back cover + IO connector problem, you will find a combined solution by sticking the IO connector to the back cover, this makes it stronger and avoids the weakness of screw areas. Also indenpendent solutions are available either for the screw areas or for the IO connector.

Reliable solutions for upper and lower posts, which are really the weakest design point of HP41 series.

I can say that, except for the display, you can find good solutions for any HP41 series at http://www.thecalculatorstore.com and at a reasonable price. I am not commercially linked to this portal, to avoid suspicions.

Kind regards

Ignacio

[brmarcant]

(06-06-2020 07:12 PM)GreyUser Wrote:  Any plastic case repairs should be performed with solvent based plastic model cement, often times called plastic solvent cement or plastic welding solvent. Using cyanoacrylates (super glues) will result in failure 100% of the time. Attached is a photo of a repaired case back, the spacers are 1mm square by 7-8mm long. The size is not critical, they only need to fit into the recess of the battery contact block.

The bottom screws that hold the processor board in place need to be the 3/8" version (9.5mm). The shorter screws will result in broken and cracked posts. I doubt your 41CX has the shorter screws as that change would have been incorporated into standard production by the time the CX was produced.

Another possible issue that popped to mind about your failure description could be contamination of some kind on the processor board. It could be on the top or bottom, anything conductive in any way will cause problems due to the extremely low operating currents of the 41.

Do you know the history of this CX or did you receive it as-is, partialy/intermittantly functioning?

Unfortunately I do not have any clue of the history. I bought it severa years ago on EBAY and now I'm wondering if it could still operate. I ordered the glue and waiting for it as soon as it arrives I'll post here my progressions (if any :-) ) .

I notice also that I have the upper left post broken. I fixed it with a double component resine glue. I hope it works otherwise I need to find a solution.

[brmarcant]

(06-08-2020 08:23 AM)isanchez Wrote:  Hello,

I recommend the parts of http://www.thecalculatorstore.com

There are smart solutions for zebra connectors and many others. The new zebra design is incredibly simple and works very well.

For the back cover + IO connector problem, you will find a combined solution by sticking the IO connector to the back cover, this makes it stronger and avoids the weakness of screw areas. Also indenpendent solutions are available either for the screw areas or for the IO connector.

Reliable solutions for upper and lower posts, which are really the weakest design point of HP41 series.

I can say that, except for the display, you can find good solutions for any HP41 series at http://www.thecalculatorstore.com and at a reasonable price. I am not commercially linked to this portal, to avoid suspicions.

Kind regards

Ignacio

Thanks a lot Ignacio.

Not sure I understood about the combined solution of the back cover + IO. Can u clarify ?

I knew about the site and maybe I'll buy something, but first I want to understand if it's a contact problem and maybe just fixing all together correctly will make it restart working :-)

[isanchez]

(06-09-2020 09:15 PM)brmarcant Wrote:  

(06-08-2020 08:23 AM)isanchez Wrote:  Hello,

I recommend the parts of http://www.thecalculatorstore.com

There are smart solutions for zebra connectors and many others. The new zebra design is incredibly simple and works very well.

For the back cover + IO connector problem, you will find a combined solution by sticking the IO connector to the back cover, this makes it stronger and avoids the weakness of screw areas. Also indenpendent solutions are available either for the screw areas or for the IO connector.

Reliable solutions for upper and lower posts, which are really the weakest design point of HP41 series.

I can say that, except for the display, you can find good solutions for any HP41 series at http://www.thecalculatorstore.com and at a reasonable price. I am not commercially linked to this portal, to avoid suspicions.

Kind regards

Ignacio

Thanks a lot Ignacio.

Not sure I understood about the combined solution of the back cover + IO. Can u clarify ?

I knew about the site and maybe I'll buy something, but first I want to understand if it's a contact problem and maybe just fixing all together correctly will make it restart working :-)

Hi,

yes I explain myself: The most frequent problem in HP41 series is the battery leakage that ruins the I/O port battery contacts. Due to the effect of the acid, in many times it also affects to the back cover plastic area around the upper screws heads, so it is impossible to screw them properly. In other cases this problem comes if you screw the screws too tight and break the plastic. In this case, when both are affected, then you can get a plain bottom I/O port (3D printed+flex) and glue it (I recommend E6000 glue) to the back cover. The plain bottom improves the contact surface and the glue effectivity. What you get is a solid union between the I/O port and the back cover, and the screw heads rest on the I/O port when tightened...I don't know if I explain myself well?

This solution has worked well for me and IMHO is better than just repairing the holes area by glueing a small plastic part to the back cover.

kind regards

Ignacio

[Jose Gonzalez Divasson]

Hi,

The domes for the upper screws are really the weakest part in the calculator - more so than the screw posts. And gluing them, even with solvent glue, is seldom good enough. What we did was to create a piece with a lot of surface but that matched the lower part of the original battery module:

   

The piece has two sides, and here is cut in two since one dome was still fine. To install it you need to grind down the remains of the dome. Then you glue the piece with E6000, and you can lie the original battery module or the 3D one.

Ignacio Sánchez has given me a lot of ideas over time. This time again: he suggested creating a 3D module with much more surface that can play the role of both pieces: the old module and the back repair part. Then you can grind down both domes and paste the battery module in place. In this picture you see the original module, our 3D part to replace the above, and the new part in case both domes are broken:

   

We have tried it in a couple of calculators with success: the piece stays put and it can be pressed hard without fear.