Repairing an abused HP41CV

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Repairing an abused HP41CV

Postby ve3oat » Fri Aug 26, 2022 1:20 am

Hello,

I recently acquired an HP41CV with two modules. The original owner reported that the calculator and modules no longer worked but had worked before. They stopped working, in stages, as he installed and uninstalled the modules.

I suspect he had forgotten to turn off the calculator before installing and uninstalling the modules! Both modules work just fine in my own '41CV. I have not opened the calculator yet as I expect the damage to it is, therefore, electronic rather than purely physical.

Is there any way to repair such damage?? Unfortunately, the calculator is a half-nut, otherwise I would consider converting it to a '41CL.

Thanks for any advice or guidance.

... Martin
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Re: Repairing an abused HP41CV

Postby rprosperi » Fri Aug 26, 2022 2:31 am

Although of course one should never plug/unplug modules while the machine is on, I've done this a number of times in the last 40+ years, with many 41C/CV/CX machines and though it has led to lockup and possibly even ML, it has never done any electronic damage, and I doubt that is the cause. It's far more likely the machine has been dropped or roughly handled and one or more of the posts have broken (the plastic has become quite brittle over the years), or that batteries leaked and the blue crud has damaged contacts or even the board.

CAREFULLY take it apart (there are guides here, on the MoHPC Forums or even in the service manual) ensuring that you don't further damage the brittle plastic posts and/or post mounts. This should reveal the cause, and if it is post damage, the Calculator Store has excellent repair parts available:

https://www.thecalculatorstore.com/epag ... P41_corner
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Re: Repairing an abused HP41CV

Postby ve3oat » Fri Aug 26, 2022 2:54 am

Thank you, rprosperi, for that informative and encouraging reply.

I should have included in my original post that the calculator appears "dead". It does not power up at all, offering no signs of life, even with fresh batteries. The terminals in the battery compartment are clean and uncorroded, and there are no rattling sounds from within the case.

So, I will proceed as you suggest. I have the repair book and have read parts of it several times, although I have never taken apart an HP41 before. Always something new to do and to learn!! Hopefully, the problem will be obvious and fixable!

Thanks again for your help.

... Martin
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Re: Repairing an abused HP41CV

Postby rprosperi » Fri Aug 26, 2022 4:12 pm

If you get this reply before opening the case, try this first:

With the 41 on a table/hard surface, press down on the two edges to the left of the keyboard just outside the top row of keys (upper post area), and try using [ON]+[<-]. IF it comes on, continue the pressure and try some simple operations to see if the unit is stable.

If you get nothing, then try the same thing but instead pressing down outside the 7th row (lower post area). If again no joy, try pressing reasonably hard in the middle (or both if 2 people are available). If the problem is broken posts, this pressure may temporarily make enough connection of the flex connectors to let it work. It won't fix anything, but will let you know a) what the prblm is, and b) that the unit works.

Good luck
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Re: Repairing an abused HP41CV

Postby ve3oat » Sat Aug 27, 2022 6:29 pm

Thanks very much for that. No, no time to open it yet. (Visiting grand children here, most times are hectic or chaotic.) But I did find time to try your suggested tricks -- No response at all from the calculator during any of the procedures, or variations of them. Looks like physical repairs might be necessary. I hope to open it up late tomorrow.
... Martin
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Re: Repairing an abused HP41CV

Postby BillBee » Mon Aug 29, 2022 11:51 am

Bob has you pointed in the right direction.

The case back where the upper screws go through are the usual culprit for halfnuts. Once those go the contact is flakey for the ribbon connector. Sometimes you can see one side of the case has a gap.

Be careful when you open it as the "buzzer"" is attached to the back and there is not much wire. :)

-Bill
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Re: Repairing an abused HP41CV

Postby ve3oat » Tue Aug 30, 2022 12:21 am

Success!

I opened the back of my dead calculator, very carefully as I have never done this before, but found nothing obviously wrong. One of the four screws did seem tighter than the others, but there were no cracked or broken posts, no visible corrosion, and everything looked fine ...

Without anything else to do, I carefully cleaned the visible contacts on the Zebra cable and the mating circuit with a little Deoxit D5 on the end of a cotton swab, being careful to not leave any cotton fibers. Then I resoldered the wire from the "buzzer" that had broken off while handling the two halves of the calculator. (Sorry, Bill, at my age my hands are a little clumsy.) Finally, reassembled the calculator without any difficulty, making sure to not over-tighten the screws.

"MEMORY LOST". Calculator works, modules work in it, BEEP works, everything works fine. Not sure what the problem was -- maybe that one tight screw was part of it, or there was unseen corrosion on one of the contacts.

Many thanks to everyone for their advice and information!! This was simple but the experience was tremendously helpful to me, and encouraging. Maybe now I will try to fix the keyboard on that semi-functional '41CX that I bought a few years ago.

Thanks again to all!
... Martin
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Re: Repairing an abused HP41CV

Postby Garth » Tue Aug 30, 2022 12:32 am

Wow, that's good to hear!

BTW, I think you're referring to a flex circuit, not a zebra strip. Zebra strips are rectangular elastomeric strips that have conductive rings in them, to make the connections between the LCD glass and the circuit board below it.
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Re: Repairing an abused HP41CV

Postby ve3oat » Tue Aug 30, 2022 3:35 am

Garth, you are absolutely right; it is the flex circuit. I had the name wrong. It is the almost folded-up part that connects the plug-in modules to the circuit board and on to the battery connections. Sorry about that.

Thanks for the correction. (I'll get all of this figured out eventually!)

... Martin
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Re: Repairing an abused HP41CV

Postby rprosperi » Wed Aug 31, 2022 2:45 am

Great news Martin, glad you ventured where sane folks rarely go, it clearly was worth the trip! :)

Most likely the issue was dirty contacts. FAR more often than I would have guessed, contacts that look clean really are not; the grime/dust/oxidation/etc, on them prevents good contact, easily fixed by cleaning. I've had plenty of machines (including more than one 41) that would not even come-on, magically restored simply by cleaning the clean-looking battery contacts.

Thanks for posting this, the technique will help future readers and your success will hopefully motivate others to try similar repairs.
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Re: Repairing an abused HP41CV

Postby ve3oat » Thu Sep 01, 2022 8:15 pm

Thanks for your wise words!

It is a fact that any success I have had with this and other calculator problems has been due to the freely-given help and very positive encouragement from expert members here, yourself included. My thanks to everyone.

... Martin
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