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Bad HP41 display? Shows @'s and whistles
03-09-2017, 10:13 PM
Post: #1
Bad HP41 display? Shows @'s and whistles
Hello everyone,

I am hoping someone else has encountered the display problem I currently have. While trying to fix a CPU zebra connector problem, my 41CV began showing all '@' characters in the display, while the buzzer emitted a high pitched tone. I swapped out the zebra connector with another known good one without effect. Thinking perhaps the CPU was damaged, I put a different board into the case and got the same exact behavior when putting the batteries back in.

With two different CPU boards and two different zebra connectors producing the same bizarre behavior, I decided to put one of the CPU boards into another working case. The CPU worked perfectly, leaving me to think the problem must be with the display. The I/O connector is in near pristine shape and worked fine earlier in another case.

As suggested in another post, I did press lightly against the display connector pins to see if any might be loose. Before going at the solder joints with an hot iron (yikes!), I wanted to check to see if anyone has seen this display behavior before. The display failure, if that is what it is, did not happen immediately. There was a short period when I would get garbage in the display and occasional tones each time I put the batteries in, which reinforced my original thought that the zebra connector was bad.

Sound familiar to anyone?
~ Mark
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03-10-2017, 07:51 AM (This post was last modified: 03-10-2017 11:33 AM by isanchez.)
Post: #2
RE: Bad HP41 display? Shows @'s and whistles
Hi,

from my experience repairing many HP41 series calculators:

-It is not a CPU board problem (I have repaired more than 30 HP41's and never found a failing board).

- It looks very much a problem in the zebra contacts: for any reason it is not doing a proper contact. You must assure a good contact. Check that the bottom screws are tight (not excessively or you will break the plastic holes! Check that they are not already broken, this makes the screws not to work properly) . Is the zebra connector the "golden" type? or is it the rubber one? From my experience, the "rubber" type does cause problems when "reused". It seems it gets a permanent deformation that makes difficult to get a proper contact. An extra "pressure" is needed to get the contact, but this is risky. It is not the case for he golden versiĆ³n.

- Could also be the upper flex connection. If it is original (not the reconstructed flex) it shouldn't be. Check that the upper screws do work fine.

- IN ALL CASES: Do not press too much the screws, or you will ruin the back plastic cover!

- About the DISPLAY: you can easily check the continuity of the contacts with a multimeter and visually by pulling up with a screwdriver each of the contacts. If there is a defect in the soldering you will find it very easily. If all is OK, then we can think in the display driver (not the LCD part).


I am pretty sure it is something in the zebra. You have to insist in finding the proper connection (try to find a golden zebra if you have the rubber version)...after some tries, then works, from my own experience.

EDITED: if it whistles, definitively this does not sound like a problem with the display.

Good luck!
Regards

Ignacio
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03-10-2017, 05:15 PM
Post: #3
RE: Bad HP41 display? Shows @'s and whistles
Sorry, maybe bad news on the LCD panel:

after following Ignacios' continuity check for the LCD solder joints and ensuring good flex contact at top and good zebra at bottom and still resulting in the same display: it will be the display driver on the LCD panel, the two square caps on the panel pre 1980 or the epoxy grey lumps also on the panel post 1980.

A donor panel can be used and easily replace the defective panel if required. If the display is still defective after replacement then it is some where else.

I have seen this on two of the systems I have replaced.

So:

-check all contacts
-check solder joints
-ensure screw posts are not defective
-check zebra strips (done)
-check foam insert on top flex contact for undue compression.
-replace LCD panel

Let us know how it turns out.

Geoff
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03-10-2017, 06:22 PM
Post: #4
RE: Bad HP41 display? Shows @'s and whistles
Struggling the same fight these days.
Have a CV that sometimes turns on all ?, sometimes nothing or invisible, sometimes only 6 or 7 ,-signs, sometimes only all annunciators, sometimes only left half or only right half of the display showing something, most often waste.
Found a loose display contact cause it was singing in the top but the buzzer. Resoldered.
Now the buzzer is wheening displays same waste as before.
It the chewing gum zebra. So you don't know it does good contact or not. Can't measure after you close it for testing.
Is there a source for golden zebra replacements or substitute for reasonable prices? Or solder all contacts from keyboard to mainboard as I have seen somewhere (Monte and the 41CL?)
I live in Europe.
Andi
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03-10-2017, 11:51 PM
Post: #5
RE: Bad HP41 display? Shows @'s and whistles
Thanks everyone for the suggestions. I'm struggling to restore three "for parts" 41CVs with questionable zebra connectors. A couple worked for a while, then went intermittent - sometimes worked, sometimes not. In swapping out suspect parts one at a time, I may have zapped one of the CPU boards. I've put that board aside and I'm using another one that works fine, which I think rules out the display. I've found the zebra connector to be a constant source of headaches, so much so that I never again open a calculator once I get it working Smile

Quote:Is there a source for golden zebra replacements or substitute for reasonable prices? Or solder all contacts from keyboard to mainboard as I have seen somewhere (Monte and the 41CL?)

I got into this fix a couple of months ago with a flaky 41C (zebra again!). I tried the soldered wirewrap approach and made a complete mess of it. Wires constantly popping off cold solder joints and/or breaking off when flexed. That calc now lies abandoned in a junk bag in hope it can be resurrected in the future, when science finds a cure for bumbled repairs.

It did inspire an effort to find a substitute for the zebra connector. After a couple of failed approaches, I have a third replacement approach that seems to work. I've managed to successfully replace the zebra connector on two of the three calculators, and I'm working on getting a repeatable and reliable replacement process worked out. The principle is simple, but the details are anything but.

       

The two images show the basic idea; In the first image, a flex circuit with gold contacts carries the signals from the keyboard PCB to the CPU PCB. The second image shows the flex installed, ready to be folded over to make contact with the underside of the CPU board. Two lengths of lamp cord are held in place over the lower contacts with two-sided tape. The cords serve to press the top and bottom flex contacts to their corresponding PCB contacts. This is where all the trial and error with materials comes into play.

The two gold zebra connectors in the first photo are now spare parts. Hopefully soon I'll reach a point where all this will be useful to others.

Well, back to the bench...
~ Mark
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03-13-2017, 09:02 AM
Post: #6
RE: Bad HP41 display? Shows @'s and whistles
Hi,
This looks very nice! Could be the definitive solution to this issue. Congrats! Please keep us informed about your progress.

I have been also querying about purchasing new rubber zebra connectors. Many companies do offer these rubber connectors. The thing is to find the proper model, size etc. and also the company that could sell small amounts. Some guys in China companies are very diligent and willing to help.

It seems that nearly all HP41 series main defects can be repaired: posts, zebra, flex, 3d printing covers set...

Only a couple of issues still to be investigated:
- The display (seems difficult)
- The broken screw holes in the back cover (is there any way to repair this? I have not found a proper glue for this kind of plastic, any idea?) I have more than six HP41 back covers with the holes area broken...


<Regards

Ignacio
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03-13-2017, 11:02 AM (This post was last modified: 03-13-2017 11:04 AM by Hans Brueggemann.)
Post: #7
RE: Bad HP41 display? Shows @'s and whistles
(03-10-2017 11:51 PM)mfleming Wrote:  Two lengths of lamp cord are held in place over the lower contacts with two-sided tape. The cords serve to press the top and bottom flex contacts to their corresponding PCB contacts. This is where all the trial and error with materials comes into play.

Well, back to the bench...
~ Mark

have a look at these, 3mm dia. should be way to go:
http://www.silex.co.uk/products/solid-silicone-cord/
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03-13-2017, 02:28 PM (This post was last modified: 03-13-2017 02:58 PM by mfleming.)
Post: #8
RE: Bad HP41 display? Shows @'s and whistles
(03-13-2017 11:02 AM)Hans Brueggemann Wrote:  have a look at these, 3mm dia. should be way to go:
http://www.silex.co.uk/products/solid-silicone-cord/

Thanks for the suggestion! I'll look for a local supplier. I've found something similar that works well:
http://www.homedepot.com/p/Phifer-0-140-.../100548686

The 0.140" diameter hollowcore spline is a bit too large, so 0.120" (3mm) would surely be the right size. A round versus rectangular shape also seems to work better, perhaps because it concentrates the compressive force for better conformance between the flex and PCB contacts.

The last item needed is a stronger double-sided tape of proper width to hold the cords in place. Then perhaps I can ask for a few volunteers to get broader test results.

isanchez: If you mean the recessed holes in the back cover that break out due to force and fatigue, I've had success gluing these pieces back in with an industrial cement meant for plastics. See here:
http://eclecticproducts.com/products/e60...esive.html

The trick for a strong bond is to use a small C clamp to apply a light pressure to the piece as the glue sets. Seems to hold up well to the force of a tightened screw.

Good luck as well in your search for proper zebra connectors! The lack of these in small quantities is what led me to try the flex approach.

~Mark
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03-13-2017, 03:33 PM (This post was last modified: 03-13-2017 04:03 PM by isanchez.)
Post: #9
RE: Bad HP41 display? Shows @'s and whistles
Thanks! At your disposal to volunteer in any kind of experiment!! Do not doubt, if I can be of help.

I am going to try the proposed glue. I found it in Amazon-Spain and E6000 is offered in different formats. The one shown in your picture shows it is "auto leveling", is it relevant? They all seem to be the same component, am I right?

Kind regards

Ignacio
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03-13-2017, 04:26 PM
Post: #10
RE: Bad HP41 display? Shows @'s and whistles
Hi Ignacio,

Self-leveling just refers to the medium viscosity of the material. It will flow to fill cracks or holes for cosmetic purposes, but to get a strong bond, you need to coat both surfaces you want to join, and then clamp them together for a few hours. If the broken piece is just pressed into place, the bonded joint will fail under pressure. With a finger-tight clamping force though, the joint seems nearly as good as new.

I think you can get the mini tube pack or smaller precision tip pack with the same formula.

Hope it works for you as well. I'll post a separate progress report message about the flex replacement in a few days when I have all the materials needed. Consider yourself first on the field trial list Smile

~ Mark
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03-13-2017, 04:54 PM
Post: #11
RE: Bad HP41 display? Shows @'s and whistles
Understood!
Thanks again and very interested in progressing on it!

Regards
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