41c weak display segments

Hi folks.
Is there a known issue with a 41c which causes display segments to be weak / faint. Mostly the upper segments on the left side.
Thanks for any ideas,


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What is the problem (other than very static-sensitive chips) in dismantling the display module of a fullnut?

You would have to desolder the connections from the keyboard PCB, then unclip the clamps (top and bottom) and seperate the hybrid board from the display. Yes, I've read the service manual, and I know HP say you can't re-use the zebra strips, but having repaired many other similar display modules, I don't understand why you can't (now that replacements are unobtainable) you can't clean everything with propan-2-ol and put it back together.

Weak segments does not sound like the driver mis-initialisation problem. It sounds like a bad contact in the zebra strip, or a defective driver (I have had that on both a halfnut 41 and on a 16C).

I've got half a mind to pull a display from a mangled 41CV apart and put it together just to prove it can be done and see what the real problems (if any) are.


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I have had a lot of success in repairing missing segments _in other devices_ by cleaning the zebrastrips. One trivial example. I saw a second-hand Fluke 85 multimeter in a second-hand tool shop. When I tried it, some of the segments were weak, but there. _In the shop_ (with the seller's permission, of course), I openied it up, unclipped the display, cleaned the edges of the display on my shirt (!), put it back, and it was fine. I did not attempt to clean the PCB there and then, as I was worried about static damage to the IC. The seller took \pounds 10.00 off the price anyway, since there was a possible fault...

I always intended to clean it properly when it failed again. But it's been fine for well over a year.

I cured one Voyager (original series, with the seperate logic module on the flexible PCB) by taking it apart and cleaning it. Another one did have a defective R2D2 chip, though.

I will admit I have never taken a fullnut HP41 display, but unless I can find good evidence that taking one apart _will_ ruin it, I would give it a go if other cures failed. Yes, I'd power down for a long time first (this is documented in Wlodek's book which every HP41 hacker should have read). Yes, I'd resolder the connections to the keyboard PCB (I've had dry joints there). But I would not assume that taking the unit apart would necessarily ruin it.

After all, the HP manual does give an official procedure for doing this. And my experience of all service manuals is that they err on the side of caution. I've had considerable success with dismantling and repairing modules that the official manuals (HP and others) regarded as not field repairable. On the other hand, there are some things that are almost impossible to repair in the field (hard disk HDAs and dot matrix printheads being examples of things I wouldn't take apart unless there was really no alternative, and with the assumption that I probably wouldn't get them going again).

Which brings me back to my original question. If there is something odd about this display where dismantling it will do serious damage, then I would like to know about it. If there is nothing known like that, I probably will take an old 41CV (the most common 41 in my experience, I have a few with cracked posts, etc) apart and see how I get on. I'll report here anything that I find out.


I would not take apart a 41C display unless I aboslutey had to. It can definitely make things worse. It can also make things better. The Zebra strips can easily be damaged by removing and reinstalling them. I have over a dozen 41C display modules. I'd say it helped 1/3 of the time, made no difference 1/3 of the time, and make things worse 1/3 of the time. If the display module is definitely bad, you have nothing much to lose by opening it, you just don't want to open a good module.

Of the ones that got worse, one failure mode that seems to occur is the zebra strips seem to have a light adhesive on one edge that helps keep them in place. This adhesive does not restick and after a while the replaced strips shift and the contacts flake out again. Cleaning the strips with alcohol can spread the adhesive residue over the contact areas and also lead to intermittent contacts.

As far as weak segments, I'd first try cleaning everything with 91%+ isopropyl alcohol. Start with the back of the LCD and the LCD contacts. Keep it off the display window or it will leave white stains behind (which can be poslished off). Also clean the back of the keyboard and the CPU board and the CPU contacts.

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