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Full Version: HP-41C bad display - where to start from ?
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ciao a tutti,

I have purchased for a fair price an HP41C, that I wanted to use as my everyday calc, thus I wasn't looking for anything mint, nos, nib ... just working...

There are a couple of small chips on the (plexi)glass, but I don't mind.

What I do mind about, is that when turned on, the display works like this : (sequence of 0, and sequence of 5) :

The second digit is missing a couple of segments, and this impair the view of few characters and numbers.

Since I had a spare "half Blanknut" I have tried to swap the keyboard/display unit, with the following result :

The board is the same, but the display is getting worst

I may have a bad "half blanknut", but I don't want to open the other 41Cs I have just for the fun of it.

What could I check to get the display working ?

Would it make sense to de-solder the blanknut display and put it on the original keyboard assembly ?

Curiosity : when opening the unit, rather than the usual gold zebra strip, I have found inside this one :

Thanks for any advice, take care, Alberto

You might check the 13 or so fingers that connect the screen. If the unit was dropped one or two of the solder joints may be loose and need to be touched up. Just pry on them to see if they move.

Ciao Alberto,

IMHO the "bacon strip" interconnector is the worst of the various interconnect variants used in the HP-41,

because it shrinks over time, and thus is prone for bad contacts.

Theoretically the gold ring interconnector (zebra strip) shown above the bacon strip is the best solution,

but your unit looks deformed, and thus may not provide proper contact between the CPU and the kbd pcb.

But let's get to the LCD. There's always a chance to get non-working segments,

especially when the damaged protective plexiglass indicates that the calc suffered rough treatment...

Did you actually put the 41C LCD into your blanknut, or did you really swap the keyboard pcb?

I ask because the latter is much work because of the many heat stakes, and isn't worth the effort in most cases.

Or does the blanknut LCD look like on the pic before any changes?

In my experience, bad interconnectors (CPU-KBD and KBD-battery) are the most common cause for display and keyboard problems.

In somewhat rarer cases, one or more solder bridges between the LCD and the kbd pcb have become bad.

The latter can be fixed using a suitable soldering iron.



Thank you very much for your replies.

This is what I have :

The unit on the left is the complete HP41C I have purchased, it has the bacon zebra strip and few (two to my knowledge) segments not working on the second digit.

Then, on the right, I have the upper half shell of a blanknut, it includes the keyboard, the display, the gold zebra strip.

This unit was purchased as a spare for my Blanknut, just in case ... It's from "Better Control Medical Computers inc." as the front label reads, it was used in a medical environment as part of a proprietary system for insuline dosage.

Now, the back of the orginal HP41-c looks like this :

It looks nice, but, some cold soldering or detached links may always be possible. I don't know what's happend to the original unit, but I would exclude it has been dropped, since the rest of the case is in good conditions, no chips, no cracks.

The back of the Blanknut looks like this :

The only thing I did, was to move the CPU board from the original HP41 to the half Blanknut, at first using the gold zebra strip, than I also tried with the bacon zebra.

Question :is there any way, to lit up all the segments (no, I don't have the test module), may be just applying a low voltage to the display contacts, and check the functionality ?

This would isolate the problem eventually to bad contacts between the cpu board and the keyboard board because of the zebra connectors

Thanks everybody !
Have a nice weekend, Alberto

Display issues in fullnuts are a real pain as the only way to trouble-shoot is to swap parts. On the upside, it is good soldering practice ;-)

Missing segments have nothing to do with soldering as they are always a bad driver or a bad lcd and in some rare cases, both. If there was no corrosion in the unit, I would first try the driver. If there was any corrosion, or gold interconnects, suspect the LCD first. Based on the greenish color of the polarizer in your problem unit, I suspect it has the more problematic connector. Pink or white LCD elastomers are far more reliable than the gold. This is the opposite of the situation with the keyboard to logic board connector.

Illuminating all segments: only possible with an HP service rom or PPC rom.

IMO, I would not mess with the damaged plastic lens. They are very, very difficult to replace/repair. Almost always turn out worse than they start.

ciao Randy,

thank you very much for your reply, do you mean that is probably a problem of the zebra strip ?

thanks for help, Alberto

And, if I well understand, that the ony way to isolate the probelm, not having a PPC module, is to use a fully working unit and begin to swap elements.

This would tell me if is the board, the display, or the connectorm right ?

Thanks again Alberto

The pink elastromeric keyboard to logic board connectors are the source of many forms of 41 trouble. They can be the source of the missing display segments although the problem is more likely to be the driver or lcd.

Troubleshoot using only the gold version keyboard to logic board connector. Change only one part per test. You changed at least three by substituting the blanknut.

Ciao Alberto, reading your words "use a fully working unit and begin to swap elements" I'm quite scared, maybe 'cause I'm not experienced like you in soldering, or maybe because always, first at school, then at work the advice for all was: "if the system works, don't touch it, let it work and it 'll work lifetime". Even if I had a dozen of 41 working HPs, never I should try the thrill...
Excuse me, maybe I've not so skilled in this matter and for this reason I've not courage enough to to risk the healt of a fully working (ancient) calculator.

Edited: 12 Feb 2012, 2:24 p.m.