Faded display on HP15-C


I dug out my HP-15C the other day (purchased new in 1982!) and turned it on. The batteries are fresh. It turned on fine but the display is faded - many of the LCD segments are not as dark as they should be. The display is still readable but of course not as clear as it should be.

I tried a different set of batteries - same problem

Has anybody seen this problem before? The calculator self tests fine and is usable but it would be nice to have this fixed if it could be




Hi Larry , hi everybody!

This is my first time here and I would like to say that I have just bought an HP15C that shows a "faded" display.The unit is like new and has been kept in a drawer or shelf for 19 years !! (its serial number is 2635B11858).Its display is not as sharp and "black" as my 11C that has 2707B05949 serial number.It is not a real "problem" but I wonder if it gets worse as time passes.
I would like to congratulate the people who writes in this so cool forum!!




Yes my 15C was bought new in 1982! I used it regularly for about 4 years and then it got put into a drawer and brought out every now and then. It always worked fine until about a few months ago I brought it out to show somebody and noticed the display had faded. Not the unit is over 23 years old so I guess something had to go but the display going seems to be a bit unusual

My serial # is 2250A00792

I don't use it at all now since I use the Mathu application for the PalmOS which basically emulates the operations and functions of the unit

But would hate to junk it :-(



E aí, tudo certo? (so, what's up?)

Tô na área desde 2000! (I'm in the neighborhood since 2000)

If you want to: lcvieira (@) quantica dot com dot br

There are a few others around here, too. Welcome!

Luiz (Brazil, também)


Hi Luiz,

Thanks for your attention! Next days I will need some help on that HP stuff ok?



Hi Larry, falks;

I once repaired an HP15C overseas (it was sent me for repairing procedures from an e-friend in Europe) and the problem was that some segments in some digits 'faded' as the display got filled with digits after the fourth one, right to left . I'm not sure yours has the same problem, maybe the fading is constant. In that case, some digits became almost unreadable after the last posible one was entered.

I soldered all terminals of the threee IC's and it did not solve the problem. In fact, had to completely disassemble the unit, remove the LCD, clean all pads and both polymers and rebuild it all again. It worked perfectly fine after that, and the last news I read from him (about one year later) were good ones.

I'm not sure this procedure will fix your HP15C's problems, mostly because I don't know if the LCD itself is actually faded. I saw some Voyagers with digits not as dark and consequently not as sharp in the edges of their segments as others. I own eight Voyagers: one 11C, two 15C, two 16C and three 12C (I also have two 12C Platinum, but it is not a Voyager in concept... I don't even know if the poor little things have a family name...). The two newer HP12C (with one single 3V, button-type, flat battery) have the worst LCD contrast, and one of the HP16C (the earliest one) either has slightly pale digits.

Maybe, just maybe, your HP15C is good as it is.

Best regards.

Luiz (Brazil)


Luiz wrote:

"I own eight Voyagers: one 11C, two 15C, two 16C and three 12C (I also have two 12C Platinum,"

What! No 10c!



Yeap! Unfortunately no HP10C!

I'm aware of the difficulties involved on "hunting" one of this, so I sadly accept not having one HP10C amongst the other ones... Also, prices are somehow high!

Best regards, John!

Luiz (Brazil)

Edited: 30 Jan 2005, 2:07 a.m.


I think my problem sounds more like the one you describe where alll the digits are not as dark as they could be - they lack contrast. Certainly it wasn't like that new (of course) and not even about a year ago when I last looked at it. So it seemed to have faded with no use?

Since I don't use it anymore I guess it's not a huge problem but I feel a certain sadness that such a great calculator is now not so usable and the repair seems to be major


Out of interest can anybody recommend a place to repair a 15C?


Edited: 30 Jan 2005, 3:23 a.m.


HP15-C => HP-15C


Larry --

Randy Sloyer at www.fixthatcalc.com repairs Voyager models and many other HP calcs. He has done fine work on two of my HP-41's, but I've never had to send him a Voyager.

He might be swaped with orders, so don't count on getting instant service...

-- KS


Maybe it isn't a problem with the calculator at all!

Everybodys eyesight fades with age.

B^) <--me wearing glasses


Hi Larry,

I am curious if there is any voltage conditioning responsible for the phenomenon? What if you just power it up more often, and maybe run the self test ahtt makes the whole display fill up at the end, once a week.

Am I completely crazy to think this could help? (I don't know anything about LCD's).




Hi Luiz,

Could you describe in more detail what you did that did make a difference--was it the cleaning of the pads? What do you mean by "polymers"

OT: BTW, my son has another new ocarine--which looks Brazilian to me--tnx again for your very interesting links!




Hi, Bill;

thanks for your info! I hope your sun is having fun and great moments with the study of music instruments. Have you checked Uakti, a brazilian band that builds their own musical instruments? It´s worth checking them out (excuse me, folks... could not help adding these words.)

About the polymers: the LCD itself needs residual current to maintain information. In fact, a small "peak" current is observed when charging/discharging the small capacitors formed by the conductors that alter the pole of the crystals when changing from "black" to transparent and back. In fact, to be more precise, the crystals without the polarizer shield show no black/transparent effect.

Because of the tiny current, there is no need for low resistence conductors to electricaly contact the LCD terminals (a transparent oxide I forget the composition) to the PCB terminals (I called them tabs... is it correct?), so some "zebra" polymers with relative low resistence is used. They are thick stripes pressed between the PCB and the LCD itself (edges), and in all voyagers this is achieved through a metal frame with four tabs on the upper and lower edges (the newer HP12C´s have seven tabs, instead). After removing the frame it is possible to remove and clean all of the surface contacts and the polymers, if needed.

I have some pictures at home (I´m in the University right now) and I´ll add them tonight so you can see it better. If someone else has additional pictures, that would be good, too.

Hope this helps a bit.

Luiz (Brazil)

Edited: 3 Feb 2005, 7:39 p.m.

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