HP 41C LCD/Display


I have a 41C that is having trouble displaying the numbers/letters clearly. The funny part about it, is that when I go outside and the sun hits it directly, it shows everything fine and plus some. As soon as I go back inside, it goes back to it's original issue. CAN ANYONE LET ME KNOW WHAT THE HELL IS GOING ON WITH MY CALC?


A few questions come to mind:

1.  Has this just started?

2. Is it coincidental with new lighting situation inside?

3. what type of indoor lighting, incandescent, florescent,
mercury vapour?

4. When viewing the LCD panel indoors what angle are you using
to view the calculator?

5. Is this angle different to the angle you use outside to view
the calculator?

- Viewing angle is important with LCD panels.
- LCD panels fade with age.
- The type of artificial lighting affects clarity and resolution.
- LCD panels on watches have a polarized filter built into the
lense and I am 90% sure that the HP 41 LCD panel is the same.

To my knowledge there is no photo-sensitive system or variable system to alter the 'darkness' of the LCD display as in subsequent models (48, 28 etc). My guess is the LCD is slowely fading as they all do. The non-polarized natural light is the best source for contrast. Try using, if you are not already, an incandescent bright light while inside.

Cheers, Geoff

Edited: 26 May 2009, 1:15 a.m.


My 41 has the same, bold, high-contrast display it did brand new 23 years ago. I have other LCDs of about that age, all of which I can say the same thing about. As long as the LCD is electrically fed correctly (which is not a concern with a calculator whose display-driving electronics were debugged long ago), the only threats I can think of to LCDs are heat and humidity (especially the combination), freezing the liquid (which I have found does not always damage the display like the manufacturer says you might), and mechanical pressure (which is also not a problem when there's a transparent protective window in front of it.

All LCDs do however have polarizers that are part of how they operate. If your light source is polarized, the LCD background may look dark, depending on the orientation compared to the polarity of the light source or even polarized glasses.

Years ago I was designing a wide-temperature-range LCD into one of our aircraft products. After it passed the temperature tests, I tried the polarized sunglasses, since many pilots in the cockpit would be using them. It failed that test. I called the manufacturer, and they said they cut the polarizer however they can get the best yield; but they understood the importance in the application and said they could cut it in any orientation we wanted.


To my knowledge there is no photo-sensitive system or variable system to alter the 'darkness' of the LCD display as in subsequent models (48, 28 etc).

In halfnut models, the display driver does have programmable contrast, but the microcode does not ever adjust it. There's no way to set it without some kind of third-party ROM containing suitable microcode.

If the display contrast setting of a halfnut gets changed, and you don't have a suitable third-party ROM to set it back, the only way to recover it will be to remove the batteries long enough that the display driver completely loses its state. Removing the batteries momentarily and shorting the terminals will probably NOT be sufficient.

I don't think this is likely to be the cause of hte problem Mr. Rios is having, especially since there are very few halfnut 41C units in circulation.


I have never had any problems with my 41 or 42S displays, except the creeping crud in one 42S.

But as a point of interest, we use HP-200LX as survey data collectors for some 1995 vintage total stations and have had to create special cloth covers to cover the display in the field. If left uncovered for any length of time the display darkens to the point where it is difficult to read. This occurs even when the 200LX is in an environmental case, and it doesnt seem to make a difference with the calculator as we have used up a number of them (maybe as many as ten). Although most surveyors use the 48SX/GX, we have found the 200LX to be faster and the alphabetics an advantage. The TDS card for the 95/100/200 is virtually impossible to get though.



To my knowledge there is no photo-sensitive system or variable system to alter the 'darkness' of the LCD display as in subsequent models (48, 28 etc).


My guess is the LCD is slowly fading as they all do.

Not so accurate.

I think you'll find that problem to be associated with backlit LCDs in which the light source begins to dim. This is not the case with the LCDs used in HP calculators, they are all reflective designs without backlight.

The most common causes for dim HP calculator displays, in the order of occurrence:

1) In models where contrast is adjustable, it is set too low. As pointed out, the 41 is not among this group.

2) Weak, exhausted batteries. Don't count on the battery low indicator to be 100% accurate.

3) A hardware fault of some kind.

FWIW, in the case of the 41 being discussed, I seriously doubt it is a bad LCD. I've seen individual segments become dim (usually due to battery leakage) but never the entire display. That would more likely be a display driver or CPU timing problem.



In the watch industry, I have been restoring watches for about 13 years, there is a definite fading of displays over time. So much so that there is a large demand on sealed lcd panels for early vintage LCD panels.

In fact, if you see old LCD seikos as well as many first generation LCD calculators you will see a yellow tint to the display. This was a UV filter to slow or prevent fading displays.

Displays have evolved since then but I still think they are subject to a fading.

Having said that, and restored a few 41's including 6 of my own dating back to 79, I have not had a problem with them at all. Some have different viewing angles but all are crisp and clear..

Cheers, Geoff



I've seen many just flat out broken displays and many "creeping crud" failures (permanent black areas) with all HP calculator products but never have I seen an HP display uniformly dim.

Most models get used outdoors for survey work (41's, Voyagers, Pioneers, 48's) in direct sunlight so if UV was the cause, we would have see failures. I have seen many 48 LCD's fail due to over-temperature where the polarizer actually melts. Discounting those obvious abuses, HP displays seem to me to be pretty darn good and reliable. Heck, all 41 displays are at least 20 years old, some now at 30 and counting.


Hi Randy,

I agree with you, the HP LCD's are excellent. I have noticed different brand watches have various quality displays.

For example, the Seiko is meant to be viewed at 45' and start to fade as the tilt angle is greater then 90' and less then 30', a viewing angle of 60 degrees.

My Omega LCD however, can tilt through 100' without any fading outside the direct best angle.

It would seem that the watch industry has differing standards of quality, and maybe this is the reason they fade. Of course they are in the sun 24/7, certainly more so then HP calcs.

I have replaced quite a few Seiko World timers shown here, some of which have the yellow UV filter. All the yellow watches are 1976 era and suffered faded displays. The displays in them now were NOS sealed in aluminum pouches.

Cheers, Geoff


I think it's a hardware problem. As far as the sunlight, I agree, most HP displays tend to fade away when brought back indoors. I appreciate all of your responses. Thanks for the help.
By the way, It's a fullnut 41c.


Geoff --

I still wear this classic 1981 Seiko H357 Duodisplay model, shown in the linked photograph (which is not of my personal watch):

Seiko H357-5030T

The LCD is angled toward the wearer, but can be read from different orientations. There has been no problem of fading. Maybe LCD technology improved during the late 1970's? (Remember that few calculators had LCD's until about 1980.)

Exceptional quality and a clean, sensible design -- just like H-P calc's of the era. It's hard to find consumer products like that anymore.

-- KS

Edited: 28 May 2009, 6:11 a.m.

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