Cleaning LCD display on 41-series?



#7

I just bought a used HP-41C. Like my own HP-41CX, it has dust and other crud between the cover plastic and the LCD. There is enough of this stuff in the 41C that it is quite annoying.

Is there any (easy?) way to get in there and clean out the crap? It got in there, which would seem to indicate that you should be able to get it out. All the discussions I've seen about getting into a '41 don't seem to mention access to the LCD. Does this mean you can't get in there? What about gently prying off the covering plastic?


#8

If it's an original (fullnut) machine (without the black border round the display) then it is possible to remove the display unit.
It's quite a job, and it risks damaging your 41. What you do is take out the battery pack, peel off the feet, and undo the screws under them.
Lift off the bottom case, remove the logic PCB and its connector, the U-shaped centre case (keep this the right way up) and the piece of black card.
Then remve the little plastic strip at the top of the display. Unsolder the connections from the display to the keyboard (note, the display module is static-sensitive, so take care), and take the display module out.
As I said, it's quite a job, and you risk damaging the display module.
If it's a halfnut, then you could try cutting away the heat-staked posts holding the logic PCB to the top case and then re-staking them afterwards, but I wouldn't want to try it. In the halfnut, the display is on the logic PCB, as is the keyboard, so you have to remove the logic PCB to get to the display


#9

I did it! - and it's easier than just described - but still a bit dicey - and I wouldn't do it unless I was willing to risk disaster. Fortunately, my '41C is a fullnut, so the LCD module is a separate, independent piece. Follow Duell's steps up to removing the "little plastic strip" (which is a retainer) at the top of the case/display. After that, with a little bit of diligent prying (with a knife blade or slim screwdriver) at the TOP (and perhaps sides) of the display LCD unit, you can tilt the display up far enough - WITHOUT unsoldering anything - so that you can insert a piece of paper to use as a wiper. (Try moistening the paper a LITTLE bit for better "grab.") I cleaned out most of the offending crud this way, and the display looks MUCH better. After reassembly, it took a few heart-rending seconds for the calculator to come back to life, but it now works and looks good. (And, I fixed a recalcitrant decimal point key at the same time, with a spray of circuit board cleaner. Many thanks to those who described previously how to fix bad keys!)


#10

I didn't realise that you could move the display module back far enough to be able to clean it like that. I've removed a couple of 41 displays (to repair them), and I've always cleaned them
at the same time. Never tried to clean the display without removing it for repair first.


#11

I didn't really "move it back" - rather I tipped it up (at the top edge) by, in effect, getting under the assembly (i.e. in between the assembly and the cover plastic through which it is normally viewed). The LCD assembly rotated with respect to the back of its holder and the solder fingers. The holder for the LCD appears to be metallic (shielding, probably) - I scraped some of the black paint off and saw shiny metal (copper or brass, judging from the color).

#12

The first thing I try when I have a dirty '41 display is try to blow the dust out with a can of compressed gas. I've had good luck just putting the tube into the card reader clip slots and giving it a blow. A couple of times I have had to open up the case on a fullnut and tilt back the display and blow.

I have cleaned a couple of half nuts by sticking the display end in a bowl of warm (almost hot) distilled water and swirling vigourously (I've also used ultrasonic cleaners). Repeat three times with fresh distilled water each time. In the last rinse I place a little Kodak PhotoFlo 200 wetting agent. This helps prevent streaking as the display drys. I shake out the excess water (quite vigorously), let it dry out completely, and pray... I haven't killed one yet. One of the halfnuts had been dropped in a muddy pool and came out looking like new.


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