Spots on display, not pixels

Discussion of hardware issues related to the DM1x/DM41. Please ensure that you specifiy whether the issue is related to a credit card size unit or a "Voyager" unit (DM1xL/DM41L).
amen
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Joined: Wed Dec 16, 2020 9:32 am

Spots on display, not pixels

Post by amen »

I dug out my DM41L in the workshop this morning and brought it indoors. I checked it in the workshop and all was fine. A few hours later I got it out of its pouch to use and saw this on the display:
spots1.jpg
spots1.jpg (95.79 KiB) Viewed 376 times
It seems to be in the plane of the LC, as far as I can tell. Anyone know what this is or seen it before? The display seems undamaged. Is this permanent?
Could it be something to do with temperature? It's cold today and there was probably a warming up of the calculator when it came indoors.

Andrew
Michael
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Re: Spots on display, not pixels

Post by Michael »

I'm sorry for the spots on the LCD.
It seems this happens to such liquid crystal displays exposed to cold temperature or pressure.
The good news is there is an easy solution to this. Put the calculator in a hot place, up to 70°C or 80°C for a few hours and the spots will be gone.
The LCD turns dark if it's hot and clears up when it cools down again. This makes the spots disappear. Sometimes this procedure needs to be repeated once or twice.
You can put it in the oven if the temperature is kept low enough, or even putting it in the car on a hot day for some hours does it.
Best would be to only bake the PCB without the housing and without the battery.
Please let me know if it worked for you, otherwise we'll find a different solution.
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PierreMengisen
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Re: Spots on display, not pixels

Post by PierreMengisen »

it makes me think of a weak battery.
Pierre
[TI59 with PC100C; TI-84 Plus CE-T; HP41CV with HP IL loop & 2*82161A DCD & 82162 TP; HP15C; HP28S; DM41; DM41L; DM42; DM41X]
amen
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Joined: Wed Dec 16, 2020 9:32 am

Re: Spots on display, not pixels

Post by amen »

Ah, OK, I'll try putting the LCD through a heat cycle. I'll let you know what happens. Don't worry about it.

Andrew
reavy
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Re: Spots on display, not pixels

Post by reavy »

I had something just like that recently, and about eight cycles of 90 minutes each at 70°C did it. Only problem I had was the foam on the back of the PCB didn’t like the high temps. Calc works fine after being reassembled, though.
amen
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Joined: Wed Dec 16, 2020 9:32 am

Re: Spots on display, not pixels

Post by amen »

Well, I cycled the display quite a bit, and the final bake at 85C for 90 minutes seems to have worked.

I have a video of the full story here:

https://youtu.be/lOEOn0heLaI

Thanks, Michael, for the advice.

If anyone knows what is going on here, I'd like to know...

Andrew
rprosperi
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Re: Spots on display, not pixels

Post by rprosperi »

Thanks for the extensive testing, sharing the video and for having the patience to find the sweet spot.

Would you guess that a single 'treatment' of 85C for 90 minutes should be enough or do you think that got it done building upon all the prior heating attempts?

It's really helpful useful to have a known-good, east-to-follow recipe, though the image of folks popping their machines into the kitchen oven is a bit frightening... DON'T DO THAT FOLKS, he's using a special oven. It's almost impossible to control your kitchen oven at temperature this low; find a friend or a tech with a lab oven.
--bob p

DM42: β00071 & 00282, DM41X: β00071 & 00656, DM10L: 071/100
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pauli
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Location: Australia

Re: Spots on display, not pixels

Post by pauli »

My DM16 exhibited this problem and I tried Michael's solution: baked in our over at around 80℃ for a couple of hours. I let it cool and the marks are gone. I've not powered it up yet but will soon.

I didn't know LCDs did this.


Pauli
amen
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Joined: Wed Dec 16, 2020 9:32 am

Re: Spots on display, not pixels

Post by amen »

If I had to guess, then i'd say that the one cycle would do it, based on the dots that were on the display at the beginning of that cycle looking just like the original dots. So i think I was back where I started when I put the PCB into the oven that last time. All the other steps had little effect.

If you do this make very sure you don't leave the battery in.

FYI I used a Miele 5000 series oven, so it isn't a specialised oven, just a microprocessor controlled one where the temperature is entered on a keypad. I think you could do this on any oven that can set a repeatable temperature and calibrate the actual temperature by finding where the LCD goes black and increasing slowly from there. Or use a calibrated thermometer :-)

I've just checked and the dots are still not present. The 41L has been indoors since the treatment.


Andrew
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pauli
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Location: Australia

Re: Spots on display, not pixels

Post by pauli »

I spoke too soon, the device came out of the oven about seven hours ago and one black pixel has returned. I'll give it another heating cycle and see.


Pauli
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