Contrast problem with a graphical LCD - Page 1
Forum section: electronics / repair

author: hli
author_rank: Frequent Contributor
timestamp: July 13, 2025, 02:07:10 pm
content: So I have been asked to look at a problem with a machine control unit (its a PLC unit). The problem is that the contrast control of the graphical LCD behaves strange. When you look at the attached pictures, you can see that the upper and the lower half now have a different contrast setting. This contrast is not controlled by a trimmer somewhere, but can be changed from the front panel, so there is a DAC somewhere (either that, or its set via registers digitally in the display unit itself - there is a LA5316 IC on the display board, which makes this likely - "variable divided voltage generator IC for multiple drive of LCD matrix").This unit is rather old (produced in 1992), so its difficult to get spare parts. It seems there are shops which sell this display (part number is LM-KE55-32NFZ), but for one they are not exactly cheap (somewhere between 200 and 400 EUR, and I guess for the cheaper ones I will need to pay taxes and customs as well), and I have no idea how reliably chinese sellers are...So: what should I look at when I get access to the machine? When I got the problem description the first time, it seemed as if it could be just a problematic solder joint with the contrast dimmer, but now I know that is not the problem. Could this be just solder joints? Could the LA5316 be faulty, or one of the caps near to it?

author: coromonadalix
author_rank: Super Contributor
timestamp: July 13, 2025, 05:30:50 pm
content: can you check under the flat cable, you see partial numbers / characters

author: Harry_22
author_rank: Frequent Contributor
timestamp: July 13, 2025, 08:06:55 pm
content: Hi!Check this capacitor. It looks kind of strange.

author: amyk
author_rank: Super Contributor
timestamp: July 13, 2025, 08:28:00 pm
content: The LA5316 is just a bias voltage generator, and a single-channel one at that. What other ICs are on the board besides it? Your symptoms suggest that the two halves, upper and lower, are being driven with different voltages.

author: Harry_22
author_rank: Frequent Contributor
timestamp: July 13, 2025, 08:52:45 pm
content: I found PCB from Power Book screen (Sharp LM64N792).Similar designs have some capacitor problems. Please look here:https://mac84.net/web/macintosh-powerbook-160-lcd-display-recapping-guide/

author: hli
author_rank: Frequent Contributor
timestamp: July 15, 2025, 08:05:59 am
content: Quote from: coromonadalix on July 13, 2025, 05:30:50 pmThere are 2 digital ICs here, see attachement.

author: hli
author_rank: Frequent Contributor
timestamp: July 15, 2025, 08:22:13 am
content: Quote from: Harry_22 on July 13, 2025, 08:06:55 pmI asked for a better picture of this part - see the attachment. It seems the problem is not the ceramic cap - from the looks the electroytics have leaked.So I will plan to replace them. This special form factor seems not to be available anymore, but there is plenty of room to use regular ones. Once I have access to the machine I will try to measure the ceramics as well and replace that one cap (or at least clean and re-solder it), just to be sure.Quote from: amyk on July 13, 2025, 08:28:00 pmThe other ICs on the visible side are a '358, a HC40103 and a HC74.I agree on the two halves. But the LA5316 has actually 5 output channels, so it might actually drive both halves. I guess replacing the caps are the best way forward, esp. since this seems to be a known problem. Once I have the display out of the machine I will look whether there is an additional driver or not.Thanks for all your help. I will report once I attempted the repair (will take a some weeks, though, first I will be on vacation ;-)

author: Harry_22
author_rank: Frequent Contributor
timestamp: July 15, 2025, 09:26:48 am
content: I like this approach (pls see the photo).https://tinkerdifferent.com/threads/powerbook-100-screen-recap.779/

author: hli
author_rank: Frequent Contributor
timestamp: August 03, 2025, 09:06:27 pm
content: So I finally got the actual board yesterday. Cleaning up all the mess took a while - the leaked electrolytic was there for too long I guess. It turned out that the leaked capacitor destroyed several traces. So I used some (rather small - 0.2mm) bodge wires to replace the missed traces and pads. Luckily I was able to determine where the PCB traces were before. Unfortunately I forgot to take a picture before soldering on the caps, but here is the result:Well, turns out the display stayed black afterwards. After some swearing and head-scratching I started to remove the caps one by one - under the assumption that I might have either missed to connect a trace to shorted two together. After the first two, I got the first picture on the display, but with rather low contrast. But since the contrast was consistent over the display (not the split version from the first post), I figured that using ceramic caps was a mistake (even though the thread linked by Harry_22 claims they would work). So I replaced all caps with electrolytics, and voila - perfect display again!I would have preferred to use SMD caps, but with regular radial caps I could use the leads for the missing wires (with SMD electrolytics this would have been a pain). I used some Kafuter K704 to hold everything in place (this is a PLC for machine control, so I expect some vibrations). Does not look as nice anymore, but does the job: