LCD display zapped by static discharge, repairable? - Page 1
Forum section: electronics / repair

author: Divarin
author_rank: Contributor
timestamp: April 05, 2022, 05:10:55 pm
content: Hi all. This is my first post on this forum I'm looking for some expert advice.Way back in the early 2000's I bought an MP3 player called a NEO-25 (here's a page describing it:http://www.djzone.net/pg/0008/tf00083.shtml)It's had a hard life despite my being as careful with it as possible. The battery no longer holds a charge, the power socket had to be re-soldered about 10 times, the power switch is flaky and needs to be halfway between on & off and the big one: One day I touched it and a static discharge went from my fingertip into the LCD screen and ever since then the screen is pretty messed up. Basically all of the "pixels" are darkened but you can barely make out what is on the display if you hold it at just the right angle.Anyway I put it away in a box and forgot about it but now I want to try to refurb it so that I can use it to load software into various 8-bit micros that use tapes such as a zx spectrum, TRS-80 MC-10, and others.I can power it by putting 5 volts into the charging port even with no battery so I plan on using a USB power pack there, probably replace the laptop hard drive with a compact flash card since my windows 7 device doesn't recognize it when plugged into USB, but before I even start any of that I'm trying to decide what, if anything, can be done about the LCD screen. I know if the screen was smashed that's pretty much non-repairable but I don't know about zapped screens.It's probably not repairable and if I am going to use it I'll have to live with the screen as is but I thought I'd reach out to some more knowledgeable folks before I give up.Any advice is appreciated, thanks.Here's some pictures for reference:

author: Haenk
author_rank: Super Contributor
timestamp: April 05, 2022, 05:16:08 pm
content: I think it might have zapped the contrast adjustment. The display otherwise seem to work (aka "good thing, the controller is alive and working").Is there a contrast adjustment key combo?

author: Divarin
author_rank: Contributor
timestamp: April 05, 2022, 05:41:27 pm
content: I don't think so, there's nothing about it in the manual.

author: Divarin
author_rank: Contributor
timestamp: April 05, 2022, 06:17:03 pm
content: on the back side of the LCD panel is a number of surface mount resistors. I wonder if one or more of those resistors got zapped if that would affect the contrast. I guess I could desolder each one and test them.

author: Bud
author_rank: Super Contributor
timestamp: April 05, 2022, 06:33:49 pm
content: Google for '20x4 LCD pinout' to find the contrast adjustment pin number and go from there.

author: tooki
author_rank: Super Contributor
timestamp: April 05, 2022, 06:39:38 pm
content: Worst case, if it’s the controller IC that’s died, this is likely a totally standard 2004 character LCD that can be replaced for peanuts.

author: Divarin
author_rank: Contributor
timestamp: April 05, 2022, 06:52:04 pm
content: Quote from: Bud on April 05, 2022, 06:33:49 pmHmm okay taking a look here:https://forum.arduino.cc/t/20x4-lcd-screen-too-dim-please-i-need-help/564520/6seems to suggest adding a pot on the 5volts (pin 3, at least for the one on that page) may give me control over the contrast. There's no pots of any kind inside this unit right now. But I wouldn't be surprised if the 5 volts going into this display can be traced back to a resistor which is not working.

author: Haenk
author_rank: Super Contributor
timestamp: April 05, 2022, 07:08:54 pm
content: Remember, that thing is 20 years old, it's very likely a different display from what can be found on Aliexpress: maybe open it up and make a picture...

author: tooki
author_rank: Super Contributor
timestamp: April 05, 2022, 08:05:20 pm
content: Quote from: Haenk on April 05, 2022, 07:08:54 pmI completely disagree. The whole joy about character LCDs is how standardized they are! No, it’s not 100%, but the vast majority are indeed one of a small handful of types (size, pinout).

author: james_s
author_rank: Super Contributor
timestamp: April 05, 2022, 08:12:50 pm
content: Quote from: Haenk on April 05, 2022, 07:08:54 pmI've replaced displays in devices much older than that with no problems at all. The HD44780 type LCD character displays have been around since at least as far back as the 80s and the vast majority of them are compatible and use the same pinout.

author: Gyro
author_rank: Super Contributor
timestamp: April 05, 2022, 08:15:46 pm
content: QuoteA classic (and rare) case of 'If it ain't broke, don't fix it'.

author: Buriedcode
author_rank: Super Contributor
timestamp: April 05, 2022, 09:46:01 pm
content: Given its age it could jsut be that - age. The contrast is often set with a trimpot because it can vary with temperature. Some displays that are designed to operate over a wider temperature range require a negative voltage on the contrast pin - close to 0V but sitll negative. Thats an STN display with what looks like 16 pins - pretty bog standard HD44780 as others have pointed out.The contrast voltage - pin 3 - will be very close to the GND/0V rail, but not quite, so removing the resistor connected to that pin and adding wires to the pads for a pot could work. if theres no room for a small trimpot (or trimmer) then you can temporarily connect a trimpot, set it so the contrast is where you want it (at the viewing angel you want - these old displays were all designed to be viewed from a certain angle, 6 o clok, 12 o clock etc..) then measure the trimmers value and replace with fixed resistors.Frankly I'd go with what the others have said and replace it. The only non-standard spec might be the dimensions of the PCB its on and the mounting holes, but all the 4 x 20 HD44780 displays I've seen over the years have been the same size.https://www.mouser.com/c/optoelectronics/displays/lcd-displays/lcd-character-display-modules-accessories/?character%20count%20x%20line=20%20x%204&sort=pricingAmazon, Ebay or Aliexpress will have plenty that are cheaper, but they don't always provide detailed dimension drawings.

author: amyk
author_rank: Super Contributor
timestamp: April 06, 2022, 01:37:42 am
content: Post some pictures of the inside. I agree that the controller is likely to be HD44780-compatible, but the pinout may or may not be different from what you can find today.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: April 06, 2022, 10:33:24 am
content: I remember many years ago finding a solution to replacing a display where the contrast voltages were different.I discovered that soldering a standard 1N4148 diode between contrast pin and ground gave around 0.6v contrast voltage, which happened to be perfect.There must have been some kind of pullup resistor or circuit on the display that enabled this to happen.

author: Divarin
author_rank: Contributor
timestamp: April 06, 2022, 12:30:30 pm
content: Quote from: Buriedcode on April 05, 2022, 09:46:01 pmSo the specific model # for my display is p-s4a20ctIt looks like there is 5 volts going *into* a resistor just before pin 3 but only about 0.4 volts coming out. The surface mount resistor is labeled "103" which, looking that up, should mean it's a 10k ohm resistor.There are no trim pots (or pots of any kind) throughout this entire device. So I'm thinking the purpose of this resistor is to set a constant contrast value and is not meant to be adjustable. Given the extremely low voltage coming out I'd say it's likely this resistor is what got zapped. I was just about to desolder it yesterday and was going to try exactly what you suggested: temporarily adding a pot, adjusting it, then measuring the resistance and replacing with an appropriate resistor. Unfortunately as soon as I powered on the soldering iron the power went out and stayed out until dinner time so I never got around to doing that.If the issue is this resistor, or somewhere in the circuit before the resistor then replacing the screen won't help as the resistor in question isn't even on the display, it's on the main board just before a ribbon cable which goes to the display.Here's some more pics:

author: Divarin
author_rank: Contributor
timestamp: April 06, 2022, 12:33:03 pm
content: Quote from: Audiorepair on April 06, 2022, 10:33:24 amJust read this after posting my last reply, if 0.6v was the correct voltage for you then my 0.4v isn't so suspect afterall, still I guess it wouldn't hurt to try temporarily replacing that with a pot and adjusting it to see if it affects the contrast.

author: Divarin
author_rank: Contributor
timestamp: April 06, 2022, 01:37:44 pm
content: Hey I had some luck! I put a small pot initially set to 10k between pin 3 and pin 2 because I noticed direct continuity between pin 2 and the "input" side of the resistor before pin 3 so this seemed the easiest solder point and would also mean I wouldn't have to remove the resistor for now, just bypass it.Powered it on, initially nothing seemed to happen I had to turn the pot quite a lot but eventually the contrast came in normal. I desoldered it and measured the resistance: 1070 ohm, so about 1/10th what the original resistor is doing.I might just add some wire and keep the pot there but position it someplace more convenient.I don't think the 10k resistor is zapped I think something changed about the screen when I zapped it which makes it now require more voltage to bring the contrast up to normal than was needed before. So replacing the screen (and removing the pot) might work but also I can try just keeping the pot in there for now and if it doesn't get worse over time then I'll be content with it.

author: amyk
author_rank: Super Contributor
timestamp: April 06, 2022, 01:47:09 pm
content: https://huizhouteneng.sell.everychina.com/p-108658268-4-x-20-characters-lcd-display-ssc4a20dlnw-e-p-s4a20ct-e1-white-led-side-backlight.htmlThis specific part is still in production, apparently.

author: Divarin
author_rank: Contributor
timestamp: April 06, 2022, 02:07:07 pm
content: Quote from: amyk on April 06, 2022, 01:47:09 pmYeah that link was the only source I could see but it looks like there's a minimum order of 500. I might be able to get it (or one compatible with it) somewhere but if I can get by using the one I have then I'd rather do that.

author: Divarin
author_rank: Contributor
timestamp: April 06, 2022, 06:30:25 pm
content: Well the pot fixed helped a lot. You can tell there's some kind of damage to the screen but after tweaking with the pot I got it to look pretty good. It went from "can barely read it" to "can barely tell anything's wrong with it":Thank you everyone for the help. There's still a lot to do to refurb this thing but this issue was the "is this even going to be worth trying" issue. Now, it's worth trying

author: Buriedcode
author_rank: Super Contributor
timestamp: April 06, 2022, 07:06:14 pm
content: Those nice PCB photos! (often we see blury ones here..).From what I can make out, theres a resistor divider, with 10k going to pin2 of the display (5v) and 330 ohm to pin 1 (ground/0V) that provides the voltage for pin3 - the contrast pin. So with 5V on the display thats 330/(10k + 330) * 5 = ~0.16V. Generally the contrast voltage on these can be anywhere from 0.1 to 0.7V so its within range, but looks quite low to me.Putting a pot in parallel with the 10k resistor - as you did, between pin 2 and 3, thats between 5V and contrast - reduces the value of that resistor, which should raise the contrast voltage and give better results - again, exactly as you did! If your value of 1070 ohm was correct (I don't doubt), then in parllel with a 10k resistor makes it pretty much 1k. With the voltage divider that would make the contrast voltage .... 330 / (1k + 330) * 5 = 330/1330 * 5 = 1.24V. Quite high, but hey, if what you've done improves contrast, it works.The faded pixels, to me, just look like age. STN LCD's tend to last a very long time, but things like humidity and wild temperature changes over long periods can damage them. I've lost a couple of old 90's 240x64 displays when I stored them in my garage. I think about 8 winters killed 2 :/If you were to replace it, it looks like they don't use the mounting holes but hold the displays PCB in place with plastic hooks - so the dimensions of the display are important (98 x 60mm). I'm almost certain the pinout with be the standard 16 pin one.The only odd thing is the backlight... it has a nice blue backlight, I haven't seen too many with that exact colour, so I'mguessingits an EL (electroluminescent) backlight - that might be what the small yellow transformer is on the PCB. EL backlights are very pretty, but a bit of a sod to drive (400Hz relatively high voltage). And have largely been replaced by LED backlights (run on 3-5V, easy to drive/dim, and are bright and efficient). Again.. I could be wrong hewre, the transformer could just be part of the units power supply.So replacing the display might mean a different backlight - theres some blue STN ones with white pixels and blue backlight, as well as negative mode (dark background, pixels the colour of the backlight) . If the unit can control whether the backlight is on or off, then it could require some mods to get this to drive an LED - but if its on permenantly, then you could just connect to the 5V line if it doesn't draw more than 50mA.But if you're happy with it, and its readable, then its a job well doneAnd if you like the device, then its always worth giving it a shot. I value things more when I've repaired them.Well done dude

author: Divarin
author_rank: Contributor
timestamp: April 06, 2022, 08:12:17 pm
content: Quote from: Buriedcode on April 06, 2022, 07:06:14 pmThe backlight is only on when you first start up or for a couple of seconds after you press a button. It has always made a high pitched wining sound while the light is on. Yeah I'm not sure I fixed it "correctly" but somehow managed to make it work and since my use case for it now isn't going to be carrying it around on walks or anything, just using it to feed data into old vintage computers it should work fine.I had this thing in a box in the closet for probably 8 or 9 years now because I knew I wanted to get it going again but had no idea what could be done. But over the past few years I've been dinking around fixing up old computers and stuff and have gradually been building up some skills (still a long way to go) but I felt confident enough to at least try. Just needed some of the experts on this forum to point me in the right direction, I didn't even consider that there might be a contrast adjustment.

author: Buriedcode
author_rank: Super Contributor
timestamp: April 06, 2022, 10:46:18 pm
content: Quote from: Divarin on April 06, 2022, 08:12:17 pmYeah that'll probably be an EL backlight then. Difficult to replace, but it seems to be working just fine, so no need.Repairing, or just "investigating" devices, taking them apart, see how they work, and googling questions it raises is a great way to learn. And this forum is great for that, getting pointers to the right direction, when one is unsure what exactly to google.Apologies if my posts were too detailed, or off topic, I get carried away with explaining stuff or trying to work out how things work. Glad its all working!

author: DavidAlfa
author_rank: Super Contributor
timestamp: April 06, 2022, 11:06:39 pm
content: Definitely EL.These displays are easy to disassemble, shouldn't be hard to transfer the EL to the new display.Just ensure to remove any LED resistors.

author: james_s
author_rank: Super Contributor
timestamp: April 07, 2022, 01:45:57 am
content: Careful with EL backlights, they'll give you a good zap. Won't kill you but it doesn't feel good either.

author: amyk
author_rank: Super Contributor
timestamp: April 07, 2022, 03:24:49 am
content: Quote from: Divarin on April 06, 2022, 01:37:44 pmI agree; resistors are rarely damaged except by overheating, it's more likely the ESD event damaged the FETs on the driver ICs so they're now leaky and don't turn off completely. By boosting the contrast voltage it works around that.

author: mkaleu
author_rank: Newbie
timestamp: October 22, 2024, 04:31:24 pm
content: Hello, are you still working on your Neo? I have one of those too and have taken it out of the junk box again. It still boots, but doesn't display anything. I'm not sure if the hard drive is faulty, or the player itself. Is the program stored on the hard drive? Can you tell me something about it?Mike