Trouble shooting a small LCD? - Page 1
Forum section: electronics / projects

author: jwhitmore
author_rank: Regular Contributor
timestamp: November 06, 2018, 03:42:09 pm
content: Hello all,looking for some advice on how to trouble shoot a very small LCD, out of a tractor's Electronic Instrument Cluster. The display is only showing a few of the segments very faded so I can't read the error code being produced by the tractor.I'll attach a photo but it appears to consist of a, now obsolete, LCD Driver chip PCF8576 . That driver just connects to a flex cable at the edge of the board. That flat flex connector cable does appear to have a few scrapes across it, but not in the actual conductor, which sill looks good. I'd assume it's a mechanical issue and that the PCF8576 would still be working, but as its obsolete anyhow it's not like I could do anything with it. Maybe it's neither the Driver IC or the cable but the actual LCD.I was hoping that this would be a two minute teardown and find the issue but no such luck. Driver IC / Flex Cable / LCD ? Not a lot I can do with any of them?Just out of interest there's a code on the PCF8576T "6735" is that the date code? Can't be 1967? Better ask brother how old this tractor is.https://www.nxp.com/docs/en/data-sheet/PCF8576.pdf

author: Buriedcode
author_rank: Super Contributor
timestamp: November 06, 2018, 04:54:07 pm
content: That could be the date code - but I very much doubt its from 1967A faded LCD could well be the driver, but could also be the glass itself. Is the flex cable soldered to a board? or is it simply pressed in place?Gently moving the flex cable whilst the unit is functioning could show if the segments work again.You could also test the Vlcd line (multimeter across pin12 and pin 11). It's possible that the Vlcd line used to control contrast isn't the correct voltage (could be set by a pot) which is making the segments faded. Why this would fail I do not know, I would need to see the whole board.

author: jwhitmore
author_rank: Regular Contributor
timestamp: November 06, 2018, 06:02:57 pm
content: Thanks for taking the time to respond. I'm afraid I have too many cameras and the one I want at the moment for close up I can't find. I'll try this photo for a start.I'm not sure why this has been done but part of the board as been painted with a red covering? There's no pot on the side of the board I can see. The bottom is covered by plastic mounting, I'll have a look at removing that. Anyhow pins 7 to 12 all seem to be connected together, and since pin 11 is Vss I assume Vlcd is tied to ground. The flex cable is attached to the edge of the PCB with a metal clip. Unfortunately this device is connected to the wiring loom with two AMP Multilock .040 connectors (16 pin and 20 pin) but I can't source any. I might have been able to just hook it up to a bench top power supply. I might actually have a crimp connector which might get me away and I'll try that.I'll try to power it up and check voltages. If the driver IC is dead there is a replacement listed so I might be able to get my hands on that, get rid of the red gunk and solder in a replacement.

author: alanambrose
author_rank: Frequent Contributor
timestamp: November 07, 2018, 06:28:40 pm
content: >>> Trouble shooting a small LCD?Get closer maybe?

author: Buriedcode
author_rank: Super Contributor
timestamp: November 07, 2018, 06:58:27 pm
content: Re: red gunk. I've only seen that once, on the PCB of a radio controlled car I had as a kid in the 90's! I vaguely remembered it flaked off like nail varnish.As for that photo, going by the PCF8576T's datasheet, it looks like VLCD (pin 12) is connected to ground. This can be the case, as it only needs to be negative (with respect to ground) with larger multiplexing, so it looks like the whole thing just runs off 5V.The "clip" that holds that flex cable in place looks like its still doing its job just fine. I would be very cautious about removing it to check for pad corrosion (a possibility on an old board!) in case you cannot get the flex cable and clip back on properly. But if you're confident, might be worth a check.I have my doubts about the failed LCD driver chip ( the PCF8576). It isn't supplying any significant current anywhere, so will never be stressed, but I really couldn't say for sure - I agree that checking its outputs would be worth checking just in case, a good check would be seeing if the outputs reach close to the power rails (0v and 5v). My vote is the glass itself. Excessive IR or UV from sunlight can age polarizers, and I have had several old watches "fade". I don't know if this is the liquid crystal or the polarizers.

author: JVR
author_rank: Regular Contributor
timestamp: November 08, 2018, 03:08:40 pm
content: Ideally we need a picture of what the LCD is capable of displaying at this moment. I'll second that the driver chip is probably fine, cracked glass or delamination from vibration are much more likely.

author: jwhitmore
author_rank: Regular Contributor
timestamp: November 09, 2018, 08:57:16 am
content: The flat flex is bonded onto the side of the LCD. I don't think that matters you'd have to find a replacement LCD complete with flex cable attached, and I can't see that happening.

author: jwhitmore
author_rank: Regular Contributor
timestamp: November 09, 2018, 09:01:36 am
content: There's nothing obviously damaged on the LCD but I'm not the expert. I'm afraid this is going to be impossible to do anything with.But thank you all for trying. I'll try and find on the connections for this device and connect it up to a power supply. If I connect it back to the tractor I won't have much access. I have some documentation but it isn't too specific about connections. These things aren't made to last.

author: JVR
author_rank: Regular Contributor
timestamp: November 09, 2018, 09:41:48 am
content: This is very close to the Replacement Part Cost < Time and Faff Cost line.If the part is no longer available from the OEM, you can probably spoof the output from the MCU. Power connections and voltages should be easy to decipher from the PCB though.