Mindstorms NXT Display Failure - Cracked capacitor value? - Page 1
Forum section: electronics / repair

author: Ziginox
author_rank: Contributor
timestamp: November 29, 2021, 02:38:35 am
content: Hello everyone, a bit of a last ditch effort on this one, but here we go:I recently bought a Mindstorms NXT kit for almost nothing, but found that the screen is very low contrast and difficult to read. According to the googles, it's a common failure caused by three small surface mount caps on the screen/button PCB that often have cracked solder joints. I checked on them, and found that the smallest of the lot, C3, was already cracked clean in two.So far, I've been unable to find the value. I did find the schematics, linked below, but they seem to not cover the button/LCD board.I can't tell where the other side goes, but the other appears to go to pin 3 of the connector labeled J15, which would be VCC3V from the main board. Ithinkit's for setting the bias of the LCD, but I'm not sure.Anybody have ideas as to what the value could be?TIASchematic:http://www.xatlantis.ch/doc/nxt/Appendix%201-LEGO%20MINDSTORMS%20NXT%20hardware%20schematic.pdf

author: james_s
author_rank: Super Contributor
timestamp: November 29, 2021, 04:49:48 am
content: VCC3V sounds like the 3.3V supply, it may just be a decoupling capacitor. You might just try sticking something in there, 0.1uF if it's decoupling, if that doesn't work you could try 1uF.

author: amyk
author_rank: Super Contributor
timestamp: November 29, 2021, 04:59:05 am
content: Post some pictures? If it's a resonably standard LCD / controller, some schematics from other devices using it and/or the datasheet could be used as a reference.

author: Ziginox
author_rank: Contributor
timestamp: November 29, 2021, 05:11:11 am
content: Quote from: james_s on November 29, 2021, 04:49:48 amThat's what one might expect, but with the cap now gone, I get nothing on the display at all. Decoupling caps usually go across the load and it should operate without, correct?Quote from: amyk on November 29, 2021, 04:59:05 amI'll get the macro out and get some shots tomorrow.

author: james_s
author_rank: Super Contributor
timestamp: November 29, 2021, 05:59:28 am
content: Quote from: Ziginox on November 29, 2021, 05:11:11 amYes, unless there's so much parasitic reactance in the wiring that it doesn't work at all without the decoupling but that's unusual. Most devices will work fine with some of the decoupling caps missing, they just get less reliable.

author: Ziginox
author_rank: Contributor
timestamp: December 02, 2021, 04:26:49 am
content: https://cloud.ziginox.com/s/k52kcH3krJHbs3AHere are the pictures, as promised. Sorry for the tardiness!Please let me know if this image host doesn't work, I tried Imgur but one of the files was too large.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: December 02, 2021, 04:44:31 am
content: LCD controllers often have capacitors that are used for voltage multipliers. Without those the higher voltage needed for the contrast circuit won't be generated, so there won't be any display. Try 0.1uF and then 1uF as james_s suggested.

author: Ziginox
author_rank: Contributor
timestamp: December 02, 2021, 04:59:51 am
content: Alright, I will do and report back. Thanks!EDIT: I do suspect it's something like that, as before I removed the capacitor the display was still visible from some angles but very faint.

author: jpwolfe31
author_rank: Contributor
timestamp: September 03, 2025, 12:56:49 am
content: We had a similar issue but were able to fix it by burnishing the lcd cable where it attaches to the board contacts with the round handle of a very small screwdriver. Consider trying this first. We just removed the small cracked cap.