HP 3478A LCD Flicker - Page 1
Forum section: electronics / testgear

author: Shark
author_rank: Contributor
timestamp: June 10, 2014, 09:18:19 pm
content: Hi guys!Recently I got an HP 3478A multimeter and I noticed there is some LCD flicker. I noticed that one segment (second from right) flickers on V DC and MV DC ranges. Strangely, when I do the self-test, all segments are okay, with full contrast and without any flicker. To make it clearer, I'm attaching a short video on YouTube of the flicker in action followed by a self-test.http://youtu.be/Dkw_6zVh5V8Aside LCD flicker, if I approach my ear, I listen to low-frequency humming coming from the top right part of the unit, where positive and negative 15V regulators are. Measured their temperatures and they were around 45°C (113°F). I don't know if flickering and humming are in any way related.Any ideas what I should examine first?Big thanks

author: cellularmitosis
author_rank: Unknown
timestamp: June 11, 2014, 10:18:51 pm
content: Try manually switching between 5.5 digit, 4.5 digit, and 3.5 digit modes.You might just be seeing the micro updating the LCD. I'd imagine on 3.5 digit mode the updates should be so fast you won't see the flicker.

author: Shark
author_rank: Contributor
timestamp: June 14, 2014, 07:46:09 am
content: Quote from: cellularmitosis on June 11, 2014, 10:18:51 pmThanks for your suggestion, but the problem remains when switching to 4.5 or 3.5 digit modes. The flickering is acually more visible, I guess it's because the LCD update frequency is higher.I noticed that there is no flicker if I go to manual trigger mode. Still, in that case, letter 'D' from 'VDC' is faded, as the picture I put together with my first post shows.Any other ideas what it could be?

author: Chipguy
author_rank: Unknown
timestamp: June 14, 2014, 08:24:52 am
content: Oh, last time I saw those flickers I got a display on its way to desintegrate.It could be a bad contact of some sort though. Depends on the way the display is contacted to the board.

author: PE1RKI
author_rank: Regular Contributor
timestamp: June 14, 2014, 10:53:06 am
content: could be the connection to the glass going high resistance on one point.if you are able to press on the glass or behind on the pcb of the lcd, you might see some difference.

author: Shark
author_rank: Contributor
timestamp: June 18, 2014, 06:27:33 pm
content: Thanks for your input! I was thinking of high resistance too. But if that's actually the case, I don't understand why all segments are completely black during self-test.I'm attaching another image (this time clearer), highlighting the odd difference between normal operation and self-test, when all segments are turned on. Any other ideas?

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: June 18, 2014, 09:32:57 pm
content: Wild guess without having seen the actual lcd. What if it is a low resistance path (leakage) to an adjacent segment that is not active, effectively pulling down the affected segments? When all the segments are active, everything looks fine.Either way, have you pulled the lcd to inspect for hidden damage?

author: Shark
author_rank: Contributor
timestamp: June 19, 2014, 08:53:10 pm
content: Good news! I was daring, took LCD apart and managed to fix it! Thanks to all you guys who contributed and gave me ideas on what it could be.It turned out that it was a poorly placed zebra strip ... I would not have imagined it for an instrument costing 1000 USD in 1983. It seems to me that it was like this when it left the factory because I saw no signs of tempering. I could be wrong though.I took some pictures while doing the job and wanted to share them with you. Even more so because I struggled to find any decent pictures of the HP 3478A LCD before I attempted the repair.

author: PE1RKI
author_rank: Regular Contributor
timestamp: June 20, 2014, 07:04:41 am
content: very nice, congratz on fixing the problem