DMM LCD segment always "on" - Page 1
Forum section: electronics / repair

author: xrunner
author_rank: Super Contributor
timestamp: March 10, 2015, 05:20:18 pm
content: Got a Simpson 467 DMM free from EEVBlog member Excavatoree, all I had to pay for was shipping (thanks!).He told me beforehand that the "B" segment on the display was always on. The "B" is supposed to tell you when the battery is low, however it is on even if no battery is present, and does not go off when the proper battery voltage is applied to the meter. There are no other faults visible on the LCD display, and it seems to work perfectly fine. I have not taken the meter apart yet. There are no other batteries in the meter.What I don't understand is how it can be "on" with no voltage applied to the circuits. Any LCD experts out there?Thanks in advance.

author: retiredcaps
author_rank: Super Contributor
timestamp: March 10, 2015, 06:58:43 pm
content: Quote from: xrunner on March 10, 2015, 05:20:18 pmJust to clarify, if you remove the battery, the "B" is displayed?

author: xrunner
author_rank: Super Contributor
timestamp: March 10, 2015, 07:06:32 pm
content: Quote from: retiredcaps on March 10, 2015, 06:58:43 pmYep, the "B" is always on. I received the unit with no battery and the "B" was on.

author: tautech
author_rank: Super Contributor
timestamp: March 10, 2015, 07:12:29 pm
content: Could a reversed installation of the battery damaged the sense circuitry?That is where I would focus my initial investigations.If the output of that sense circuit is wrong that might turn on the segment.

author: SeanB
author_rank: Super Contributor
timestamp: March 10, 2015, 07:18:41 pm
content: Short the battery leads together inside the meter ( take battery out first of course) with the meter turned on, and leave for about 5 minutes like that. That should turn off the segment, provided the display has not been damaged by the application of DC for long periods, which will mean it will stay on always.Most probable is a broken trace to the segment, allowing static to charge up the segment to turn it on, and you will have to remove the LCD and clean it and the board with the zebra strip to fix it.Otherwise the CD4077 quad Xor gate inside the meter either has a failing low voltage detector, or the chip has a blown output or a broken track to the display, allowing it to float with stray voltage.

author: xrunner
author_rank: Super Contributor
timestamp: March 10, 2015, 07:41:25 pm
content: ^^^ Good ideas. I will have to take the thing apart and see if the connection to the LCD from the driving circuit is OK.

author: xrunner
author_rank: Super Contributor
timestamp: March 11, 2015, 01:49:45 pm
content: I removed the display. Indeed, when you push around the "B", it tends to go "off". However, the part that holds the LCD to the board is the black plastic "frame", and it's held because the four plastic studs are melted down on the back. I'm going to go ahead and carefully cut around the studs so as to be able to remove the frame, and worry about how to re-attach it later. Hopefully I can clean and re-assemble it so it will work properly.Reference for searches: Simpson 467 LCD Display

author: gfiber
author_rank: Contributor
timestamp: March 11, 2015, 10:12:55 pm
content: you might be able to drill some small holes in the studs and use a sheet metal screw maybe #4 or smaller and washer to hold it back together.

author: electr_peter
author_rank: Unknown
timestamp: March 11, 2015, 10:39:15 pm
content: I would say forget it. You will likely damage display while repairing cosmetic defect. If it is not broken, don't mess with it.

author: miguelvp
author_rank: Super Contributor
timestamp: March 12, 2015, 08:27:24 am
content: Agreed, it is probably supposed to be on when no power, maybe because they cut a square the size of the symbol to have the same polarization as the back end, and only when power is there it turn off.I would not mess with it.

author: Zucca
author_rank: Unknown
timestamp: March 12, 2015, 11:05:59 am
content: "B" means Broken. Reverse Eng the thing and put a red led in the from panel if you want to signalize the battery low state..

author: xrunner
author_rank: Super Contributor
timestamp: March 12, 2015, 12:50:24 pm
content: Quote from: miguelvp on March 12, 2015, 08:27:24 amNo, I have another of the exact same meter which operates properly. The "B" indicates low battery voltage. It requires the circuit to be operating at a low level in order to turn the "B" on when the battery gets to a certain level; however, even the "B" will go out if the battery isdead.Quote from: electr_peter on March 11, 2015, 10:39:15 pmYou need to get in touch with the theme of this forum and blog -"Don't turn it on - take it apart"Besides - "If it is not broken, don't mess with it" isn't a philosophy I've ever held, I don't care if what I take apart breaks in the process, I'll just fix what breaks and learn as I go. But thanks for thinking about me.

author: Excavatoree
author_rank: Frequent Contributor
timestamp: March 12, 2015, 03:55:06 pm
content: I'm just glad someone is getting some use out of it. Tear-down, repair, whatever, it's better than the poor thing setting in a box on a shelf.I will add that it's been like that since I got it. At first, I thought "it just has a bad battery in it." Then "well, maybe there's residual charge." When it was still like that after years of setting with no battery, I figured it was some strange defect with the LCD, but I've never seen one fail like that. Most will get the "black Blobs" or a segment will be unresponsive.When I checked it, years ago, I wrote "intermittent flaky readings" or something like that on the meter in water based marker.

author: electr_peter
author_rank: Unknown
timestamp: March 12, 2015, 08:59:14 pm
content: Quote from: xrunner on March 12, 2015, 12:50:24 pmIf you have luxury to use it foreducation/hobby/fun/etc. purposes, have a good luck with repair and update us with more pictures.

author: xrunner
author_rank: Super Contributor
timestamp: March 12, 2015, 10:22:41 pm
content: Quote from: Excavatoree on March 12, 2015, 03:55:06 pmWell here it is, can't go any further. I can press around the "B" and make it go out, but it comes back shortly. It indicates that the LCD does have a defect regarding the "B". So I'll put it back together and clean up the meter. If I see any "for parts" 467's on Ebay I'll get one and swap out the LCD.

author: ModemHead
author_rank: Frequent Contributor
timestamp: March 13, 2015, 12:36:03 am
content: You may want to scope out the "B" line and see if it is stuck. Could be the root cause of the damaged LCD segment?