HP 3476B Multimeter LED Segment fails - Page 1
Forum section: electronics / repair

author: ra-ma
author_rank: Contributor
timestamp: January 31, 2018, 07:30:49 pm
content: Hello, ich just purchased an used HP 3476B Multimeter. What I already knew is that one segment of an LED digit is failing. If I read the schematic correctly it's segment "e" of digit "4". Unfortunately I'm pretty new in electronics and can't interpret the schematic completely.Can anyone give me a hint which transistors/what else to check and how to measure in order to get to the root cause? Un- and re-plugging of the display made no change, it seems it's not an intermittent contact.Link to complete manual:http://www.hparchive.com/Manuals/HP-3476B-Manual-Operating-Service.pdf

author: The Soulman
author_rank: Super Contributor
timestamp: January 31, 2018, 07:50:33 pm
content: Yeah that schematic isn't the best when you are new to this, e 4 is correct.The top row of transistors control the segments of all digits at the same time, the bottom row controls the corresponding digit.Digits are turned on one at a time, and sequenced (multiplexed) fast enough so it looks like they all are on at the same time.Given the fact that all other digit are ok it is unlikely that the problem is with the driving circuitry, probably a failedsegment led or a bad solder joint.

author: ra-ma
author_rank: Contributor
timestamp: January 31, 2018, 09:01:34 pm
content: Thanks a lot for the easy to understand explanation of the way how the display is driven. As in general the digit 4 and the segment e work a defect in the LED itself seems to be the issue. I resoldered the LED connector on the board but no change. Is there any way to repair a single LED segment?Here a picture from this thread which I just found.https://www.eevblog.com/forum/testgear/hp-3476a-dmm-repairrefurb/msg626331/#msg626331

author: The Soulman
author_rank: Super Contributor
timestamp: January 31, 2018, 09:45:26 pm
content: Quote from: ra-ma on January 31, 2018, 09:01:34 pmSadly not.However looking at the pictures, it would probably be doable to build a complete plug-in replacement display module with common parts.It depends a bit on the space available.You'd learn about mechanical design/fabrication, pin spacing, common cathode/anode, forward voltage, soldering and much more.And if all goes well a very rewording and useful result.

author: ra-ma
author_rank: Contributor
timestamp: February 01, 2018, 06:48:44 pm
content: Actually a great idea! And if it doesn't work I'll at least have still the current display as it's connected via the pin-socket and easy to change.Before I start looking for possibly fitting parts: If I read the schematic correctly, I need to look for common cathode LED as this is then the connection of all segments to the collector of the transistor which steers the actual digit, is it?

author: lowimpedance
author_rank: Super Contributor
timestamp: February 01, 2018, 11:40:24 pm
content: Quote from: ra-ma on February 01, 2018, 06:48:44 pmLooks like it. If you do build up a replacement do share some pictures of the new display.Have fun.

author: The Soulman
author_rank: Super Contributor
timestamp: February 01, 2018, 11:48:31 pm
content: Quote from: ra-ma on February 01, 2018, 06:48:44 pmCorrect.

author: ra-ma
author_rank: Contributor
timestamp: March 14, 2018, 08:53:54 pm
content: Hi Community! After a couple of weeks I finally got my self designed replacement PCBs and started to solder the replacement board. As LEDs I'm using Lite-On LTS-2301AP (Datasheet:https://media.digikey.com/pdf/Data%20Sheets/Lite-On%20PDFs/LTS-2301AP.pdf). From a mechanical point of view it fits and looks nice, unfortunately it doesn't display what it should. It displays "P. 8. 8." whereby digits 2 and 4 are almost off. The original display still works as before. Does anybody have a hint for me what to check first?

author: floobydust
author_rank: Super Contributor
timestamp: March 15, 2018, 03:07:31 am
content: This sounds frustrating, after all that work.Pins 3, 8 on the LTS-2301AP LED display are shorted together internally (for common cathode) but I see tracesanda connector to it, so maybe LED pinout is wrong?I would double-check the new display board pinout with a multimeter on diode-test, on the bench.Maybe check if the LED pins are shorting to the main pcb, it looks very tight.Code:b seg.P1-1g seg.P1-2e seg.P1-3N/CP1-4CC digit 1 leftP1-5CC digit 2P1-6CC digit 3P1-7CC digit 4P1-8CC digit 5 rightP1-9N/CP1-10dpP1-11d seg.P1-12c seg.P1-13a seg.P1-14f seg.P1-15

author: ra-ma
author_rank: Contributor
timestamp: March 15, 2018, 04:24:34 am
content: Quote from: floobydust on March 15, 2018, 03:07:31 amYou are right, there is a connection between the common ground pins. I will test to not connect one of the pins tonight and report.Besides that I checked all connections in the board before soldering and that was ok. So maybe really the fault is in the design.Thank you, I will let you know.Gesendet von meinem Moto G (4) mit Tapatalk

author: ra-ma
author_rank: Contributor
timestamp: March 17, 2018, 09:04:32 pm
content: Hello, good news. I was actually finding two issues: a) connection between common ground pins b) connection between a pin/solder and a trace which was not covered completely by solder stop as my design was potentially a bit too tight (first PCB I designed).Luckily I got 3 of the PCBs therefore I cut the connection between the common ground pins on the second board, relocated all LED to the second board and was very careful soldering not to touch any traces again.And, now it works just fine! Next step is to calibrate it and then to have fun! :-)Thanks for all your support, having such an international community helping each other is great!