Fluke 123 not turning on - Page 1
Forum section: electronics / repair

author: larsstave
author_rank: Newbie
timestamp: December 21, 2020, 05:34:39 pm
content: I just got a fluke 123 that i am trying to throubleshoot.the old owner had rewired the dc plug and reversed the polarity. I have rewired it correctly, but i am only getting a steady «clicking sound» seems like a relay who opens and closes continioulsy. When i am coneccted both to dc input and the battery, it does turn on the lcd for a second or so. Just white light before it turns off again.Have anybody experienced this before?

author: larsstave
author_rank: Newbie
timestamp: December 21, 2020, 06:10:19 pm
content: I am able to boot it when i short between ground and TP531. That also stops the relay im hearing..

author: fordem
author_rank: Regular Contributor
timestamp: December 22, 2020, 01:34:45 am
content: Do you have the manuals that match that instrument? There are two or three different versions on the instrument, and the manuals document not powering on tests.

author: bdunham7
author_rank: Super Contributor
timestamp: December 22, 2020, 02:33:42 am
content: If you can, post a really good picture of the circuit board near the DC plug and battery connections. Inspect all of the larger FETs in that area for having holes blown in them and look for any low-value resistors that might be the current-sense type. Hopefully someone digs up a schematic.

author: fordem
author_rank: Regular Contributor
timestamp: December 22, 2020, 02:52:39 pm
content: Quote from: bdunham7 on December 22, 2020, 02:33:42 amFull schematics and parts listings are at the back of the service manuals - I have at least two different versions, a google search will find them.

author: bdunham7
author_rank: Super Contributor
timestamp: December 22, 2020, 03:14:39 pm
content: Quote from: fordem on December 22, 2020, 02:52:39 pmYep, I found one that had schematics.https://www.manualslib.com/manual/1701255/Fluke-Scopemeter-123.htmlOP: Check R501, it should be 0.1R. The inputs are polarity-protected by Schottky diodes V501 and V504.

author: larsstave
author_rank: Newbie
timestamp: December 22, 2020, 06:45:43 pm
content: Thanks for the input. After several hours of fault-searching it turned out to be R504, R506 and R507 which were defect.Probably burned out when the previous owner had switched the polarity.It is now 100% functioning.

author: larsstave
author_rank: Newbie
timestamp: December 22, 2020, 06:48:54 pm
content: And for later use.. Are most of the resistor kits on ebay okay for normal use? Especially thinking of different smd components.. Or is it better to buy from a specific brand?

author: bdunham7
author_rank: Super Contributor
timestamp: December 22, 2020, 07:41:16 pm
content: Quote from: larsstave on December 22, 2020, 06:45:43 pmGood...but which DC plug did the former owner reverse???

author: larsstave
author_rank: Newbie
timestamp: December 22, 2020, 10:54:54 pm
content: The DC plug for the charger was reversed.He has also changed the batterypack with new cells, so he might have switched up the polarity there as well.I changed the cells with new ones this evening, so there is no more problems there. Battery life seems to be good, but i might do a battery refresh tomorrow since that is recomended.