Fluke 87 III - current measurement and backlight problem - Page 1
Forum section: electronics / repair

author: cairn7
author_rank: Newbie
timestamp: September 08, 2021, 03:40:35 pm
content: Hi all,I bought a Fluke 87III just to find out that it doesn't work properly,Multimeter's in good condition, mV, V (AC/DC), continuity, diode test, buzzer, resistance - no problem at all with these ones.The problem is with:1). Current measurement doesn't work properly (same issue for the AC and DC range).For example, I tested the lamp and other two Fluke 87V units showed proper value at 47mA, this 87III unit showed 1,05 mA (stable 1,05mA but incorrect value),What I've checked:- When this unit arrived, both fuses were blown and both were not the original ones, I replaced them with the new ones.- I checked the fusible resistor, seems to be okay (1k).- Replaced the battery.- Cleaned the rotary switch.- Visually checked whole PCB from the both sides, everything seems to be fine,2) Backlight:This function doesn't work at all.I tested the diode under the display and it's also okay,Any suggestions would be much appreciated, I can take some photos of the PCB if needed.

author: cairn7
author_rank: Newbie
timestamp: September 09, 2021, 01:16:14 pm
content: I did some more tests this morning,mA/A issue:Tested the shunt resistance, seems to be fine (for A and uA),Tested the bridge rectifier with the diode - both components are shorted, I need to order replacements (DF06S + VISHAY S1D), hopefully only these two components needs to be replaced.Backlight issue:MMBT3904 transistor is damagedUPDATE:When I was looking for a backlight issue - I've checked three transistors + few resistors (around the backlight connector) and compared resistance values from good and bad 87III unit, now backlight doesn't work on good 87III unit, how could I damage any of these components only with a simple resistance test? I tested all of the components with a Fluke 87V, battery was disconnected.

author: m3vuv
author_rank: Super Contributor
timestamp: September 09, 2021, 08:24:16 pm
content: how come your english grammar is so bad,are you a native?,or an import??

author: cairn7
author_rank: Newbie
timestamp: September 09, 2021, 09:16:48 pm
content: None of the above "mate", English is not my first language, not even second... hope you can speak 3 languages perfectly. Cheers.

author: james_s
author_rank: Super Contributor
timestamp: September 09, 2021, 09:25:51 pm
content: Why is that even important? Not everyone is a native English speaker, so what, I could understand him just fine. It is not remotely helpful to chime in just to complain about a person's grammar.Anyway, unfortunately I do not know what's wrong with the meter or I'd be happy to assist. Does Fluke offer a schematic for the 87? I've never had a problem with mine so I've never had a reason to look.

author: srmahaffy
author_rank: Contributor
timestamp: September 11, 2021, 04:31:47 pm
content: Hi Cairn,I wanted to share a couple ideas on your backlight problem of your 87iii. I purchased an 87iii meter a few months ago and it had a backlight problem. It turned out the root cause was the little 2 pin ribbon cable wasn’t pushed down all the way to the J2 connector. Before I discovered that, I did the following steps to troubleshoot:With the battery disconnected:- did resistance check of Q5 and Q7.- continuity check from Q5 emitter to J2-1 mating ribbon cable- tested the LED by probing across the two pins of J2 using the diode mode of another meter, to make sure the LED lights up. ( you said yours is good)With the battery connected:- looked at voltages of Q5 and Q7 voltage levels. from my notes. VSS = -3.2V, VBT- at Q5 collector = 5.5V. VBT+ at J2-2 = 3.5. I verified/ checked to see if voltage state changes at J2-1 and Q5 emitter when shorting across backlight push button pads. With J2-2 (VBT+) being 3.5, J2-1 needs to switch to a low voltage to make the LED turn on.I hope your ribbon cable was just not pushed on all the way like mine, and also with your other meter!I attached part of the schematic.

author: srmahaffy
author_rank: Contributor
timestamp: September 11, 2021, 05:09:19 pm
content: Here’s a pic of voltage level information from the 83, 85, 87 service manual.

author: james_s
author_rank: Super Contributor
timestamp: September 11, 2021, 05:42:44 pm
content: That's a strange arrangement, why are they using a PNP transistor as a low side switch? I don't think I've ever seen it done like that before.

author: bdunham7
author_rank: Super Contributor
timestamp: September 11, 2021, 05:48:38 pm
content: Quote from: james_s on September 11, 2021, 05:42:44 pmVBT- is 3-4 volts lower than VSS, if that explains it. I don't know if it does, I didn't try to work it out...