Fluke 77 iv - erratic behaviour - Page 1
Forum section: electronics / repair

author: cardosoluis
author_rank: Newbie
timestamp: November 21, 2024, 03:57:40 pm
content: Hello everyone,i bought a fluke 77 iv for parts/repair, and i thought it would be a fun and simple project to repair since in the pictures it look like it was working ok, only strange part was that in the volts selection, would show ohms, a few days later and i have it with me, and the multimeter has a really weird behavior.when i turn it on, despite what measure i chose, i always get the same screen in ohms and it beeps randomly, like i was measuring continuity, after taking it apart i saw that the previous owner soldered a wire in the fuses (probably that's why it got in this state).Visualy i don't see anything wrong apart from that, if someone has any ideia of what the problem could be, i would apreciate your help!i'll attach some pictures of the meter

author: cardosoluis
author_rank: Newbie
timestamp: November 21, 2024, 04:00:39 pm
content: Forgot to mention, the bridge rectifier looks like was resoldered, also the battery connector doesn't look oem.also added pictures of the meter running and a video:https://youtube.com/shorts/DXS7XyqXIfE?si=lsCTkQ1AN_zlttl2

author: lowimpedance
author_rank: Super Contributor
timestamp: November 21, 2024, 10:26:12 pm
content: Are the switch contacts still all there and in correct positions, (in the function switch in top case half!), given this unit has been worked on before you ?You may find the below thread useful.https://www.eevblog.com/forum/repair/fluke-77-repair-184173/good luck

author: cardosoluis
author_rank: Newbie
timestamp: November 22, 2024, 11:36:58 am
content: yes they are, i saw a video where one of the contacts was broken so was my first check, i already saw that thread and tested all that, and its fine on mine.i ordered a new bridge rectifier to test, lets see.Thanks!

author: cardosoluis
author_rank: Newbie
timestamp: November 26, 2024, 08:11:20 am
content: i'm guessing the issue is not with the bridge rectifier, i removed mine from the board, and the strange behaviour persists, if anyone has any idea please let me know

author: lowimpedance
author_rank: Super Contributor
timestamp: November 26, 2024, 10:57:42 pm
content: Quote from: cardosoluis on November 26, 2024, 08:11:20 amOne thought is a circuit that is used to select the appropriate function might be the fault ?! ?..Referencing the suggestion from that other thread I linked with the vague possibility that the 87V might have some similarities, that meter uses a resistor tree to select via the main switch a voltage that's read by the micro to determine the switch position. Looks to use the same MCU !. Worth checking out the 87V circuit.Do the soft buttons do anything sensible, back light for example?Really the best way is the 77iv/17X series service manualPerhaps PM @mzacharias to see if he can email the schematic to you, as I suspect its going to be rather a tall order to potentially fix this meter with out it.

author: Ugur
author_rank: Regular Contributor
timestamp: November 27, 2024, 05:34:21 am
content: I suggest you to check your regulated voltages, if all are good, most probably SL10327 IC is faulty.

author: cardosoluis
author_rank: Newbie
timestamp: November 28, 2024, 08:17:30 am
content: Will have a better look today!The buttons all work/do something, the backlight works also