[SOLVED] - Fluke 79 III handheld DMM - Dead - Page 1
Forum section: electronics / repair

author: shakalnokturn
author_rank: Super Contributor
timestamp: November 03, 2019, 06:51:59 pm
content: I've just started trying to fix a Fluke 79 III TRMS multimeter.When I received it the function switch was desoldered, I checked contacts before resoldering. No short on battery terminals but no display either.Can anyone confirm I have the switch in the right way as it is possible to get it wrong by 180°.No service data available for this model, well at least I was unable to find any.Can anyone point me to other models using the "Fluke CLASSIC 0006" single ASIC solution?

author: retiredcaps
author_rank: Super Contributor
timestamp: November 04, 2019, 01:01:59 am
content: Quote from: shakalnokturn on November 03, 2019, 06:51:59 pmFluke 26 III = Fluke 79 III.Using modemhead's pictures, you have the switch installed incorrectly. Seehttp://mrmodemhead.com/blog/fluke-26-iii-corrosion-repair/

author: shakalnokturn
author_rank: Super Contributor
timestamp: November 04, 2019, 09:37:06 am
content: Thanks for that retiredcaps, I actually found it before reading your post. (Thanks for ModemHead for going to the trouble of bloging)So, yes although it wsn't soldered when I received it the switch had been placed with the wrong orientation so no joy. How sneaky...Once back in the right way the display came back up, during my first tests I was experiencing bad power with switch on VAC position, due to the contact "10" used for power switch being too loose.More worrying, ohms was reading negative overload or negative values depending on range, DCV was working but jumping all over the place when left open.No obvious PCB contamination so I went to measure the at the V/Ohm input, that was reading >40M.The fluke 29/79 II schematic has many similarities including: Function switch, input signal conditioning, ASIC. A couple of ICM7555 timers, an AD737 have been added between versions II and III, and of course the whole PCB layout has changed.Going by ModemHead's blog I'll assume that the ASIC CLASSIC 0006 is equivalent to part number 103728 that also appears to have been used as far back as the model 76.Anyway I had at least a partial schematic to go by so started testing continuity from both ends of the 9.996Mthick film resistor (also checked resistor itself of course). All that measured as expected from input on one side and to ASIC's pin99 on the other.By now I was starting to believe the ASIC was at least partially shot but still wanted to troubleshoot further by measuring at pin99 with DMM powered. The function switch needed removing once more. After bending the contacts enough to get them to make a clean contact on VAC, the switch was re-fitted.Surprisingly the previous symptoms of 10M resistor being out of circuit had disappeared, DMM was working normally... (Bugger!)First thought was that maybe power glitches due to bad switch had previously troubled the ASIC, I've also been caught out more than once testing for continuity or voltages right at IC pins. Applying pressure when probing is sometimes enough to press a broken solder joint together, and when this happens you're never looking at the defective equipment to see that it is working when you make that precise measurement...The ASIC got a full reflow just in case... Unless it's something intermittent the DMM is since working normally.

author: shakalnokturn
author_rank: Super Contributor
timestamp: November 04, 2019, 10:51:47 am
content: Currently checking calibration...VAC needed touching-up a little, otherwise

author: shakalnokturn
author_rank: Super Contributor
timestamp: November 05, 2019, 09:27:51 pm
content: Edit 2 posts up:Was previously "26/79 II" corrected to "29/79 II", service manual is around online.I realised my mistake while sorting recently downloaded PDF's.Attached ASIC pinout extracted from it.