Fluke 77 III Resistance measurement fault - Page 1
Forum section: electronics / repair

author: bicyclerepairman
author_rank: Newbie
timestamp: September 16, 2019, 08:53:02 am
content: My Fluke 77 III has an issue with resistance measurement - when making a measurement, it displays some value slightly above the actual value, and then slowly reduces that value, approaching the actual resistance. For example, when it is shorted out, it displays 12 ohms, and after about a minute it's near 2 ohms. When I measured a 5k6 resistor, it started at 6k, and after about a minute it's near 5k6. Would appreciate any advice on this issue!

author: benj38
author_rank: Frequent Contributor
timestamp: September 16, 2019, 08:44:16 pm
content: Usually, DMMs push a constant current through the unknown resistance and then measure the DC voltage drop on it using essentially the same circuit used in the DCV mode. Hence, the first question is: do DC voltage measurements behave completely normal?Another thing I would check is if the added resistance indicated is constant when measuring different resistorswith the meter in the same range(i.e., not in auto-range).

author: shakalnokturn
author_rank: Super Contributor
timestamp: September 16, 2019, 09:34:33 pm
content: Even if added resistance is constant in value at one point it will be hard to compare as it isn't constant in time. Do check how DC mV range reads with leads shorted though.Before doing any serious troubleshooting you may want to first consider cleaning the input banana sockets, cleaning rotary switch, checking PCB for battery spew/corrosion.

author: retiredcaps
author_rank: Super Contributor
timestamp: September 17, 2019, 04:26:31 am
content: Quote from: shakalnokturn on September 16, 2019, 09:34:33 pmThis.A Fluke 77 III with shorted probes should show close to 0.2 ohms in less than 1 or 2 seconds.An IPA bath might also help? It helped me. Seehttps://www.eevblog.com/forum/chat/fluke-77-ii-incorrect-readings-on-all-ranges/

author: bicyclerepairman
author_rank: Newbie
timestamp: September 17, 2019, 07:29:49 am
content: Quote from: benj38 on September 16, 2019, 08:44:16 pmForgot to say - the voltage measurement doesn't seem to share this issue. The measured voltage stays constant from the moment the probes are applied.It seems the added resistance is in proportion to the scale in use. I didn't try it on manual mode, but measuring a short added about 12 ohms (initially), and measuring a 5k6 resistor added about 400 ohms.

author: bicyclerepairman
author_rank: Newbie
timestamp: September 17, 2019, 07:31:53 am
content: Quote from: shakalnokturn on September 16, 2019, 09:34:33 pmI'll double check, but I'm pretty sure I tried shorting the leads on mV, and it went to zero as expected.

author: shakalnokturn
author_rank: Super Contributor
timestamp: September 17, 2019, 10:51:49 am
content: Well,ifit is behaving on DCV, try noting resistances on each range for shorted leads then with a value close enough to full scale. (i.e: 270, 2k7, 27k...) We'll try to make some sense of that.

author: goaty
author_rank: Regular Contributor
timestamp: September 17, 2019, 10:57:44 am
content: Maby and try to measure the voltage across the resistor while it is being connected with an oscilloscope or different multimeter to see if it drifts. If so, the constant current regulation is faulty.

author: xmetal
author_rank: Contributor
timestamp: September 17, 2019, 12:22:18 pm
content: I had a Fluke 77 playing up on the resistance range. It turned out to be the test leads.

author: bicyclerepairman
author_rank: Newbie
timestamp: September 22, 2019, 01:03:43 pm
content: So a correction to start with - I looked closer, and I have a 73 III, not a 77 according to the thread title.I have also tried different leads, and cleaning the sockets, with no change.A battery has previously leaked a little inside, but it's been cleaned up and there's only some surface marking on one battery terminal.Anyway, I found some time to work on this suggestion:Quote from: shakalnokturn on September 17, 2019, 10:51:49 amI found the problem wasn't related to near full-scale values, but low resistance values (in all ranges). For the following results, the resulting value is the value displayed after 10 seconds.320ohm range - shorted leads - 5ohm320ohm range - 15ohm resistor - 19ohm320ohm range - 75ohm resistor - 80ohm320ohm range - 100ohm resistor - 105ohm320ohm range - 270ohm resistor - 274ohm3k2 range - shorted leads - 30ohm3k2 range - 15ohm resistor - 35ohm3k2 range - 75ohm resistor - 95ohm3k2 range - 100ohm resistor - 120ohm3k2 range - 470ohm resistor - 480ohm3k2 range - 2k7 resistor - 2k7320k range - shorted leads - 2k6320k range - 15ohm resistor - 1k3320k range - 75ohm resistor - 1k8320k range - 100ohm resistor - 1k1320k range - 270ohm resistor - 1k5320k range - 2k7 resistor - 4k320k range - 27k resistor - 29kI did try measuring the voltage across the probes with another multimeter while taking measurements, but couldn't make much sense of the results.

author: retiredcaps
author_rank: Super Contributor
timestamp: September 23, 2019, 04:39:20 am
content: Quote from: bicyclerepairman on September 22, 2019, 01:03:43 pmHow?

author: shakalnokturn
author_rank: Super Contributor
timestamp: September 23, 2019, 07:05:37 am
content: Fluke 23 II schematic should be very close to your 73 III.Your main culprits are the switch and R1 fuse resistor.If I'm reading the schematic right you should be measuring 1,001M\$\Omega\$ between V/ohm input and U1 pin 29, 1Mbetween V/ohm input and U1 pin 20 with switch on ohm position.Knowing how well the battery leakage has been cleaned is important, can you be sure there is nothing left under the switch?

author: bicyclerepairman
author_rank: Newbie
timestamp: September 30, 2019, 08:31:05 pm
content: I cleaned up the battery leak with IPA. There wasn't much at all - it's mostly surface corrosion to the body of the negative battery terminal, and a little damage to the negative trace. Pic:https://photos.app.goo.gl/DC5xc9Wo2J7p5LfGAI couldn't find a 23ii schematic online - if one is available, that would be very useful. I got 1.008MOhm from V/ohm input to pin 29, and 998kOhm to pin 20 (although my other multimeter is nowhere near as good as the Fluke...).Also, most of the components on the PCB are not labelled, so I'm not sure which is R1.

author: shakalnokturn
author_rank: Super Contributor
timestamp: September 30, 2019, 09:24:03 pm
content: http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Fluke/FLUKE 77, 75, 73, 70, 23, 21 Series II Service.pdfAt the worst U1 is defective, I recently saw the same fluke PCB as yours on evil bay selling with Ohm function not working...The good thing is this IC was used in many Flukes of the time, if you really want to save it you should be able to find a low-end model for parts.

author: shakalnokturn
author_rank: Super Contributor
timestamp: October 02, 2019, 09:43:07 pm
content: Edited bad SM link...

author: bicyclerepairman
author_rank: Newbie
timestamp: October 05, 2019, 02:40:16 pm
content: Thanks for your help and the schematic - I will see what I can do from here!