ohms issue with a Fluke 87 - solved - Page 1
Forum section: electronics / testgear

author: istovit
author_rank: Contributor
timestamp: August 15, 2021, 01:25:05 am
content: my Fluke 87 won't read resistors under 6 ohms,shorting the leadsshows about 6 ohms where before it was less then 1. probing the ma.input in ohms reads 1005 ohms and 6 at the 10a input. I recently checkeda car batteries voltage while in ohms mode. everything else seems ok.its an older model 87 before they had battery doors. TIA

author: BrokenYugo
author_rank: Super Contributor
timestamp: August 15, 2021, 01:51:56 am
content: I accidentally put like 100V through my Fluke 75 in ohms mode once and had the same problem. I put it in a drawer for about a year, and it magically works fine now.

author: Fungus
author_rank: Super Contributor
timestamp: August 15, 2021, 02:37:00 am
content: Quote from: istovit on August 15, 2021, 01:25:05 amThat wouldn't have done anything bad.To me it sounds more like a problem with the leads.

author: bdunham7
author_rank: Super Contributor
timestamp: August 15, 2021, 03:21:13 am
content: Does it read volts correctly? Were there any sparks when you misconnected it? Are the fuses good and of the correct type? (there's a reason I'm asking even though fuses aren't involved in ohms...)It seem unlikely that you would damage the meter in ohms as it is protected against that sort of thing, and it is even more unlikely that such damage would cause it to read 6 ohms high. Unless, of course, you actually had the leads in the 10A sockets with the switch on ohms mode and your 10A fuse has been replaced with a solid copper bar, then you might do some damage to the common socket, test lead or PCB where the current shunt is kelvin-connected.Assuming you have a plain old original Fluke 87, here's the relevant part of the schematic.

author: floobydust
author_rank: Super Contributor
timestamp: August 15, 2021, 03:43:46 am
content: With 12V on ohms it would not damage anything. 400ohms range test current should be 0.700mAIt might even be a dirty rotary switch adding the 6 ohms.

author: istovit
author_rank: Contributor
timestamp: August 16, 2021, 12:43:42 am
content: Quote from: BrokenYugo on August 15, 2021, 01:51:56 amlast tuesday it was 6 ohms,before I checked the leads on another meter today it was just over 4 and .3 on the other meter. if the same thing is happening here what could be doing that?thanks for the replies

author: BrokenYugo
author_rank: Super Contributor
timestamp: August 16, 2021, 01:29:19 am
content: Quote from: istovit on August 16, 2021, 12:43:42 amLike I said, magic. My wild ass guess is something in the chip taking a charge and slowly bleeding off.

author: J-R
author_rank: Super Contributor
timestamp: August 16, 2021, 01:32:15 am
content: My thoughts on this:Test each lead separately, so for example just the black lead plugged into COM and place tip into V, do same for red. Test all 4 possible combos to see if you can glean any info about a poor jack contact.Clean the jacks with contact cleaner.If you move the rotary switch back and forth on the Ohms position a bunch of times does the resistance go down any?Disassemble the meter and clean the rotary switch and the PCB rotary contacts with contact cleaner.

author: Fungus
author_rank: Super Contributor
timestamp: August 16, 2021, 05:57:31 pm
content: Quote from: BrokenYugo on August 16, 2021, 01:29:19 amI don't know every last detail of the internals but it seems incredibly unlikely. Other people would see it regularly, there would be a lot of complaints.My theory: If the leads check out when you wiggle them then look for some foreign body on the contacts of the range switch.

author: istovit
author_rank: Contributor
timestamp: August 17, 2021, 03:54:34 am
content: Quote from: Fungus on August 16, 2021, 05:57:31 pmtoday's reading is 3.2 ohms before and after swabbing the input jacks.I'll wait a little while before any disassembly if this continues.this may happen more often than we think and could go unnoticed beingonly a few ohms,and if its the case here eventually fading back to normal.

author: LazyJack
author_rank: Frequent Contributor
timestamp: August 17, 2021, 06:02:29 am
content: Check the input jack where it is soldered to the pcb. On older 87s the solder or the jack can crack.

author: Fungus
author_rank: Super Contributor
timestamp: August 17, 2021, 08:17:11 am
content: Quote from: istovit on August 17, 2021, 03:54:34 amNope. Not around here.This is a bad contact, bad lead or bad solder joint in the input jack. Wiggle the leads, press on the input jacks, see of it varies.

author: bdunham7
author_rank: Super Contributor
timestamp: August 17, 2021, 03:52:31 pm
content: Quote from: Fungus on August 17, 2021, 08:17:11 amI have had several meters, including bench models, with intermittent input connection problems and they can be devils to track down depending on where the issue is. I wonder if it is possible for a cracked solder joint to creep back together given enough time in a desk drawer?

author: AVGresponding
author_rank: Super Contributor
timestamp: August 17, 2021, 04:02:34 pm
content: Quote from: bdunham7 on August 17, 2021, 03:52:31 pmProbably, through the formation of tin whiskers.

author: istovit
author_rank: Contributor
timestamp: September 09, 2021, 01:14:24 am
content: just finished cleaning the rotary switch and were still between 2-3 ohms with leads crossed1003 ohms at the ma. input and 2.3 at the 10a input. with the delta function I was able to reada 2 ohm resistor. I could live with this but would rather not,what protection components areinvolved when a voltage is applied in ohms mode? TIA

author: kcbrown
author_rank: Frequent Contributor
timestamp: September 09, 2021, 01:58:27 am
content: Quote from: istovit on September 09, 2021, 01:14:24 amAt this point, it's time to take the meter apart.Use one of your other meters and start measuring resistances in the ohms path inside the Fluke. Because you're able to properly measure a 2 ohm resistor in delta mode, that means you probably don't have a scaling issue, but rather an offset issue.My money's on something like a cracked solder joint, so you should certainly at least examine the joints in the measurement path, and might consider reflowing them anyway even if you don't see anything.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: September 09, 2021, 01:38:35 pm
content: sighs... it sounds a bit like you've stretched one or both of the multimeter's leads.have you tried shorting together the multimeter's input terminals directly using a short piece of copper wire? if this reads (close to) zero ohms, then the problem is in your leads. do you have a spare set to try?if you stretch a multi-stranded length of cable (such as a multimeter lead) that has fine strands, various of the strands within the cable will snap. they will NOT all snap at the same place. the end result is that you can, after a few minor stretches of even fewer major stretches, end up with a length of cable withno single strandbeing connected at both ends. this will result in... well, i hope yo can imagine the result! volts will usually read fine, ohms will have a disconcerting offset, and amps will cause the damaged lead to get unexpectedly hot.cheers,rob :-)

author: LaurentR
author_rank: Frequent Contributor
timestamp: September 10, 2021, 05:20:29 am
content: Quote from: robert.rozee on September 09, 2021, 01:38:35 pmI have had a several Fluke and Pomona TL71 leads that suffered from this. They were retail-bought so probably genuine, but I was surprised to find that they were definitely conducting but had high resistance.

author: kcbrown
author_rank: Frequent Contributor
timestamp: September 12, 2021, 12:59:41 am
content: Quote from: istovit on August 16, 2021, 12:43:42 amIf you've got another meter then you should also have another set of leads. Is this a problem that occurs withallof your leads or just a single set? Obviously if it's with all of your leads then the problem is almost certainly in the meter. And if it follows the leads then the problem is with the leads.The single-lead test is also a good one to do. If you find that one of your leads generates a significantly higher resistance than the other then you have a faulty lead. If both of them generate the same resistanceandthat resistance is the same resistance you've been reading by shorting both leads together, then you've got a problem in the meter.If the problem is with your meter then you'll need to take it apart as I suggested previously.

author: istovit
author_rank: Contributor
timestamp: September 16, 2021, 06:18:43 pm
content: the leads read about .5 ohms in other meters but I did notice RT1 was replacedby flux around the pads. it reads 1536 ohms. R2 is 917k,almost 8% high. I'll put somethingtogether to sub it and see what happens.

author: istovit
author_rank: Contributor
timestamp: September 23, 2021, 10:40:20 pm
content: I just replaced r2 and shorted leads now measure .3 ohms.trouble is its 1/2 watt and can't find a vendor for 2 watts 1% in quantities less than 100.anyway its back in business,thanks for the replies.

author: bdunham7
author_rank: Super Contributor
timestamp: September 23, 2021, 10:50:10 pm
content: Quote from: istovit on September 23, 2021, 10:40:20 pmI think your math is off by about a factor of ten? (917k -909k (spec) = 8k, 8k/909k = .0088 or 0.88%, within spec.You might just put R2 back in and liberally reflow the solder in the area between R2 and the input jack because I'm pretty sure that is where your problem is/was.

author: istovit
author_rank: Contributor
timestamp: September 27, 2021, 03:33:22 am
content: Quote from: bdunham7 on September 23, 2021, 10:50:10 pm

author: istovit
author_rank: Contributor
timestamp: October 19, 2024, 09:27:04 pm
content: if you're still reading I just took mine out of a box its been in since then,installed a new battery,and got .4 ohms with the same probes. why does this happen? anyway the 77 is in that box now.