ohms issue with a Fluke 87 - solved

[istovit]

my Fluke 87 won't read resistors under 6 ohms,shorting the leads
shows 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 checked
a car batteries voltage while in ohms mode. everything else seems ok.
its an older model 87 before they had battery doors.  TIA

[BrokenYugo]

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.

[Fungus]

I recently checked a car batteries voltage while in ohms mode.

That wouldn't have done anything bad.

To me it sounds more like a problem with the leads.

[bdunham7]

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.

[floobydust]

With 12V on ohms it would not damage anything. 400ohms range test current should be 0.700mA
It might even be a dirty rotary switch adding the 6 ohms.

[istovit]

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.

last 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

[BrokenYugo]

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.

last 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

Like I said, magic. My wild ass guess is something in the chip taking a charge and slowly bleeding off.

[J-R]

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.

[Fungus]

Like I said, magic. My wild ass guess is something in the chip taking a charge and slowly bleeding off.

I 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.

[istovit]

Like I said, magic. My wild ass guess is something in the chip taking a charge and slowly bleeding off.

I 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.

today'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 being
only a few ohms,and if its the case here eventually fading back to normal.

[LazyJack]

Check the input jack where it is soldered to the pcb. On older 87s the solder or the jack can crack.

[Fungus]

this may happen more often than we think and could  go unnoticed being only a few ohms

Nope. 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.

[bdunham7]

Nope. 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.

I 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?

[AVGresponding]

I wonder if it is possible for a cracked solder joint to creep back together given enough time in a desk drawer?

Probably, through the formation of tin whiskers.

[istovit]

just finished cleaning the rotary switch and were still between 2-3 ohms with leads crossed
1003 ohms at the ma. input and 2.3 at the 10a input. with the delta function I was able to  read
a 2 ohm resistor. I could live with this but would rather not,what protection components are
involved when a voltage is applied in ohms mode?   TIA

[kcbrown]

just finished cleaning the rotary switch and were still between 2-3 ohms with leads crossed
1003 ohms at the ma. input and 2.3 at the 10a input. with the delta function I was able to  read
a 2 ohm resistor. I could live with this but would rather not,what protection components are
involved when a voltage is applied in ohms mode?   TIA

At 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.

[robert.rozee]

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 with no single strand being 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   :-)

[LaurentR]

sighs... it sounds a bit like you've stretched one or both of the multimeter's leads.

I 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.

[kcbrown]

last 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

If you've got another meter then you should also have another set of leads.  Is this a problem that occurs with all of 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 resistance and that 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.

[istovit]

the leads read about .5 ohms in other meters but I did notice  RT1 was replaced
by flux around the pads. it reads 1536 ohms. R2 is 917k,almost 8% high. I'll put something
together to sub it and see what happens.

[istovit]

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.
 

[bdunham7]

R2 is 917k,almost 8% high.....
I just replaced r2 and shorted leads now measure .3 ohms.

I 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.

[istovit]

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.

I think so too. it measured .3 ohms a few minutes after soldering r2,today its just over 2.
heating up the area could have made the improvement. back to the bench.

[istovit]

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.