Fluke 87V - Faulty ohms range

[mazeemm781]

I hope everyone is doing great!!

I have a fluke meter (which I was given for free). This meter works fine in all the ranges (including continuity and capacitance ranges) except for Ohms. When I touch the leads, the meter starts from a random reading and then starts to go down.

I have tried to trouble shoot (from the input protection side) but sadly I did not get any lucky. All the movs and resistors seem fine to me.

And, I am aware that fluke offers lifetime warranty, but I believe that is limited to only the first owner of the product. And I can not get my meter repaired by Fluke in my country.

Here is the video on Imgur: https://imgur.com/a/0XUqAZn

[bdunham7]

Do you have another DMM handy?

Here's the manual and schematic:

[mazeemm781]

Yes I have another DMM.

[rsjsouza]

I would inspect all parts that comprise the ohms path - I would first look for signs of grime and residue and thoroughly clean the board. I would then check the input jacks for any signs of bad contact or residue.

After that, I would go to look for the parts. Are you sure the varistors RV2 and RV3 are in good shape? You can lift one of their legs and see if the issue disappears. Also, does ohms read higher resistances accurately? If not, I wonder if C3 is leaking or even Z1 might be faulty.

[bdunham7]

With your other meter measure the current output of the 87V in ohms mode (should be 1mA) and the open-circuit voltage of the same (should be about 7-ish volts).

If that is OK and if the DCV ranges work properly I think your problem is most likely right in the jack area.  The current source and voltage measurement circuits are independent except for the parts right near the jack. 

[mazeemm781]

I have measured the current output of Fluke 87V with my other meter and it is 1mA. The open-circuit voltage is 2.58 volts in ohms mode and 7.36 volts in continuity mode. The DCV ranges work fine as well and the meter measures everything within its spec.

I have cleaned the jacks and checked the input protection (which seems fine to me), but this did not fix the problem.

[mazeemm781]

I would inspect all parts that comprise the ohms path - I would first look for signs of grime and residue and thoroughly clean the board. I would then check the input jacks for any signs of bad contact or residue.

After that, I would go to look for the parts. Are you sure the varistors RV2 and RV3 are in good shape? You can lift one of their legs and see if the issue disappears. Also, does ohms read higher resistances accurately? If not, I wonder if C3 is leaking or even Z1 might be faulty.

I checked RV2 and RV3 and they look fine to me. I did not remove them to check the multimeter but will do so today.

Reading 10M ohm is fine. The multimeter does not drop down but when reading 560k Ohm, the multimeter behave the same way and starts dropping down.

[bdunham7]

I have measured the current output of Fluke 87V with my other meter and it is 1mA. The open-circuit voltage is 2.58 volts in ohms mode and 7.36 volts in continuity mode. The DCV ranges work fine as well and the meter measures everything within its spec.

I have cleaned the jacks and checked the input protection (which seems fine to me), but this did not fix the problem.

Next step would be to measure the voltage from TP9 to TP10 in the ohms mode with the leads open and then shorted.  I don't have an 87V so perhaps someone who does can chime in and confirm that the OCV in ohms mode is correct.

[mazeemm781]

Next step would be to measure the voltage from TP9 to TP10 in the ohms mode with the leads open and then shorted.  I don't have an 87V so perhaps someone who does can chime in and confirm that the OCV in ohms mode is correct.

Measuring voltage from TP9 to TP10 with leads open = 2.48 volts
Measuring voltage from TP9 to TP10 with leads shorted = 16.6 milli volts (and dropping by .1 milli volt every second) This reading is now stable at 15 milli volts.

[bdunham7]

Measuring voltage from TP9 to TP10 with leads shorted = 16.6 milli volts (and dropping by .1 milli volt every second) This reading is now stable at 15 milli volts.

And what does the 87V display read?  If you see 15mV, I would expect it to read 15 ohms.

[mazeemm781]

And what does the 87V display read?  If you see 15mV, I would expect it to read 15 ohms.

Yes, the voltage readings from TP9 and TP10 are same as the ohm readings shown on the fluke 87V.

[bdunham7]

There has to be a bad connection somewhere there.  Are you sure the test leads are good?  It looks like the jack assembly has been resoldered at some point.  Try soldering in a wire on the backside of the board from the V/OHMS jack to the COMMON. 

[mazeemm781]

Yes the test leads are good. I have tested the test leads on other multimeters and I have also replaced the leads.

I already tried soldering the wire to V/Ohms and COM but sadly I got the same results.

I think there is something wrong with the protection circuitry but I can not fully diagnose this on my own.

[bdunham7]

Yes the test leads are good. I have tested the test leads on other multimeters and I have also replaced the leads.

I already tried soldering the wire to V/Ohms and COM but sadly I got the same results.

I think there is something wrong with the protection circuitry but I can not fully diagnose this on my own.

The protection circuitry would cause the sensed voltage to be low, not high.  If the current is correct, the only thing that should cause high sensed voltage is some sort of resistance in the path of the circuit, or possibly leakage from the battery circuit.  If you already tried soldering a wire across the V/OHM and COM connections, then the next thing would be do desolder and remove the jack assembly so you have access to the battery wires and connections.

[shakalnokturn]

Service manual Table 11 gives you the differences in path between measuring >6MΩ (working) and <6MΩ (wonky).
The difference that shouts out at me is the switch contacts, I don't think they've been mentioned, have they been checked/cleaned? It's usually one of the first things to look at before even thinking.

[mazeemm781]

So here is where I am at currently.

I have checked and cleaned the the switch with IPA and everything seems fine in there.
I have removed the MOVs but the problem is still there so I just re-soldered them.
I have checked the jacks and cleaned + re-soldered them.

I found that when shorting COM to R1's right lead, the Ohm reads less than when I short through V/Ohms jack.

Still, I am scratching my head.

[bdunham7]

I found that when shorting COM to R1's right lead, the Ohm reads less than when I short through V/Ohms jack.

I'd expect that in this case.  Try this:

Install two test leads in the V and COM jacks.  Remove the battery from the meter and turn the switch to off.  Use your other DMM to measure the resistance from both sides of RT1 to the end of the positive (V) lead and then from the emitter of Q3 (the 'common' designation) to the end of the negative (COM) lead. 

Also you mentioned some issues in the higher resistance ranges, can you test with some resistors 100 to 10M ohms?  Just compare the results with your other meter.

[mazeemm781]

I would surely do this in the morning and report the results here.

Is there any chance that a bad calibration would be causing this issue or this is purely hardware related?

And I am very thankful to bdunham7 for helping me out with this. 

[hunter_sk]

I have similar problem with my Fluke 179 described here:
https://www.eevblog.com/forum/repair/fluke-179-off-readings-(vdc-ohms)/

Open ohms range shows about 30M instead OL and shorted ~30 ohms and dropping to ~1 ohms

[adam_mizer]

Thank you bdunham7 for posting schematic.
Repaired my 87V simple with schematic, under a dollar for part.
Problem applied over 1250+ DCV in my industry thats easy, by mistake.
Then meter showed when ohms selected 442k ohms when diode seltcted showed 0.775vdc
Fix zener diode Z2 failed or should I say protected the circuitry. EDIT; OOOOPS, VR1 was replaced.
Now back to normal.

Thank you.

[Hamelec]

zener diode??
according circuit diagram Z2 is a 1M resistor..?

[adam_mizer]

Thanks Hamelec, newbie here wrote to quickly thinking Z2 was a zener diode. VR1 saved the circuit for me.