FLUKE 87-IV Really Strange Readings Negative Ohms Without measuring

[UsedFlukeStrange]

Hi everyone,

I am writing you because I bought one week ago a used FLUKE 87-IV, its my first fluke but i already started to regret it  , it had a blown 440ma fuse, so i replaced it, problem is that now if i touch the two probes together in Ohm reading i have negative ohm on display, minus 2-3 ohms, and it goes even under that if i let the probes together for much more time,

even in mV DC position i have negative readings from -10mvDC  to -60mvDC, i understand that Autorange Multimeters haven't fixed values on screen while not measuring, but is it normal to go only on Negative DC while not measuring anything, Does it need Calibration or does it have some bad components to replace, if so i can replace them.

So I have already cleaned the pcb with IPA, already cleaned the connector sockets also, desoldered and resoldered it in place, checked the Fusible Resistor of 1Kohm in series and its value is correct,

Sending it to a Fluke service center is not really a possibility as here in Belgium there arent any,and sending it internationally will cost me more than the price that i payed for it, which is 100 euros

I will post some photos to you guys, please help me as i currently have no idea of where to look at 

Also i didnt find any schematics for this multimeter, on Fluke website I only found a user manual and a calibration pdf for servicing

PS. I have brand new batteries inside

PSS I have started going crazy, I thought of the emf maybe being the problem due to old components being more vulnerable to them, and i have just wrapped the multimeter in Aluminium sheets, and twisted the two probes togheter, with no  positive change whatsoever

[MosherIV]

Hi

Welcome to the forum.
Nice score on the dmm.

Sorry it does not work properly.

Does the meter read dc volts properly?
Does it read resistor values roughly correct?

If so, yes it is likely that it needs a calibration.

Can you measure the input impeadance (put it on volts mode and measure the resistance of the meter with another dmm)?

[UsedFlukeStrange]

Hi, thank you for your reply i appreciate it 

I just measured the Impeadence with my other chinese dmm, I measured 1.050 Mohms

For the Voltage measurement on a aa battery that i had on my bench i had 1,337V measured in Dc V on the fluke vs 1.320 V on my Chinese DMM

I am currently looking for a resistor on my bench i will add the measurements when i find one  thank you very much

[UsedFlukeStrange]

Just measured a 4 ohm resistor, measured 4ohms on my chinese dmm while measuring 2 ohms on my fluke 87-iv

[MosherIV]

I measured 1.050 Mohms

That is too low. It should bt 10Meg

1M means that it will measure the battery volts ok. In circiut, it will depend on the resistance in the circuit.

From your description, that the 440mA fuse blown, sounds like the input protection circuit is damaged in some minor way. It might be as simple as the MOVs are blown/leaky, but this usually causes the volt readings to be way off. However, the 440mA fuse and MOV are in 2 different parts of the circuit, one protects the current sense path and the MOV protects the volt/resistance path.

Hopefully, MrModemhead will come along soon and give more insight.

In the mean time, have a look at Dave's video on how dmm input protection works.

[MosherIV]

Just measured a 4 ohm resistor, measured 4ohms on my chinese dmm while measuring 2 ohms on my fluke 87-iv

That would be expected if the calibration is off.

Can you measure the volt drop of a standard silicone diode (eg 1n4148 or 1n4001) or volt drop of npn transistor between base and emitter?
It should read around 0.7V

This will give us an idea if the input is leaking or whether the constant current generator is off a bit.

[UsedFlukeStrange]

I really wish its just a reference resistor, Thank you very much MosherIV

 i will go see Dave videos right now, for the MOVs you talked about, i did check them, they all display OL readings, the fusible resistor reads 1kohm, but i have no idea of where to find the the 10 Mohm resistor

Thank you for the good advice about the readings 

[UsedFlukeStrange]

I finally found a silicone diode , it reads on my multi function tester TC1 a voltage drop of 0.680v and on the fluke a drop of 0.576V

I dont know if it could help, because i would not bet on the accuracy of the TC1 and my other dmm does not have a diode function

[UsedFlukeStrange]

the 2 RED movs read all OL,

The PTC read 1k

 the fusible resitor read 1k

The 2 brown resistor read all 1 M   so they are ok with their Value

the black resistor read 1ohm as written on it so it is ok,

The 3 green resistor of 100 k all read around that value +1 -2 k

the smd resistor with written 102 on it reads 1 k so it should be good

[UsedFlukeStrange]

others pics

[mzacharias]

Does the 87IV have the deposited carbon track switch on the rear of the board like the earlier model 87's, or use it for the main switch?

If so clean the tracks; carbon particles have been known to cause similar problems.

[UsedFlukeStrange]

on the rear it has one button with carbon track but it is for the calibration, , i took off the button, and tested the dmm, still having those weird readings on it, the pcb had already been cleaned with isopropyl alcool the tracks also

[retiredcaps]

I just measured the Impeadence with my other chinese dmm, I measured 1.050 Mohms

On most Fluke meters, when it is set to DCV and turned on, input impedance should be around 11.11M ohm using another working meter.

[giosif]

Although very unlikely this is the problem, but maybe check the white capacitor and the area around it.
In one of my repair attempts, there was some carbon deposit around a capacitor (most likely the effect of arcing) that was affecting the input impedance.
https://www.eevblog.com/forum/repair/fluke-83-multimeter-repair-attempt/msg828142/#msg828142

Also, is it just me or the Amps fuse looks a bit burned on one side (effects of arcing maybe)?

[UsedFlukeStrange]

I am looking forward on this, but how does that resistance add up as i dont have any typical 10 M resistor in there, from what i am seeing, does anyone know where is it located, from what component this resistance comes?

[UsedFlukeStrange]

Hi goisif,
I just checked no leakeage to be seen,, i might desolder it for further inspection if it can help, and for the 11 amp fuse, yes you really have a good eagle sight, it is indeed lightly burnt , but just the exterior plastic around it, thats my fault , i was desoldering the connectors socket with hot air and accidentaly touched the fuse 

[shakalnokturn]

Contamination (including battery electrolyte) is a sensible thing to look for.
Consider that the input banana sockets could be a cause.

For comparison here's what I get on my Fluke 187 once "warm":

mV range, leads shorted: -8µV (was more like +10µV powering up.)
leads fed to another meter on mV range: both read in the -100µV area (this could help in discovering if the problem is the Fluke making wrong measurements or contamination somewhere creating a battery)
mV range input resistance is 9.990 Mohm (The resistor you're looking for is the flat ceramic under the shielding...)
Ohm range test current (other DMM shunt resistor is 101ohms)
500ohm range: 1mA
5k ohm range: 100µA
50kohm range: 10µA
500kohm range: 1µA
5M, 50Mohm ranges: 100nA (abut as low as I can measure)
500Mohm range: jumps around between 3nA and 7µA

The first place I'd start looking for problems really is the banana sockets... Especially the V/ohm one having IIRC two different contacts in it.

[ModemHead]

leads fed to another meter on mV range: both read in the -100µV area (this could help in discovering if the problem is the Fluke making wrong measurements or contamination somewhere creating a battery)

Note to the OP, don't miss this bit of wisdom.

If there is leakage current (sounds likely), then the input impedance test reading of 1.050 Meg certainly indicates a problem, but the number itself may not be significant, because the leakage current will interfere with the working multimeter.

[daisizhou]

Please check the ADC chip pins for white contaminants.Including the 2 chips behind the display.Cleaning the DIP PCB circuit must be clean and immersed without actual effect