Fluke 77 III Resistance measurement fault

[bicyclerepairman]

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!

[benj38]

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 resistors with the meter in the same range (i.e., not in auto-range).

[shakalnokturn]

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.

[retiredcaps]

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.

This.

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.  See

https://www.eevblog.com/forum/chat/fluke-77-ii-incorrect-readings-on-all-ranges/

[bicyclerepairman]

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 resistors with the meter in the same range (i.e., not in auto-range).

Forgot 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.

[bicyclerepairman]

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.

I'll double check, but I'm pretty sure I tried shorting the leads on mV, and it went to zero as expected.

[shakalnokturn]

Well, if it 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.

[goaty]

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.

[xmetal]

I had a Fluke 77 playing up on the resistance range. It turned out to be the test leads.

[bicyclerepairman]

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:

Well, if it 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.

I 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 - 5ohm
320ohm range - 15ohm resistor - 19ohm
320ohm range - 75ohm resistor - 80ohm
320ohm range - 100ohm resistor - 105ohm
320ohm range - 270ohm resistor - 274ohm

3k2 range - shorted leads - 30ohm
3k2 range - 15ohm resistor - 35ohm
3k2 range - 75ohm resistor - 95ohm
3k2 range - 100ohm resistor - 120ohm
3k2 range - 470ohm resistor - 480ohm
3k2 range - 2k7 resistor - 2k7

320k range - shorted leads - 2k6
320k range - 15ohm resistor - 1k3
320k range - 75ohm resistor - 1k8
320k range - 100ohm resistor - 1k1
320k range - 270ohm resistor - 1k5
320k range - 2k7 resistor - 4k
320k range - 27k resistor - 29k

I did try measuring the voltage across the probes with another multimeter while taking measurements, but couldn't make much sense of the results.

[retiredcaps]

A battery has previously leaked a little inside, but it's been cleaned up

How?

[shakalnokturn]

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, 1M between 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?

[bicyclerepairman]

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/DC5xc9Wo2J7p5LfGA

I 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.

[shakalnokturn]

http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Fluke/FLUKE 77, 75, 73, 70, 23, 21 Series II Service.pdf

At 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.

[shakalnokturn]

Edited bad SM link...

[bicyclerepairman]

Thanks for your help and the schematic - I will see what I can do from here!