Fluke 726 handheld calibrator repair

[Krampmeier]

I recently got a defective Fluke 726 handheld calibrator. Lucky me!

The reported problem was that the resistance measurement was not working right in 3-wire mode. The instrument had already been adjusted by using the calibration menu via RS-232. In fact, it measured more or less correctly in 4-wire and 2-wire mode, and 1.47 times too high in 3-wire mode. Like with a constant gain error, which varied slightly over time.

I noticed that in 4-wire and 2-wire mode, the test current from the 726 was 135 µA, while it was 200 µA in 3-wire mode. Some internet search showed that the correct current is 200 µA.
As the unit is adjusted in 4-wire mode only, a wrong current in 4-wire mode would break measurement modes where the current is correct.

The circuit board looked alright, so I had to dig deeper. Without a service manual, I had to figure out how the ohms measurement circuit works in order to find the error.

I buzzed out the attached schematic of the ohms measurement circuit, which may still be partly wrong and incomplete. See attached fluke_726.pdf. I also checked the analog mux settings for the various resistance measurement modes, and verified that all  muxes worked fine.

The constant current source built with U7 and Q4 seemed to stop working properly in all cases where no current was sourced from the "3W" terminal (marked "1" in the attached overview).

It turned out that the zener diode D25 got very hot in those cases, and the voltage drop at D25 got much higher than the nominal 18 V: up to 21 V. This reduced the negative supply voltage for U7 so much that it could not deliver enough current.

But why was U7 sourcing so much current (>20 mA) into negative supply rail (which should have been at 24V-18V = 6V below the positive supply)?

I removed D21 and it suddenly behaved correctly. Very, very strange.

In the end, I replaced U7, and everything was fine. I could adjust the instrument without any problems and it is now working fine in all resistance measurement modes 

It seems like U7 was partly defective, and started to overstress the negative supply when U11D was inactive and Q3 should have been completely off.

I hope someone finds this interesting, or has a use for the attached information!

U11	2W	3W	4W
Pin1	H	H	H
Pin10	L	H	H
Pin11	L	L	H
Pin20	L	H	L

U11	2W	3W	4W
Connecting pins	2-3	2-3	2-3
Connecting pins	7-8	8-9	8-9
Connecting pins	13-14	13-14	12-13
Connecting pins	17-18	18-19	17-18

U9	2W	3W	4W
Pin1	L	L	L
Pin10	L	L	L
Pin11	L	L	L
Pin20	H	H	H

Edit: Added reference to attached schematic fluke_726.pdf