34401A Ohm measurement issues. Solved.

[rolkinas]

Hello to all!
I got my first 34401A. It has impedance measurement issue using front terminals. in lower ranges 2W mode readings are much lower than 4W mode.

I checked front/back switch and it is OK

The problem appears to be switch command FXR swithed by front/back switch (telling microcontroller which terminal is activated - front or back) simply connecting U500 pin 25 to gnd (front) or +5V (back).

when FXR is tied to gnd - impedance readings are messed up. Conecting FXR to +5V - readings are more or less real.  There are measurements in bottom of my post with FXR tied to gnd.

Finally I confirmed that only FXR causing measurement failures after soldering 1Ohm resistor past front/back switch and switching FXR by external switch soldered to cut traces.

FXR goes directly to uController (U500) passing only 215 Ohm resistor (R512)

is seems not much can be done there.
Does anyone have experienced such problem?

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Measurements results with FXR tied to gnd are folowing:

0,001 Ohm 4W mode (leads shorted)
0,388 Ohm 2W mode (leads shorted)

0,990 Ohm 4W mode
0,529 Ohm 2W mode
1.092 Ohm checked with Fluke45

7,472 Ohm 4W mode
7,012 Ohm 2W mode
7,569 Ohm checked with Fluke45

176,46 Ohm 4W mode
176,01 Ohm 2W mode
167,56 Ohm checked with Fluke45

1,78584 kOhm 4W mode
1,78542 Kohm 2W mode
1,7857 kOhm checked with Fluke45

17,3600 kOhm 4W mode
17,3596 kOhm 2W mode
17,359 kOhm checked with Fluke45

177,567 kOhm 4W mode
177,565 kOhm 2W mode
176,56 kOhm checked with Fluke45

1,56382 MOhm 4W mode
1,56375 MOhm 2W mode
1,5601 MOhm checked with Fluke45
________________________________________________________

Any thoughts are welcome.

[dacman]

Front/Rear zeros are separately adjustable.  I use a Fluke p/n: 884X-Short. I also treat the 34401A terminals with a minuscule amount of mineral oil to both clean them and to help preserve the gold plating on the short.  (Metal to metal contact = metal shavings.)

Try searching for 34401A threads.  There are a few threads on the 34401A with resistance problems.

[rolkinas]

Hi dacman, many thanks for hint that back and front are separated calibration wise. It seems that my 34401A was poorly calibrated in past. Calibration sticker on all screws were stopping me from thought about poor calibration (: .  After proper zero adjust multimeter works as it should.
Regards!