Fluke 45 - Low ohms measurement error

[AndersJ]

On the lower Ohms range it shows about 12 Ohms too high.
The error decreases slowly, up to about 4700 Ohms where the error is gone.
Continuing up towards 10K it starts showing a too low resistance.

The measurement current is 740 uA.
On serveral other 45's the current is 800 uA, and the resistance measurements are spot on.

Thinking current may be lost/shunted on the way from the current generator in the Fluke ASIC,
I have measured the voltages across resistors on the way to the front panel.
All mesurements are consistent, and it seems the ASIC outputs 740 uA.

Perhaps the current is not the issue here.
I did however find it peculiar that all others (I have another five instruments) use 800 uA.

I have tried a front panel calibration,
which slightly improved the situation, but the issue remains.

In all other respects the instrument measures correctly, Volts, Amps, DC, AC etc.

Any ideas, anyone?

[AndersJ]

I have nothing to add.
Just trying again, hoping someone might have a comment this time.

[The Soulman]

I have nothing to add.
Just trying again, hoping someone might have a comment this time.

What low Ohms ranges? 300 and 3000 ? both the same fault?
Did you measure the input resistance when set to dc-Volts?

Attached is the service manual.

Edit: Adding attachment doesn't work, link:

http://media.fluke.com/documents/45______smeng0100.pdf

[Armadillo]

Yes, I can confirm that the current is 0.8mA @ 1.5V constant voltage source at output for ohms measurement.

Check the input protection components A1RT1&A1R5, A1R6 & A1R7.

Verify that U1 constant voltage source is OK [U1 is OK] by measuring the voltage at K2 Relay common [before the input protection]. If this voltage is OK, then the protection components are suspect.