8846A Fluke DMM has a proplem with the reading and it shows as bellow

[Gen.Eng]

Voltage DC Open Loop Value         
Range    Unite   Reading    Uint
100   mV   OL   mV
1   V   0.128   V
10   V   0.128   V
100   V   4.08   V
1000   V   3.95   V
Current DC Open Loop Value         
Range    Unite   Reading    Uint
         
100   µA   OL   µA
1   mA   0.41   mA
10   mA   OL   mA
100   mA   41.5   mA
400   mA   41.5   mA
1   A   OL   A
3   A   OL   A
10   A   4.22   A
Resistance Short Loop Value         
Range    Unite   Reading    Uint
10   Ω   8.08   Ω
100   Ω   40.73   Ω
1   KΩ   0.0406   KΩ
10   KΩ   0.0406   KΩ
100   KΩ   4.04   KΩ
1   MΩ   0.0039   MΩ
10   MΩ   0.039   MΩ
100   MΩ   0.039   MΩ
1   GΩ   0.039   GΩ
         
The Reaging of the applied  value on the range adds to half the applied value + the readings in the above tabels         
For Example at range 10 ohm the value show up 8.08 if we applied 1 ohm it will equal (8.08 + (applied Value /2)) and it shows 8.58 ohm         
For Example at range 100 ohm the value show up 40.73 if we applied 100 ohm it will equal (40.73 + (applied Value /2)) and it shows 90.73 ohm          
For Example at range 100mA the value show up 41.5 if we applied 10 mA it will equal (41.5 + (applied Value /2)) and it shows 46.5 mA         
         
Voltage AC Open Loop Value         
Range    Unite   Reading    Uint
100   mV   7.3   mV
1   V   0.0072   V
10   V   0.0072   V
100   V   0.0475   V
1000   V   0.638   V
Current AC Open Loop Value         
Range    Unite   Reading    Uint
100   µA   0.09   µA
1   mA   0.0006   mA
10   mA   0.005   mA
100   mA   0.06   mA
400   mA   0.575   mA
1   A   0.00087   A
3   A   0.0057   A
10   A   0.0062   A

[MathWizard]

Voltage and Ohms often share some of the same resistors in their divider networks. But they don't usually share divider resistors with the current sensing. All 3 might also share some of the same connecting switches, either electronic switches, or mechanical.

I've only worked on a few DMM's, but yeah I wonder what's wrong with the current measurement, and if it's related to the others ? Usually, the amp signal into the DMM, takes a different path right away, than a voltage signal. But then their sampled signal, might go through the same analog switch IC's.

If only the ADC section had a problem, I'd expect errors on AC too.


I'd trace the path of the V/R into the DMM, and check their divider networks and switches, and then see where Amp signals go.

Here's a link to some manuals, but I don't see a service manual, maybe it's too new a DMM by the late 90's ?

https://www.manualslib.com/products/Fluke-8846a-2791364.html

[coromonadalix]

8845 8846 meters  schematics never leaked, and i'm not aware of any reverse ...

only heard about dim display, and many network related problems

[Kleinstein]

As the AC part seem to work somewhat this suggests that the ADC part could still work and the problem is more with the input amplifier or signal switching at the input.
The ohms part usually is a combination of a switchable current source and a voltage reading. So the gain from the voltage part is usually shared. The current source is usually separate. However in some of the better meter Fluke also has a somewhat non stanard way to implement the ohms modes.

The current part would have shunts and than usually use the lowest DC range, in some cases the 1 V range.

A simple point one could test is the test current from the ohms source. So measure the current with an external meter when in the ohms ranges.
The may give a clue if the factor 0.5 comes from there.

Another simple point is checking if there is significant input current when in voltage mode.

[coromonadalix]

don't double post .... in the beginner thread

[Gen.Eng]

@Kleinstein
I checkd the current during Resistance mode as below
1- range 10 ohm is 5.000 mA.
2- range 100 ohm is 1.000 mA.
3- range 1 Kohm is 1.000 mA.
4- range 100 Kohm is 0.100 mA.
5- range 1 Mohm is 0.010 mA.
6- range 10 Mohm is 0.001 mA.
7- range 100 Mohm is 0.001 mA.
8- range 1 Gohm is 0.001 mA.

During Voltage mode there is no significant  to current

Could you please assist me on this matter

[Kleinstein]

The currents from the ohms source look OK.
How was the input current in volts mode measured, which range is the current ? It should be somewhere below 50 pA near zero voltage.

The factor of 0.5 in the scale factor is a bit odd. A factor of 2 may come in also from the digital side / calibration constant. Is the coin cell still OK ?
This would be the easy way to fix and woth a check first.
Another simple point would be to check for some offset settings by resetting to factory defaults for the turn on settings.

Is the activation / dactiviation of the analog filter make a difference ?
I don't know very much about this meter, but there could well be some AZ and non AZ mode setting that could also make a difference (e.g. with a short in the 1 V and 10 V range).

For further measurements it could help to have a bit more information and at least a crude plan.
There are quite some test points but AFAIK not much information what the testpoints actually are. An interesting point would be the output of the main amplifier / input to the ADC. Could be TP11, maybe TP14 or TP28.

[Miti]

What’s the history of the meter? Did you get it like that or it just went crazy? Is it both, front and rear? Make sure the F/R switch is not broken or not pushed firmly in one position. The switch is a weak point in these instruments.
Someone may have messed up the zero calibration. I have an 8845A and 100V and 1000V ranges had an offset. Did a zero calibration and everything is bang on now.
Check the calibration manual at the end for the zero calibration procedure, AFTER you confirm that the readings seem ok but with offset. Unless you have a calibrator STOP before ### Calibrate AC Linearity.
You’ll need a good 4 wire short.