8846A Fluke DMM has a proplem with the reading and it shows as bellow  - Page 1
Forum section: electronics / repair

author: Gen.Eng
author_rank: Newbie
timestamp: November 30, 2024, 01:17:29 pm
content: Voltage DC Open Loop ValueRange Unite Reading Uint100 mV OL mV1 V 0.128 V10 V 0.128 V100 V 4.08 V1000 V 3.95 VCurrent DC Open Loop ValueRange Unite Reading Uint100 µA OL µA1 mA 0.41 mA10 mA OL mA100 mA 41.5 mA400 mA 41.5 mA1 A OL A3 A OL A10 A 4.22 AResistance Short Loop ValueRange Unite Reading Uint10 Ω 8.08 Ω100 Ω 40.73 Ω1 KΩ 0.0406 KΩ10 KΩ 0.0406 KΩ100 KΩ 4.04 KΩ1 MΩ 0.0039 MΩ10 MΩ 0.039 MΩ100 MΩ 0.039 MΩ1 GΩ 0.039 GΩThe Reaging of the applied value on the range adds to half the applied value + the readings in the above tabelsFor 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 ohmFor 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 ohmFor 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 mAVoltage AC Open Loop ValueRange Unite Reading Uint100 mV 7.3 mV1 V 0.0072 V10 V 0.0072 V100 V 0.0475 V1000 V 0.638 VCurrent AC Open Loop ValueRange Unite Reading Uint100 µA 0.09 µA1 mA 0.0006 mA10 mA 0.005 mA100 mA 0.06 mA400 mA 0.575 mA1 A 0.00087 A3 A 0.0057 A10 A 0.0062 A

author: MathWizard
author_rank: Super Contributor
timestamp: November 30, 2024, 11:42:29 pm
content: 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

author: coromonadalix
author_rank: Super Contributor
timestamp: December 01, 2024, 03:00:52 am
content: 8845 8846 meters schematics never leaked, and i'm not aware of any reverse ...only heard about dim display, and many network related problems

author: Kleinstein
author_rank: Super Contributor
timestamp: December 01, 2024, 11:00:27 am
content: 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.

author: coromonadalix
author_rank: Super Contributor
timestamp: December 03, 2024, 03:12:10 am
content: don't double post .... in the beginner thread

author: Gen.Eng
author_rank: Newbie
timestamp: December 03, 2024, 08:36:05 am
content: @KleinsteinI checkd the current during Resistance mode as below1- 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 currentCould you please assist me on this matter

author: Kleinstein
author_rank: Super Contributor
timestamp: December 03, 2024, 03:51:03 pm
content: 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.

author: Miti
author_rank: Super Contributor
timestamp: December 04, 2024, 06:51:13 pm
content: 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.