Fluke 73III ohms wrong-Solved - Page 1
Forum section: electronics / repair

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: January 07, 2022, 11:02:49 pm
content: My 73 III has a problem with high ohms. In auto mode reads 185K with no load. Will not read any higher. Will read lower ohm values OK.On manual it will read will read open on lower bands until it can read 185 on the upper bandsVolt readings and diode readings appear to be fine.I do not have the proper 73 III schematic but I have lots of other similar schematics.I suspect Z1 impedance board or the processor U1.Since I do not have the proper schematic for Z1 board, I am having trouble evaluating it.I have checked most of the other components.Any Hints?Wally

author: bdunham7
author_rank: Super Contributor
timestamp: January 07, 2022, 11:14:20 pm
content: It sounds like there's 185K worth of leakage somewhere.If you measure a known 100K resistor, what do you get?What is the OCV output of each of the ohms ranges, measured with a high-impedance voltmeter?What is the test current of each of the ohms ranges, measured with another DMM?

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: January 08, 2022, 12:39:18 am
content: Thanks for the responseYea, I suspected a leak in the Z1 impedance boardHere are some readings, volts are on a calibrated HP 3456, MA on an old analogScale Volts AmpsOhms 1.9 .6 ma100 R 1.8 .13 ma1K .89 12 micro ampsK .222 Almost zero1 M 26mV Almost zero10M 26mV Almost zeroI can read 100 ohm resistor fine on the first three ranges, not at all on the last three

author: bdunham7
author_rank: Super Contributor
timestamp: January 08, 2022, 01:08:19 am
content: Quote from: Wallace Gasiewicz on January 08, 2022, 12:39:18 am100K, not 100R. 100,000 ohms!Why do you think a leak on Z1? Is there a schematic for this model? What is the input impedance on the DCV ranges?

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: January 08, 2022, 12:14:02 pm
content: I have some schematics. but the 73 III is not among them. The board layout is a bit different, I suspect the schematic is close.I do not have the correct schematic. That is the reason for the post.Z1 has more leads than pictured in the 73 schematic pictured on page 52 of the service manual I have.Here is the link to the Service Manualhttps://ia800201.us.archive.org/0/items/FLUKE_77_75_73_70_23_21_Series_II_Service/FLUKE_77_75_73_70_23_21_Series_II_Service.pdf

author: SeanB
author_rank: Super Contributor
timestamp: January 08, 2022, 12:24:12 pm
content: Going to bet one or more of the MOV devices, or the overvoltage clamp transistors, is leaking. Check input impedance in the DCV range with another meter, with the Fluke manual ranged to the 100V range (any range really but the bottom, just to get out of autorange and the ultra high impedance millivolt range) and you will be able to see if it is in the voltage side or the resistance side. To test further you will have to unsolder the 1k input protection resistors and test again, as they will isolate each section from the input.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: January 08, 2022, 02:36:44 pm
content: You are absolutely correct!I disconnected the MOVs and the meter works.Now to replace the MOVsI have these MOVs (and bigger ones which would not fit)ERZ V10911 910V 1500A ZNRComes from Digikey with Cust Ref "for repair of Fluke Meter"I think I will use these. Sound OK??I have a bunch of other new MOVS