Electronics > Repair

Fluke 79 III -OL- in vdc,mvdc and won't measure resistance

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Mark24:
Greetings

A fluke 79 III that show -OL- on Vdc and mVdc and won't measure resistance all without probes using a new battery

Vac show (0.00) normal and Buzzer , Diode mode works

Attached photos for the DMM

I dont know to what to check next

Any suggestions?

m k:
One possibility is that ohms voltage source is on all the time.
You should be able to check that.

Mark24:
Thank you for the information

on ohms mode the probes fluke measure 1.23v if the probes  shorted the DMM display -OL- and the "-" flashes
on vdc mode they measure 0.99v the DMM display -OL-

is that what you mean by (ohms voltage source)?

m k:
Yes.
In theory probes can have what ever voltage when ever, but it would be overly complicated if VDC mode had an initial probe voltage.
So normally Ohms measurement style is active and VDC passive, means that the same internal current is measured but for that VDC mode uses external voltage.

When probes are shorted and Ohms is -OL- it means that the measured value is negative.
For that there must be incorrectly balanced internal measurement and reference voltages.
Without VDC mode problem it could be just an adjustment of zero level.

When VDC mode has probe voltage it means that disconnection of internal measurement voltage for Ohms has failed.
Can be just a bad switch somewhere, but the machine has many integrated switches.

Series II manual has schematics, maybe series III is close enough.

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