Fluke 79 Series II shows 10.5 megohms when open circuit

[Tim M]

Hi All:

The display shows 10.5 (this can vary from 10.5 to 10. ohms open circuit and when the leads are shorted it displays .4 ohms.  Low ohms works correct as do ACV and DCV when compared to another meter.  When shorting the leads in ACV and DCV modes the display is 0.00  When measuring ohms the result is the parallel combination of resistor you are measuring with 10.5 megohms in parallel.  I have measured the fusible resistor with another meter and it measures 1.12K.  I am not sure what the tolerance is on that part.  I have downloaded the service manual but I am not sure where to go from here or what further tests I could perform.  Any assistance appreciated.  Thanks in advance.

Edit:  The battery is new and I now see that the fusible resistor is speced as a 1% part so my fusible resistor R1 is out of tolerance at 12% high.  I paralleled it temporarily to bring it back into spec and the reading did not change.

Tim

[floobydust]

Welcome to the forums

I would say it's not the fusible surge resistor R1 1kΩ or the PTC causing the open-circuit Ohms reading 10.5MEG because they are so small in comparison to the input resistance and their purpose is to protect the Ohms-source. So if they vary 20% it's still nothing for a couple kΩ to the 10MEG in the divider.

Usually it's a damaged protection clamp transistor Q1 or Q2 that is leaky. You can lift one leg to disconnect them and see if the reading persists.
It rarely can be the protection MOV's leaking RV2, RV1.
A dirty PCB or rotary switch, especially around the Z1 9.999MEG, Q1&Q2, across RV1,2 etc. Use IPA and a Q-tip to clean up old flux and any battery leakage etc. and let it thoroughly dry out.
Beyond this I think the DMM IC can get zapped and a damaged CMOS MUX internal switch give the leaky Ohms reading.

Another way to flush out the leaky part is to apply heat - with your finger, soldering iron etc. and find what makes the reading jump significantly.

[Tim M]

Thanks very much for the response.  These sound like excellent ideas.  I will investigate and report back.

Tim

[Tim M]

I want to close the loop on this.  The problem was in fact the MOV  RV1.  I replaced RV1 and RV2 with Littelfuse V575LA10P as suggested in other posts here.  The meter is working  well again.  Thanks again floobydust for your suggestions.