Author Topic: Puzzling Fluke 85 problem  (Read 309 times)

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Offline LazyJack

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Puzzling Fluke 85 problem
« on: February 06, 2019, 09:40:06 am »
I just got an old Fluke 85 (one that predates the CAT rating). It measures voltage all right, and resistance sort of, but with open inputs it is fluctuating around  negative 200kohms. First it looked easy, I checked all the usual suspects, that is everything has been already bathed in IPA, no burn marks anywhere, rotary switch cleaned, checked the VDRs, the fusible resistor, the input protection circuit (RV1, RV2, Q1, Q2, Q6, CR7, CR8, even removed these) but the problem persists.
So it is puzzling now, as I was suspecting one of these components to be failed after some overload, but apparently not.
Any tips on what else I should be looking next?
 

Offline ModemHead

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Re: Puzzling Fluke 85 problem
« Reply #1 on: February 06, 2019, 03:49:10 pm »
This seems very similar to a recent thread: https://www.eevblog.com/forum/repair/dmm-cemetery-fluke-appa-metex/

It sounds like you've done some very good trouble-shooting already. If you haven't yet, I would recommend studying the service manual section that shows the "signal flow" for the different modes.  You'll find that the ohms mode voltage sense leads to U4P93 (AP1).  Something must be influencing this path in the negative direction.  It should only be influenced by the test current source coming through the fusible resistor, and the unknown resistance on the input jacks.

There are no deliberate connections to the analog power supply in the front end, so it is unlikely to be component failure.  The vast majority of the time I have seen this, it is leakage from contamination. It isn't always on the PCB, I have found leakage between wafers of the rotary switch before, and it is a maddening adventure to locate it.  Otherwise it will probably come from inside U4 itself, probably as a result of damage to the internal voltage clamps.

Hope this rambling helps.
 

Offline LazyJack

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Re: Puzzling Fluke 85 problem
« Reply #2 on: February 06, 2019, 07:29:15 pm »
Hey, ModemHead, I was hoping you would respond  :)
Thanks for the info, the signal flow charts are vry useful indeed. Time to do some more investigation.
 


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