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Fluke 8840A slowly drifting with no input connected?
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uski:
Hi there!
I just received a Fluke 8840A.
The voltage readings compare pretty closely with an in-cal Keithley DMM6500, so I would say its accuracy is very good, considering the last cal was 13 years ago (thank you LTFLU-1 reference)
BUT, something that surprises me, is that with no input connected, the voltage does not read zero. It starts at around 70mV when I start the meter. I can bring it to zero by connecting a resistor across the terminals. I left the meter for a few minutes on the mV reference and it's been climbing steadily by a few mV, from -5mV to around -9mV now.
I think it means there is probably some leakage somewhere internally. Any idea where to look at? I have noticed several pictures of 8840A on eBay with no input connected and also showing a few tens of mV, so I am assuming this is a common issue and hoping that someone here knows about this...
PS: The rear frame has been broken during shipping and it triggers my OCD - if someone has a parts 8840A and would be willing to sell me a rear frame for a reasonable price, I may be interested
PS2: I was able to remove most of the scratches on the smoked screen cover using Displex display polish, very impressive stuff (keep it away from the silkscreen)
Thanks ^-^
uski:
I found the answer! Apparently this is normal. The behavior of this meter is different than other meters. I am not the only one surprised.
https://www.eevblog.com/forum/repair/fluke-8840a-is-this-normal/
Still looking for a rear bezel - in case someone has a parts unit...
Gyro:
Yes, perfectly normal for any high input impedance meter, not just the 8840.
Kleinstein:
A drifting open input is normal and the DMM6500 should also show this in high impedance mode. It is a bit unusual that the input stabilzed at only a few mV. This means either an unusually low bias current (which would be good - a lucky find) or a somewhat lower than normal input impedance (e.g. a dirty PCB, a spider inside, high humidity). It may be worth doing another check here: charge the input so some 9 V or so and than record the time it takes for the voltage to come down (e.g. 1 V level). Ideally repeat with an extra low leakage (PP or PS type) capacitor of some 1-10 nF.
For the time of the RC decay one can get an estimate on the input impedance. AFAIK there are schematics available for the 8840 and the internal capacitance in known.
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