multimeter reading creeping up

I picked up a Mastech benchtop true RMS DMM the other day since I wanted
something with a PC interface and so far it's just illustrating why I
like my Fluke so much, this thing is defective right out of the box.
Before I return it, I'm curious if anyone can shed some light on what
it's doing, I'd love to open it up and have a look, but I don't want to
void the warranty since it's new. Still, my curiosity is aroused.
The issue I'm seeing is the reading creeps up over time. I left it
monitoring the AC mains and it was working fine, then a couple hours
later I came in to see it reading 145V! Thinking I had some serious
wiring issues I grabbed my handheld meter and it read almost exactly
120V. I power cycled the new one and the reading returned to normal so I
left it going and by morning it read 345V. I reset it again and tried,
this time recording the readings and found that it worked fine for about
2 hours, then started increasing at quite a steady pace. Now I've got
the probes unplugged entirely and it reads a bit over 190VAC and still
slowly climbing. Anyone more familiar with the design have some insight?
DAC? RMS conversion? Reference? I wondered if the voltage reference to
the DAC might be heating up and dropping over time, but the instant
recovery by cycling the power points elsewhere.
Reply to
James Sweet
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Bad firmware comes to mind first. Some register is not getting reset to zero and slowly accumulating bits.
There's also the possibility that it is a thermal drift problem, but that some power up calibration routine compensates for it initially.
Try running it with shorted probes (zero volts input, low impedance) and see what it does.
Reply to
Paul Hovnanian P.E.

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