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.