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Agilent U1242A "problem" ?
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PwrElectronics:
I was working on a old capacitor analyzer that uses tubes.

I went to check for excessive ripple on the electrolytic filter caps after the rectifier tube....  This unit is near 60yrs old and I don't think its been powered up since possibly the early 90s.

So, my Agilent U1242A measures ~450VDC on this cap.  I then switched the meter to AC voltage.  The meter is "going crazy" in that it was momentarily showing >1kV of AC and then dropping down to a few hundred.  Even weirder, the value rises and falls on a interval of a couple of seconds.

Needless to say, very puzzling and concerning.

So, I get a scope out with a 2kV, 100x probe and check the same with that.  I get about 5V p-p with scope on AC coupling and the same DC as the meter reported in DC mode.  Also, there are no spikes or other waveform stuff going on that would possibly explain the DMM from having a fit.

Any ideas ?
PwrElectronics:
So, I got back to investigating this issue....

Seems like its something to do with the way the DMM firmware calculates the RMS ??? and the autoranging ???

Setting a specific range on the AC would seem to prevent erratic readings.

Posted some test results comparing this to my other meters on the 'tube...

https://youtu.be/pZBK4lut3Gs
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