Hey guys,
so I just bought a Uni-T UT139C. Seems actually like a nice multimeter. However, there seems to be one little snag: the frequency measurement does some weird things. When I set the dial to 'Hz/%' and hook up a 1MHz signal from a can osciallator (i.e. about 5Vpp), the display just shows "000.0". But! As soon as I press the Hz/% button to switch to duty cycle measurement mode, the display shows the correct duty cycle (around 50%), but only for a second or two, and then goes to "00.0"! When I press the Hz/% button again to switch to frequency mode, the display shows "999.9 kHz", but again only for one or two seconds, then it goes back to "000.0"! So I can only measure frequency/duty cycle by pressing the Hz/% button all the time ...
Now for the really weird thing: when I run the oscillator signal through a 100n capacitor, and then to the meter, the frequency is displayed correctly, all the time!
What gives? Is my meter broken? Or does it really expect an AC coupled signal? I've attached the relevant page from the manual, I'm not quite sure what to make of the info.
I opened the meter up and at least optically, there is nothing obviously wrong. In fact, the quality of the soldering is excellent.