Appa 503 non-zero (Aka Voltcraft 930/950, IDM503/505 etc) Updated

[Bakaman]

Hi All,
I wanted a second meter for offline datalogging, this nice meter came up from a retiring electrician on EBay, good price including USB cable, and various bits.  It seems mostly good, maybe autoranging is a bit slow.

However, with leads shorted, it reads:
- a steady 67mVAC,
- a steady 97mV on AC+DC
- and it flickers between 56 and 90mVDC.  Sometimes it flickers, sometimes not, it isn't always exactly the same offset amount, but a low voltage offset is always there.

Doing a 10-second min/max/average test at various voltages on a cheap regulated plugtop PSU (the best I can easily get right now) shows a 62mV offset regardless of voltage (3.3-12.1V), and fluctuation is most noticeable in 40,000 mode.  The meter changes range 3 times for the following 3 test voltages.  Using the min/max mode is a handy way to record the fluctuations:
3.3V (3.2652V Average) gives 62mV max-min fluctuation very noticeable;
4.5V (4.457V Average) gives 5mV fluctuation), fluctuation is not noticeable
12.1V (12.096V Average) gives 24mV fluctuation, fluctuation is not noticeable

The offset and flicker may be related, but the flicker is most noticeable at the lowest voltage.

A very low battery, reads 0.24VDC or 0.33V with lead reversal. My Fluke 77 reads 0.284V rock steady regardless of lead reversal.  Reading another battery, it reads 1.352V, it also flickers around twice per sec between 1.35x and 1.33xV.

I can send it back but the price was very good, and the meter will do what I need quite nicely, so if there is a way to fix it then I'll be happy.  It logs at twice per second, faster than my Fluke 289, and whilst it doesn't do event capture like my 289, where I do need a fast regular log, then it may be more useful than the Fluke with its logging features in some circumstances.

So, my question for the learned EEVBlog people with loads of experience, is whether it is worth trying to sort out.  Having done a bit more testing, it does appear to be range-sensitive, and the offset appears to affect AC and DC modes.  There aren't seemingly as many caps in meters these day that might go leaky/noisy, so faults might tend to be either complete failure in a particular mode, or something like dirt/contamination/leakage tracking that might provide a non-constant reading error/offset.
 
There is no battery corrosion, but I did notice some clear grease around battery terms in the back panel, so I cleaned this off and used fresh batteries but no change. At these low voltages, and very high impedances, I know that the slightest bit of leakage could cause a major read errors.  Whether it could cause fairly regular fluctuations, I'm not so sure.

Another query is re diode test bias voltages.  On diode test on the Appa, the OC lead voltage reads a non-steady voltage between 0 and 0.25VDC on the Fluke, but it measures a diode's forward voltage similarly to the Fluke.  If I put the Fluke into diode test with OC leads, the APPA reads around 2.7V which is what I'd expect from a forward-biasing voltage potential.  Perhaps this is nothing to worry about, and maybe some meters use a fast test pulse for diode testing rather than a constant bias voltage?

Short videos here https://photos.app.goo.gl/H8rEpexEcvgqUd8d8

Thanks in advance,
Brian

[Bobson]

My APPA 506 uses a pack of 370ms rectangulars with 2.5V amplitude, 5V p2p for diode tests.

[shakalnokturn]

I don't think it's going to fix itself in any case...
Have you tried measuring at ADC input for the offset?
If it doesn't exist there then a quick conclusion (in place of extensive troubleshooting) would be that ADC is defective which could easily explain both offset and variations.