Sometimes a small victory makes you feel just as good as a big one.

Picked up a somewhat non-functional Micronta 22-220A multimeter. A little
rough but the FET meter circuit worked - voltage readings weren't too far
off and the zero control did it's job so I knew all that stuff was OK.

But the resistance function acted as though there was a 4 ohm or so resistor
across the leads all the time, and the battery was draining at about 100 mA
in Ohms function even with no leads attached.

Of course the 9.1 ohm Rx1 resistor was bad, but replacing it did NOT change
the symptom.

After finding a schematic (not many out there...) I did find a component
labelled "SA1" shorted at 4 ohms or so. The item resembled an MOV and I can
only assume SA stood for spark or surge arrestor.

Removing it mostly fixed the ohms function, and I decided a couple of
back-to-back 25 volt zeners would offer enough protection to satisfy my
needs.

Still the ohms zeroing was erratic. Cleaning the function / range switch and
ohms pot til I was blue in the face did not resolve the problem. It was
kinda usable but it kept bugging me.

I tried putting a current meter in series with the test leads but couldn't
really get a usable correlation between pushing, poking wiggling the
function switch etc and the action of the meter which might zero fine, then
show up to several ohms even seconds later with probes shorted.

It occurred to me that I could put a resistor (say 4.7 ohms on this range)
across the probes and put a 'scope across that resistor to better see what
the DC voltage there was doing.

Oh, yeah. the voltage as viewed on the 'scope varied wildly and looked
"noisy" as the funtion switch was wiggled or tapped.

But I had cleaned that switch umpteen times.

Well, there was another switch - a leaf switch, going to the negative
battery terminal hiding under the front face and also actuated by the
function knob.

A quick cleaning of those contacts and the meter works like new.

A small victory to be sure, but made me feel as good as a big one.


Mark Z.