GB GDT-3190 Reads low in all modes...

[knockbill]

That's a a good thing to know about the AC ranges,,, Never heard it before, but it makes sense...
Thanks again...

[pilobond]

Problem solved...?

Just for people who will be perusing in the future... I had same problem with GDT311. I am sure it's pretty much the same circuit board inside.
R1, pot, R2 looked the same as the picture from OP. With the pot turned all the way to left, I was only able to read about 3.9V for 5.0V source.

R2 was 1002 SMD, thus 10kohm. I replaced this with 22kohm through-hole resistor, as I didn't have any SMD in that range handy. Then I was able to adjust the pot so the reading matches my Fluke DMM. Since then all reading in DC voltage and ohm reads with in 1% of my Fluke 117. AC voltage from my main reads 120V (+ 1 decimal who value I forget) so it seems reasonable.