I recently purchased a used GDM-8034 bench DMM. It's a basic DMM with 2000-count LCD display, manual ranging, and 0.5% basic DC accuracy. It seems to be fully working, and the functions I've been able to test seem to still be in calibration (though I only have an even older Fluke 4.5 digit meter as a reference). Not bad for $25.
However, it has a slight DC offset error. For example, measuring a 0.250 V reference on the 2 V range, it reads +0.251 or -0.249 depending on how I connect the leads. I'd like to trim out this offset if I can. I have downloaded the manual for it from the Instek web site, but it seems that the manual includes just end-user instructions - nothing about how to calibrate it, no circuit diagram, not even a board layout that identifies what the adjustment pots and cap actually control.
This is in contrast to the manual for the older GDM-8135, which has tables for performance checks and calibration, and a drawing of which calibration controls are where. Now, the GDM-8034 is obviously simplified somewhat compared to the GDM-8135. The 8135 has 14 calibration adjustments (11 pots and 3 trimmer caps) while the 8034 has only 5 adjustments (4 pots, 1 cap). But I don't want to go changing the adjustments blindly while trying to figure out what they do, particularly when its calibration seems OK at the moment and I don't have the procedure to recalibrate it properly.
So I would appreciate it if anyone can send me (or point to) a schematic for the 8034 (which would let me figure out what the adjustments do. Or, even better, the real calibration procedure for this meter. Note that I do have the 15-page users' manual already, but I need something beyond that to start changing calibration adjustments.
The adjustment points on the 8034 are:
VR401: left side near U401
VC401: near U402 and J401, near left end of U606
VR501: left edge of board, behind input jacks
VR502: adjacent to U501 and near VR501
VR601: between U606 (large chip) and R515 (power resistor)
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