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GW Instek GDM-8034 bench DMM: looking for schematic or calibration notes
dmmartindale:
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)
Poroit:
The accuracy for the DC Volts ranges on that model are:
+/- 0.5% of reading + 1 digit.
It is in spec.
dmmartindale:
Yes, I know that the one count error is within the published spec. And there may not be any way to adjust this anyway; the GDM-8135 does not have any adjustment for this. But I would like to adjust out this offset if it is possible, and having a circuit diagram would tell me if there is an adjustment.
Plus, having a circuit diagram and/or calibration procedure is useful to keep the meter running in the long term, particularly if some component eventually needs to be replaced.
coromonadalix:
have you tried at GWInstek ??
other options is to measure the trimpots values and try fiddle them ... witch is a last course of action ...
or trace down witch pots go the each ic's, search ic's data sheets and try to figure out the circuit ...
since this is a basic 0.5% meter 3 1/2 digits, should not be hard ...
nctnico:
I'd leave it as is. Likely the offset will vary with temperature & humidty as well so trying to adjust will be futile. Fiddling with the adjustments is more prone to breaking something than making an improvement.
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