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GW Instek GDM-8034 bench DMM: looking for schematic or calibration notes

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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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