Is there a typical DMM calibration procedure?

[tron9000]

I have some DMM's I want calibrating - Blackstar 3225. I know its not exactly up to date but...

I have the user manual and there is no calibration procedure and can't find a service manual.

I've had the top off one and there is a shielded can (that can be removed) with holes cut out to trim some pots. This kind of indicates that there would have been a factory calibration procedure.

If I could reverse engineer the circuit, then I could determine what trimmer did what. But what order should I adjust things in? Is there a typical way to adjust a DMMs or is it manufacturer/product dependent?

[The Soulman]

But what order should I adjust things in? Is there a typical way to adjust a DMMs or is it manufacturer/product dependent?

Every dmm uses a internal dc reference voltage to reference the measured voltage against.
If you find that dc volts/amps and ac volts/amps are all reading the same percentage offf then this reference voltage could be the likely cause and needs readjusting also a dirty trimmer is not uncommon in these type of dmm's
If only ac volts/amps are off then the gain for the ac amplifier has drifted and needs adjusting.
The ohms ranges typically use fixed value reference resistors but on some meters there are also trimmers added to those.

Take a look as to what dmm chip is used in your dmm (probably some intersil icl Type) the datasheet of it should contain much useful documentation.
Or look for the schematic of a dmm that uses the same chip (keithley, fluke) from that same era.

[james_s]

The first thing to do before you start twiddling adjustments is compare it against a known good meter and see if it needs to be calibrated in the first place. Next the advice above is good, a LOT of meters were based on the same chips and are electrically almost identical. If you can find a manual for one that has the calibration procedure you can probably work out how to calibrate yours. Heathkit manuals in particular tended to be pretty good in those areas.

[tron9000]

I have one that appears to be spot on when compared to the rest, so I do have a point of reference.

[jeroen79]

Mind the difference between calibration and adjustment.

Calibration means determining how much the instrument is off with respect to some standard.
You can do this by going through all the ranges with various high and low readings and comparing these to a meter you trust.

Adjustment means changing the instrument so it's reading is no longer off from the standard.
If there are no obvious trimmers for this then you should look for a service manual or contact the manufacturer.

If there are then you should trace where they go to.
Some may trim the reference, others may trim the input from the individual ranges.

[tron9000]

Mind the difference between calibration and adjustment.

Calibration means determining how much the instrument is off with respect to some standard.
You can do this by going through all the ranges with various high and low readings and comparing these to a meter you trust.

Adjustment means changing the instrument so it's reading is no longer off from the standard.
If there are no obvious trimmers for this then you should look for a service manual or contact the manufacturer.

If there are then you should trace where they go to.
Some may trim the reference, others may trim the input from the individual ranges.

I tried looking for a service manual, but I can't find one and the manufacturer seems to have disappeared. There is a Blackstar electronics in the UAE, but I think that's coincidental as it appears they are not in the instrumentation business

I think then I would be Adjusting the instrument so it is no longer off from the standard.