Gossen Metrahit 15S calibration

[Buzz-01]

Hi there,

I'm newly registered to this forum after reading for over a year.
Today I've bought a Metrahit 15S multimeter, which after testing seems to be way off at DC Volts measurement.
Putting 12V on the input terminals gives a reading of 10.5V on the display. I'm kind of hoping this can be fixed with calibration of the meter.
The circuit board has several potentiometers, but unfortunately no silkscreen to indicate which pot is for what purpose.

Here is a photo of the inside:



I've searched the internet for a service doc, but all I could find was a spec sheet / user manual.
Is there anyone who knows a little more about calibrating this multimeter?

[Buzz-01]

I've been doing some measurements and until now this is what I've found:

• Current measurement (300mA setting) is spot on from 0 to 150mA, upper limit of my precision current source so didn't test beyond 150mA.
• Current measurement (10A setting) is fine up to 10A as far as my cheap uncalibrated 2nd multimeter shows.
• VDC is way off (measures too low) and the error is linear with the measured voltage, tested from 0V up to 24V DC.
• Resistance is OK for lower values up to 50k, above that I see a linear error, eg 100k measures as 97k and 1M measures as 820k.
• Diode measurement shows an incorrect Vf, always too low. Where I expect 0.7V I see 0.45V.
• Capacitance measurement appears to work reasonably. Because of high tolerance on the caps I have available I can't say anything about its accuracy
• Continuity test works as expected

So either the calibration is way off, or something has damaged the input circuit which affects every measurement but current measurement (all but current measurements share the same positive input bus).
Any thoughts?

[PedroDaGr8]

 Why  bother,  I'd return it to  the seller and ask for a new one.  This one is clearly out of spec.  No need to mess  with it yourself ( and possibly make things  worse)  when it's a  new device.

[Buzz-01]

Oh, if I had bought the meter being new then I'd definitely return it! 

I've bought it second-hand in an unknown state at an electronics flea market, for just a few bucks.
To me it would be a win either way; I could have bought a top notch multimeter for a great price, or it would end up being an interesting project to take apart and repair...

It ended up being the latter... 

[Wytnucls]

'Calibration
The multimeters of the METRAHit S series can be calibrated automatically
via the RS-232 data interface with the aid of the multifunction
calibrator METRAtop 90C and the METRAwin 91 software.'

It looks like it has 3 trimpots, probably VDC, VAC and Ohms. Try fiddling with them, until you find out which is which.

[MatthiasBerlin]

I managed to get hands on a service manual (in German and in English, or the later more a crude mixture of German and English  ). If it's any help for you, write me a mail and I'll scan it.

The adjustment procedure is:

Electrical Adjustments:
1. Apply supply voltage 8V to battery clip
2. Turn on device
3. Attach measurement device to supply + and contact area E
4. Adjust LCD voltage to 4,3 V using pot R105.

Reference voltage adjustment:
1. and 2. like before.
3. Main switch to V=
4. Apply 250 mV (source resistance < 100 Ohm) between input mass and test point A (input socket?)
5. Adjust to 2500 digits using R36

Capacity:
1. and 2. like before.
3. Main switch to F
4. Apply 250 nF between 250 mV between input mass and V socket (input socket).
5. Use R72 to adjust to 2500 digits.

That's all. The rest of the manual is just an explanation of the schematic, the signals expected at some test points - and the schematic itself, of course.

[retiredcaps]

The rest of the manual is just an explanation of the schematic, the signals expected at some test points - and the schematic itself, of course.

A scan of the explanation and the schematic would be great.