Author Topic: Looking for service manual/schematics for the Microohm meter Schuetz MR 300 C  (Read 1781 times)

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Online AndersJTopic starter

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Does anyone have a schematic diagram or other service literature?


« Last Edit: January 17, 2019, 08:42:41 pm by AndersJ »
"It should work"
R.N.Naidoo
 

Online AndersJTopic starter

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Perhaps someone has a donor device?
Or some other input to help push this repair project forward?
"It should work"
R.N.Naidoo
 

Offline Doctorandus_P

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Gosh,
That looks like a really nice piece of vintage equipment.

Not in the same class of instruments, but Mark Driedger wrote an Article in Circuitcelllar #314 (2016-09) of a pretty nice and simple to build milli ohm meter, based on an "arduino", an ADS1115, a few resistors and a little display.  That meter has a resolution of 10uOhm.
The thing can be built for EUR 20 or so, and it is what modern electronics does, but it still does not have the range of your meter.

As you probably know, those meters often consist of a current (voltage) source, and a measurement circuit. When applying an external voltage you can probably determine which of those is faulty. But always start with checking / repairing the internal power supply voltages.
What is the failure mode of your meter?
If it "does nothing" it probably is in the primary power supply circuit and easy to fix, even without a schematic.
 

Online AndersJTopic starter

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The instrument lights up and is controllable from the front panel,
but it reports overload regardless of measurement range and attached resistance.

On the inside there is evidence of overheat/burning on the power supply board and the fuse is blown.
I get the feeling that this is the test current generation circuit.
By following the PCB traces I can see that this is fed from the rectified secondary of the toroidal mains transformer,
and the red and blue cables are attached to the main instrument chassis.
It feels difficult attempting  to repair this without the schematic.
The right hand part of the PCB seems to be the general purpose power supply which is working.






« Last Edit: January 20, 2019, 10:05:57 am by AndersJ »
"It should work"
R.N.Naidoo
 

Online AndersJTopic starter

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The meter is fixed.
It was easier than expected.
All it took was replacing the visibly damaged components.

"It should work"
R.N.Naidoo
 

Offline Doctorandus_P

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It looks like those pictures are made after (or halfway through) the repair.
The PCB is blackend, but the fuse holder is very clean.

If it still looks like that, you should clean the black stuff of the PCB as much as you can.
This stuff often is conductive and can easily lead to reliability or stability / accuracy issues.
You can use simple water and soap with a brush to clean it.
Just let it dry thouroghly before applying power.
 


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