Schematic for Micronta 22-200 Multimeter

[Dopey_Robot]

Greetings,
I picked up this old Micronta multimeter(model # 22-200) at a yard sale for $1 and after opening it up I discovered that it has a component with a big chunk missing and I can't identify it. The only markings on it are "ULT" and "CS-3". It sure looks like a resistor, but I don't know it's value as it's internally shorted out I think and reads infinite Ohms on my meter. I'm hoping to find a diagram of this circuit and work through that to determine what it is, but finding one has proved fruitless so far. I've even emailed Radio Shack Inc. but that was pointless so now I'm here hoping someone might know the answer. Here's a picture if that helps:

http://i960.photobucket.com/albums/ae81/dopey-robot/Misc/DSC02212.jpg?t=1284778186

 

[(*steve*)]

OK, "internally shorted out" and "reads infinite ohms". I'm guessing you're not overly experienced in electronics.

Another issue is that you seem to have pulled away part of the track the component was connected to when you removed it. That will also require fixing.

At a very rough guess, I'd say it is a shunt resistor and it has failed open circuit because someone trued to measure to high a current on the meter. That's a very common fault.

A photo of the inside of the meter, showing where you removed this component may be of assistance. If there are other potential faults someone may be able to spot them too (say burnt tracks).

 

[Dopey_Robot]

Sorry for the lack of proper terminology. What I meant to indicate was that it read beyond 2M Ohms on my meter. Here's a look at the board, it was located near centre in the picture.

http://i960.photobucket.com/albums/ae81/dopey-robot/DSC02218.jpg?t=1284816319

Here's a look at the side of the component that has a chunk missing:

http://i960.photobucket.com/albums/ae81/dopey-robot/DSC02221.jpg?t=1284816319

(I've scraped away some of the material, which seems almost carbon or graphite like to me, just to satisfy my own curiosity.)

 

[(*steve*)]

OK, it's clearly a shunt resistor for a current range. And you've torn off a pad and pulled out a plated through hole by the look of it.

Carefully check all the current ranges. At least one (now without a shunt) will probably be vastly more sensitive that it should be. If other current ranges work, then the shunt resistors for them will probably indicate what the failed value was.

Yo test it you need a current source that will give about mid-range on the most sensitive scale. A battery with a suitably high value resistor should work fine. Then try this on all other ranges. Most will read a very small value (or 0) but the one missing the shunt will behave (probably) like the most sensitive range.

 

[Dopey_Robot]

Thank you, that's very helpful and I'll work on that now. But, you mention that I've damaged the circuit board. Is this difficult to repair?

 

[Dopey_Robot]

Hi Steve,
I've tried your method of deducing the value of the shunt resistor by process of elimination, but I think now that the meter is totally malfunctioning. I can't seem to attain any accurate readings of current, voltage or resistance. Oh well, a dollar spent is a dollar spent as I always say. Thanks though, I'm sure I'll be back asking more questions soon enough.

 

[(*steve*)]

The problem with finding an open circuit shunt is that it's almost certain not to be the only thing wrong.

When the shunt died, the current could have further overloaded other parts of the meter. Depending on the type of meter, the movement itself may have been damaged. That may lead to very odd readings on other scales (mostly very low I would suggest)

Fixing the meter is probably (at best) a project for a rainy day, more of a challenge than an exercise in getting a working piece of test equipment. You'd always have to ask yourself "how much can I rely on the readings I get from this meter?"

You can determine if the movement is damaged by figuring out it's sensitivity (typically the most sensitive current range, or from the ohms/volt rating), and placing the disconnected meter in series with another known good meter to see if they read the same value. If this test fails, then you have a $1 doorstop.

If the meter movement is good, then you have a puzzle. If you like puzzles, then that may be a good thing

 

[Dopey_Robot]

I think I'll leave it be for now, it's a little beyond my capabilities at this point.