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Extech Ex505 (also Craftsman 82003) calibration/ mA ammeter issue

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hunter:
Hello, I have a trusty Craftsman 82003 DMM which happens to have the same board with Extech Ex505. Recently, I just found out that the mA current measurement mode is way inaccurate ( eg: reports 50mA while it should be 150mA as checked with other DMM). It seems to have oddly large internal resistance in mA mode (5 ohms when measured  with another DMM) . This is an issue with mA mode, the 10A mode is fine and accurate. All other functions work ok.  Fuse is ok.

I wonder what sort of problems could lead to this inaccuracy, say I am trying to find out if this DMM is simply out of calibration or the mA mode is toasted. I wasn't sure if this was all along, but the only I could remember I once put a Li-ion cell across the mA mode, as soon as I saw the OL display I quickly disconnected, after checked, the fuse wasn't blow and the meter appeared to have no damage.

I see there are about 7 micro pots on the board, how do you about figuring out which one is for the mA , if that is likely possible ?
Thanks !

amspire:
It is extremely unlikely a pot would adjust from 50mA to 150mA - they probably only adjust +/- 10% or less.

It is more likely something has happened such as the mA sense resistor got cooked and tripled in value. If this is the case, the resistor probably should be something like 1.5 ohms instead of 4.5 ohms (or whatever the resistor is now).

Is the 40mA range also out or is it just the 400mA range?

I would look at the fuses in the meter and see if someone has replaced the 0.5A ceramic fast blow fuse with a higher value such as a 2A fuse.

This would allow the mA sense resistor to be overloaded. If it has been, it probably should look a little burnt.

Richard.

ModemHead:
Did you compare meters simultaneously, with both meters in series with the current source? Otherwise differences in burden voltage may have altered the current you were trying to measure.

4.99 ohms is a reasonable mA shunt value.

Wytnucls:
As mentioned earlier, there is probably something wrong with the shunt resistors or the fuse. Check both.

Lightages:
Another failed Extech EX series? What a surprise! Open it up and see if it has cold solder joints, or is falling apart, or a bodged repair job like Dave had.

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