121GW - 12.52mA Phantom Current Issue

[sambartle]

Sorry for the lack of technical knowledge I'm sure to show here, I'm a software guy who occasionally needs to measure things..
I've owned a 121GW for a few years now, and while its been very lightly used, its always worked perfectly.

I came to use it this morning to measure current on a 4-20mA device, and I have a strange issue (to me).

When I select mA on the dial, and select DC, the display shows ~12.52mA (sometimes 12.50 or 12.51) with nothing connected except the leads, if I then short the probes I get 0mA as expected, but if i try to measure a 20mA signal being generated by my device, I still only get the 12.52 mA shown.

If i select AC, I see 0.0mA on the display.

If I remove the probes, it still sits at 12.52mA

I've tried new batteries, and also re-loaded the calibration from SD from a backup I took when I first bought the device. With no improvement.

Resistance and Voltage Measurements all seem fine from what I have tested.

Anyone know what else I could try?  (V 2.05 (this is what it shipped with), S/N: 210403148, Maybe 5 Hours Use total!)

[J-R]

That does sound like something is broken, but I suppose I have some questions and comments:
- Are you using the A/500mA or mA/uA input jack?  Probably best to use the mA jack only for this.
- Any noisy RF equipment around?  Move the test leads around in the air to see if the numbers change.
- Does the display correctly change ranges when moving the test lead between the two current jacks?  It uses jack detection to do this.

[sambartle]

- I'm using the mA/uA jack (though its the same with nothing connected)
- No nothing particular, I've also had a wonder out into the garden this morning and its exactly the same there as well, movement doesnt affect the value. Waving the leads (or the meter) around has no effect
- No, moving the lead between the current jacks doesnt do anything on the display at all. It still reads 12.52mA in the A jack, with no range change (on Auto).

If I manually select the A range.. its reading correctly in the A jack though, so it does seem to be related to the mA jack.

Interestingly (maybe) with the lead in the A jack, but the meter in Voltage mode, it gives me the "LEAD" warning, but in the mA jack, no warning. 


 

[J-R]

OK, for sure that is not correct behavior.  Probably start by cleaning the mA jack, then beyond that there may be some disassembly and checking of some components against the schematic.  Search the forums for more info on this.

[sambartle]

After all this (Ive disassembled it completely and been fiddling and cleaning).. I think I have solved this.. and its stupid on my part/Non clear to a software guy.

I think the 400mA fuse (the back label specifies 440ma but this is definitly the original) is blown. As shorting out the fuse seems to resolve the issue.

I double checked the manual and nowhere does it mention that this fuse being blown will result tin 12.52mA of current (presumably from the Input detection circuit).. If the chip is expecting to see that current for a brief period to detect the lead.. could it not maybe say FUSE if thats sustained and unchanging?

Anyway.. definitly a me fault for not checking that.. Sorry for what turned out to be a waste of your time, but thank you for the advice.