Products > Test Equipment

121GW strange range/socket problem

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Morg:
Hey,

my 121GW has done splendidly for some time now (190505xxx serial), but today I got irritated -

while measuring current (expected ~3.5 mA), it consistently showed somewhere between 300 and 400 mA. A second analogue multimeter confirmed the current being around 3-3.5 mA.

Later I tested the 121 by connecting to my PSU (current limited to 50 mA), and independent of selected range the displayed current always was at around 5 A. (Mind you, connected to the socket with a 400 mA fuse...)

After using the leftmost socket, it hat the same reading as the PSU at 48 mA over all ranges.  A quick crosscheck again with the second socket showed the error again.

I did accidentally enter CAL mode by holding MODE on switching on, but I didn't press REL to enter calibration but immediately shut down. Just to be sure, I restored an old calibration backup with no effect.

Any ideas?

electr_peter:
Just an idea - did DMM showed wrong current reading when mode selector switch was set to different range than current input socket? Say, uA range on selector switch and "A" current socket or A range and "uA/mA" current socket

J-R:
The 121GW uses jack detection to switch between current modes.  Maybe the jacks need to be cleaned?

Morg:
What irritates me is that it displays almost exactly the 100-fold value, so 5000 mA instead of 50 mA. Led me to believe this could be a range problem as Peter mentioned, but there is no 100x range jump between sockets, only 1000x.

Maybe I'll try reflashing the software...

J-R:
Due to a lack of other ideas, I still would suggest investigating the jack detection.  I was able to generate some results adjacent to your issue by having leads connected to both current jacks at the same time.   I put 50mA through the A/500mA jack then inserted a lead into the mAuA jack and it displayed 5mA instead.
Here is the 121GW schematic: https://www.eevblog.com/wp-content/plugins/download-attachments/includes/download.php?id=11618

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