121GW strange range/socket problem

[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

[Morg]

Jack detection works fine; plugging an additional jack (not connected) while measuring on either jack doesn't change anything.

Not surprisingly, flashing firmware again didn't change anything.

Concerning the schematics - I couldn't find an obvious defect causing this behaviour. Any voltage divider would have to be way out of spec to result in "proper" 100-fold voltages (as in 50 mA -> display 5 A, 25 mA -> display 2.5 A etc).

[J-R]

I reproduced the issue exactly by sending 50mA through only the upper half of the mA/uA jack while leaving the lower half open. 5A displayed.  So it's definitely the "Input Warning Circuit" not working correctly on the mA/uA jack causing it to think you're connected to A/500mA.  Inspect the jack itself and then expand to the surrounding PCB if necessary?

[Morg]

You're absolutely right - connecting only the upper half yields 100x the value; connecting only the lower half doesn't yield anything. Not sure what I'm achieving by inserting a 4mm banana plug and bending the top end towards the 121GWs lower end, but with enough - soft - force I get correct readings.

So this sounds a bit like a bad contact, having problems connecting both sides at once? Any experience which (mechanical?) work might remedy this? Bending the "half tongues" (or the lower one) - but how? It's quite tight in there, and I'm not sure how to remove the main pcb without damaging the central knob fastener.

[J-R]

The PCB has a tendency to hang on to the selector shaft, so just wiggle it while pulling and it should come free.  Good teardown photos here: https://lygte-info.dk/review/DMMEEVBlog%20121GW%20UK.html
Essentially, you remove the battery cover, back cover, the black PCB screws (not the LCD ones), then the PCB.
And to clarify, you do not remove the black portion of the selector from the PCB.

[Morg]

Did some disassembly, but found no apparent error. Tried resoldering the jack and the black thing next to it. Could not find a way to bend the jacks inside contacts, especially as I didn't find a tool slim and strong enough to get it between the metal and casing.

Symptom persists, but can - partly - be remedied by bending the jack upward.

Any further hints? Knowing source and symptom, it will surely be possible to just work around the issue, but for equipment of this class and price, this seems unsatisfactory...

[electr_peter]

Have you tried different banana plugs? Some 4mm plugs are stiffer and wider, maybe it will help.
Also, try inserting unshielded 4mm plug while DMM is disassembled - cracks could be visible, if there are any.

[J-R]

Which test leads are you using, the Brymen ones with the lantern style or the UEi ones with the 4-way split?

[Morg]

I have only test leads with these jacks -> https://media.rs-online.com/image/upload/bo_1.5px_solid_white,b_auto,c_pad,dpr_2,f_auto,h_399,q_auto,w_710/c_pad,h_399,w_710/F3469064-01?pgw=1

At the moment I don't have alternatives to test with, but up to now I never had contact problems (also not with the 121GW...)

I'll try the hint with the cracks...

[J-R]

Mechanical issue, should be easy to resolve with some more tinkering.  If it's a cracked jack Dave has replacements available.

[Morg]

I can't see cracks on the jack in question for sure. I can see the whole jack fixture is cracked right in the middle, though I'm not sure if this is relevant to my problem.

Is there a special way to ask for replacements or should I just write to Dave?