Electronics > Repair

Microcurrent gold - Reprise

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Moriambar:
TIFU (well yesterday but it's the same) shorting VCC to gnd through my uCurrent Gold albeit for a couple of seconds.
Although I set a limit of 20mA on the psu, it seems that I have damaged the thing.
Since I have no skill in diagnosing and repairing such things (and have a limited supply of tools), I thought this could be a good "project" but I need your guidance.
Here's a recap:
Symptoms
After doing the wrong thing, the microcurrent (it was in the milliamp range) ALWAYS shows up negative values upon powerup with the PSU disconnected.
Nothing appears blown or damaged upon visual inspection. The LED still turns on when switching on.
I tried a couple of measurements and they seem kinda off, but I don't know how to determine whether the microcurrent is really cooked or if it's fine.
First of all it gives very low readings without taking the delta for the negative starting offset. Even after doing the delta It is off wrt my EEVBlog BM235 and Keithley DM6500 of 2-3mA over a range from 5 to 20mA. Both the other meters show the same value more or less.
The same applies for the microcurrent range where the uCurrent shows values of ~100uA less than the other multimeters (around 350 uA vs 470uA). The measurements have been done using a npn current sink roughly programmed.
Anyhow the microcurrent values are always wobbly and never stable.
They were not this way before.
Questions
Can this be my paranoia and the discrepancies justified by burden voltage of the meters? I probed with either meter and the micro current separately.
What can I do to see whether there's something wrong?
Can you help/guide me step by step plz?
Tools I have

* DM6500
* BM235
* Scope->SDS1104X-e (iirc, it's the 100MHz 4 Channel Siglent)
* Linear siglent PSU (I don't recall the model unfortunately)
* DER EE LCR meter
* Hakko FX888D soldering iron with some tipsI think other stuff don't matter, but notably i do not have a hot air gun/station.

Cheers

LA7SJA:
The schematic: https://www.eevblog.com/files/uCurrentRev5schematic.pdf

If you short the input pin, what do you measure on the output in the different  ranges?
I do remember a problem with one batch of the ground-splitting op-amp oscillating with some multimeter.
Can you measure the AC component (use the scope, Luke) on the negativ output with reference to the negative pole on the battery when using one of the "BAD" reading multimeter?
You should read this tread: https://www.eevblog.com/forum/testgear/questions-regarding-ucurrents-rail-splitter-subcircuit/msg2756182/#msg2756182 and this https://www.eevblog.com/forum/blog/eevblog-1057-current-murphy/?all

Johan-Fredrik

Moriambar:

--- Quote from: LA7SJA on May 16, 2021, 05:04:03 pm ---The schematic: https://www.eevblog.com/files/uCurrentRev5schematic.pdf

If you short the input pin, what do you measure on the output in the different  ranges?
I do remember a problem with one batch of the ground-splitting op-amp oscillating with some multimeter.
Can you measure the AC component (use the scope, Luke) on the negativ output with reference to the negative pole on the battery when using one of the "BAD" reading multimeter?
You should read this tread: https://www.eevblog.com/forum/testgear/questions-regarding-ucurrents-rail-splitter-subcircuit/msg2756182/#msg2756182 and this https://www.eevblog.com/forum/blog/eevblog-1057-current-murphy/?all

Johan-Fredrik

--- End quote ---

Hello and thanks for your reply.
I'm very aware of the issue but I wasn't effected either because I got it way before or way after (couldn't remember) the incident.
Anyhow:

--- Quote ---If you short the input pin, what do you measure on the output in the different  ranges?

--- End quote ---

* mA=>-0.21
* uA=>-0.7
* nA=>-4
and also

--- Quote ---Can you measure the AC component (use the scope, Luke) on the negativ output with reference to the negative pole on the battery when using one of the "BAD" reading multimeter?

--- End quote ---
I get a high frequency noise basically, ~100mVpp

Thanks

LA7SJA:
Hi "Moriambar"

Is this noise present if you remove the multimeter?
Do you have a picture of the scope image with and without the multimeter?
Any way you could try to remove the rail splitting op-amp and use a split powersupply (2 x 2V or two 1.5V battery)?
I think it is the LMV321 thats broken.

Johan-Fredrik

Moriambar:

--- Quote from: LA7SJA on May 16, 2021, 08:12:51 pm ---Hi "Moriambar"

Is this noise present if you remove the multimeter?
Do you have a picture of the scope image with and without the multimeter?
Any way you could try to remove the rail splitting op-amp and use a split powersupply (2 x 2V or two 1.5V battery)?
I think it is the LMV321 thats broken.

Johan-Fredrik

--- End quote ---
Hello, so here are the two waveforms:
with multimeter attached
[attach=1]

without multimeter attached
[attachimg=2]

I used the BM235 btw.

I can try to remove the splitting opamp later tomorrow I think, and I can attach it to a split power supply.

Thanks for now.
Just to learn: what makes you think it's the LMV321 that's gone bad?

Cheers

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