UT139S Multimeter diode mode shorting in open circuit

[JACOPO99]

Hi all,

a couple of days ago I tried to check a capacitor and noticed that my UT139s couldn't read it, as if it was too big (470uF). Later on I had to check for a short and the leads where shorting even if not connected to anything.

I then put the diode mode on, and it reports an interesting 0.830V in open circuit.
The AC/DC measures are still ok, but I can't check for impeadance, capacitance and short-circuits anymore.

Any idea on what it could be and if is it fixable?
I inspected the PCB inside but nothing seemed browned-out.

Thanks in advance.

[Wallace Gasiewicz]

Check the input protection MOV?

[Per Hansson]

I then put the diode mode on, and it reports an interesting 0.830V in open circuit.

What happens if you put the probes together when it shows the 0.83v diode drop?

[joeqsmith]

 

It's been my experience that UNI-T are not known for their electrical (or mechanical) robustness.   The 139's I have looked at, I damaged beyond repair.  This has been typical of the brand.   My guess is you damaged one or more of the small SOT23 parts.  If you are lucky, this was the only thing damaged.  Normally, I have lost the measuring IC as well.   

[JACOPO99]

What happens if you put the probes together when it shows the 0.83v diode drop?

It goes to 0.002 and stays there. If I disconnect them, it goes back-up.

[Per Hansson]

What happens if you put the probes together when it shows the 0.83v diode drop?

It goes to 0.002 and stays there. If I disconnect them, it goes back-up.

See this thread, it's not the same meter but the input protection could be similar as the readings are:
https://www.eevblog.com/forum/repair/uni-t-ut61e-diode-mode-repair/

[JACOPO99]

must meter the cap. off-board

must meter only after fully discharge, ie. not only has been shorted but also must ensure its capacitor dielectric relaxation has been really gone off

must has tool to meter ESR determine cap. R which must be,  after its V division with series R.thev. is so low negligible  good is less than 0.7-1 Ohm

Hi, I desoldered the green caps. I only have a BM869s to test them. The resistance is between 1.2k and 1.38k for the three of them.
The blue components (they are not caps, aren't they?) report fully open circuit and infinite resistance.

I don't have an ESR meter at hand.

[Per Hansson]

must meter the cap. off-board

must meter only after fully discharge, ie. not only has been shorted but also must ensure its capacitor dielectric relaxation has been really gone off

must has tool to meter ESR determine cap. R which must be,  after its V division with series R.thev. is so low negligible  good is less than 0.7-1 Ohm

Hi, I desoldered the green caps. I only have a BM869s to test them. The resistance is between 1.2k and 1.38k for the three of them.
The blue components (they are not caps, aren't they?) report fully open circuit and infinite resistance.

I don't have an ESR meter at hand.

If you are talking about the green and blue components near the input in this picture: https://lygte-info.dk/pic/UNI-T/UT139S/DSC_0002a.jpg
Green= PTC, they are resistors that increase in resistance when they get hot (positive temperature coefficient) and around 1.2KΩ sounds in the ballpark for them.
Blue = Sparkgaps, they only conduct when the voltage is over their breakdown, like 700v or 1000v or whatever they are rated, otherwise they should read open.

P.S: Measuring actual capacitors like the SMD capacitors to the left in the picture is only useful in capacitance mode of your meter, that is located behind the diode mode by pressing the select key of the BM869s.

I would measure the components I have marked in the attached photo, from collector to emitter and then emitter to collector and reporting the result for each of them. Then from base to emitter and emitter to base in diode test mode with your meter, or better if you have one of the cheap "component testers" aka "transistors testers" test them on that.

[JACOPO99]

If you are talking about the green and blue components near the input in this picture: https://lygte-info.dk/pic/UNI-T/UT139S/DSC_0002a.jpg
Green= PTC, they are resistors that increase in resistance when they get hot (positive temperature coefficient) and around 1.2KΩ sounds in the ballpark for them.
Blue = Sparkgaps, they only conduct when the voltage is over their breakdown, like 700v or 1000v or whatever they are rated, otherwise they should read open.

P.S: Measuring actual capacitors like the SMD capacitors to the left in the picture is only useful in capacitance mode of your meter, that is located behind the diode mode by pressing the select key of the BM869s.

I would measure the components I have marked in the attached photo, from collector to emitter and then emitter to collector and reporting the result for each of them. Then from base to emitter and emitter to base in diode test mode with your meter, or better if you have one of the cheap "component testers" aka "transistors testers" test them on that.

Thanks Per for the explanation,
yes, I'm aware of the basics of the multimeter haha, but I never had hands on the PTCs and the sparkgaps.

The measurements in Diode mode requested:
Vce 0.54V
Vbe 0.0V
Veb 0.0V

This is true for all 4 of them.

Jacopo

[Per Hansson]

Alright, referring to the attached schematic, this is from the UT61E and I removed everything that is not relevant.
If we measure on any of the transistors from base to emitter with the positive probe on base we should see a diode drop of 0.6v, and if we reverse the probes it must be open.
Unless I'm misunderstanding you from your numbers all transistors are shorted, if that is so you should remove them and test if the meter then works fine.

[JACOPO99]

Alright, referring to the attached schematic, this is from the UT61E and I removed everything that is not relevant.
If we measure on any of the transistors from base to emitter with the positive probe on base we should see a diode drop of 0.6v, and if we reverse the probes it must be open.
Unless I'm misunderstanding you from your numbers all transistors are shorted, if that is so you should remove them and test if the meter then works fine.

Just checked again.
it shows 0.55 of voltage drop Vbe! On all of them.
I must have got the pinout the other way around before, sorry for that.

[Per Hansson]

Just checked again.
it shows 0.55 of voltage drop Vbe! On all of them.
I must have got the pinout the other way around before, sorry for that.

Ok, the important bit is that it must be open with the probes the other way around.

[JACOPO99]

Ok, the important bit is that it must be open with the probes the other way around.

Can confirm, they are all open the other way around.

[Per Hansson]

Ok, the important bit is that it must be open with the probes the other way around.

Can confirm, they are all open the other way around.

Ok, since schematics don't seem to exist the only way is to check the input yourself by hand, there must be a bad transistor somewhere based on your details in your first post.