UT139S Multimeter diode mode shorting in open circuit - Page 1
Forum section: electronics / repair

author: JACOPO99
author_rank: Newbie
timestamp: July 19, 2022, 07:14:47 am
content: 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.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: July 19, 2022, 11:07:47 am
content: Check the input protection MOV?

author: Per Hansson
author_rank: Unknown
timestamp: July 19, 2022, 02:57:17 pm
content: Quote from: JACOPO99 on July 19, 2022, 07:14:47 amWhat happens if you put the probes together when it shows the 0.83v diode drop?

author: joeqsmith
author_rank: Super Contributor
timestamp: July 19, 2022, 03:57:40 pm
content: 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.

author: JACOPO99
author_rank: Newbie
timestamp: July 21, 2022, 02:48:59 pm
content: Quote from: Per Hansson on July 19, 2022, 02:57:17 pmIt goes to 0.002 and stays there. If I disconnect them, it goes back-up.

author: Per Hansson
author_rank: Unknown
timestamp: July 24, 2022, 09:55:54 am
content: Quote from: JACOPO99 on July 21, 2022, 02:48:59 pmSee 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/

author: JACOPO99
author_rank: Newbie
timestamp: August 04, 2022, 04:09:51 pm
content: Quote from: abdulbadii on July 21, 2022, 11:28:17 pmHi, 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.

author: Per Hansson
author_rank: Unknown
timestamp: August 04, 2022, 06:21:23 pm
content: Quote from: JACOPO99 on August 04, 2022, 04:09:51 pmIf 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.jpgGreen= 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.

author: JACOPO99
author_rank: Newbie
timestamp: August 05, 2022, 11:48:11 am
content: Quote from: Per Hansson on August 04, 2022, 06:21:23 pmThanks 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 0.54VVbe 0.0VVeb 0.0VThis is true for all 4 of them.Jacopo

author: Per Hansson
author_rank: Unknown
timestamp: August 05, 2022, 12:27:07 pm
content: 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.

author: JACOPO99
author_rank: Newbie
timestamp: August 06, 2022, 01:37:15 pm
content: Quote from: Per Hansson on August 05, 2022, 12:27:07 pmJust 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.

author: Per Hansson
author_rank: Unknown
timestamp: August 06, 2022, 02:54:08 pm
content: Quote from: JACOPO99 on August 06, 2022, 01:37:15 pmOk, the important bit is that it must be open with the probes the other way around.

author: JACOPO99
author_rank: Newbie
timestamp: August 11, 2022, 02:26:37 pm
content: Quote from: Per Hansson on August 06, 2022, 02:54:08 pmCan confirm, they are all open the other way around.

author: Per Hansson
author_rank: Unknown
timestamp: August 11, 2022, 03:29:36 pm
content: Quote from: JACOPO99 on August 11, 2022, 02:26:37 pmOk, 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.