Author Topic: Uni-T UT139C repair beeping continuously  (Read 271 times)

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Offline Falkra

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Uni-T UT139C repair beeping continuously
« on: April 08, 2021, 03:49:55 pm »
Hello,  :)

I have a Uni-T UT139C and I am trying to repair it.
It is faulty : it beeps continuously on every position of the contact switch and the display has every segment ON or does not display anything at all (but the beeping continues).

I reprogrammed the 24C02A eprom with factory code (it was modified, probably with a hack that went wrong) using code from this topic, using an eprom programmer).

I checked for shorts and found Q2 Q3 Q8 and Q9 transistors are shorted when on circuit. Out of circuit they check good. Removing all four didn't clear the short on the motherboard.
C16 cap is shorted when meter is set to OFF using selector switch, but not when ON. Contacts of the selector switch are clean and I cleaned them, same for the zebra strip screen contacts.
Pictures (not of mine) can be found here : https://lygte-info.dk/review/DMMUNI-T%20UT139C%20UK.html


I'm a hobbyist, and I'm making progress, I unsoldered and resoldered the SOT-23 transistors correctly with an iron (no hot air this time) and that makes me happy, it's the way to learn, trying on inexpensive stuff.  :)

I'm thinking maybe the main chip/cpu is dead.
But I don't now how to check this. I'm looking for clues.


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I don't know if I'm in the good topic but since the forum is back again, I cannot be logged in my latest topic, and only that one (I needed to edit an error).
https://eevblog.com/forum/repair/ut139c-repair/
I can reply in every other topic, including my old topics, except that one. When I go to that specific topic I logs me out and I can read but never post. If I manually log in, it doesn't work either and the "www" disappears in the url.  :-//

Topic can be deleted if needed and I start another one. Sorry for the duplicate.
« Last Edit: April 15, 2021, 07:54:24 am by Falkra »
 

Offline Falkra

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Re: Uni-T UT139C repair beeping continuously
« Reply #1 on: April 20, 2021, 04:07:11 pm »
Any recommendations ?  :)
 

Offline ogdento

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Re: Uni-T UT139C repair beeping continuously
« Reply #2 on: April 21, 2021, 09:47:51 pm »
I don't know much about the Uni-T meters but it seems to have a DTM0660 dmm chip.  Kerry Wong translated the datasheet to English a few years back when he hacked a meter with this chip.

http://improwis.com/projects/reveng_multimeters/#DTM0660LDreamTech

Use the chip info to help you find any signs of life... e.g. make sure the clock oscillator is working (probably is if you're seeing all the lcd segments), and if you have a scope or a buspirate you can probably see if there's any i2c activity between the rom and the chip.
 
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Offline ogdento

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Re: Uni-T UT139C repair beeping continuously
« Reply #3 on: April 21, 2021, 09:55:29 pm »
and Q2, Q3, Q8, Q9 are part of the input protection... if you think they're blown, I would remove them while you're doing your testing. (just realized you already removed and tested them)  They can appear shorted because they're wired up something like this:



« Last Edit: April 21, 2021, 09:59:27 pm by ogdento »
 
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Offline Falkra

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Re: Uni-T UT139C repair beeping continuously
« Reply #4 on: April 23, 2021, 04:27:10 pm »
Thank you, I will give it a try.  :)
 

Offline ogdento

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Re: Uni-T UT139C repair beeping continuously
« Reply #5 on: April 23, 2021, 06:28:00 pm »
hey you're welcome!

the photo of the back of the board (https://lygte-info.dk/pic/UNI-T/UT139C/DSC_2731a.jpg) shows an unpopulated header in the top right that might have the i2c sda/scl lines already broken out for you.  One of the pins is labeled "sda" (presumably i2c data line), and another is labeled "slc" - I'm wondering if "slc" is a mis-print and should be "scl" (the i2c clock)?
 


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