Dead UT71E? Help needed to repair or discard it.

[Flyfish]

Hi there.

A few days ago my UT71E died suddenly during some measuring. I was measuring mains (230V AC), and by itself, after a beep, it changed to DC, and after that the screen went off and it didn't powered on again.

Obviously, I cheked the battery and fuses, everything ok. And I also checked the board in order to find any blown component or path. Everything seemed ok, too.

So my question is if I can check something else, in order to fix it, or if I should think about to buy a new one.

Any hint?

Thanks in advance for any asistance.
Regards.

[PedroDaGr8]

Based on Joe Smith's research check the negative five volt regulator (and possibly the positive five volt regulator). These seem to get damaged when an over voltage is experienced. Likely you had a high voltage transient come down your line (very common) and Uni-T do not have great input protection against those.

[joeqsmith]

Sorry but I don't know anything about the 71E.  As Pedro mentioned, they are not one of the more robust brands I have looked at.  Agree.  Check the power supplies.  I got lucky with the 181A. 

[Flyfish]

Many thanks for your answers.

Can you please point me to those particular components? I've found this schematic, which although it's from a Volcraft 940 is technically the same than the 71E.

https://www.eevblog.com/forum/testgear/uni-t-ut71e/?action=dlattach;attach=30953

I'm more an electrical than an electronic guy, so I'm afraid that I need a little more of guidance through the components and the way of checking them.

Again, thanks in advance.

Edit: Joe, I have just watched your 181A part 2 video. Exciting! I hope I'll be as lucky (and skilled) as you.

[retiredcaps]

Your regulator is probably U7 in the schematic.  The ES51966 says it uses a 5V power supply as per

http://www.cyrustek.com.tw/spec/ES51966.pdf

A quick search then leads to 7201U55 (U7) which is a 5.5V regulator.

http://datasheetz.com/data/Integrated%20Circuits%20(ICs)/Voltage%20Regulators%20-%20Linear/NJU7201U55-datasheetz.html

[joeqsmith]

It's a totally different from the 181a I had damaged.  U8 is your 3, U7 your 5.  You will need to trace everything out in that are.   I'm going to guess U1 at a minimum.   Hope I am wrong.

[Flyfish]

Many thanks for your answers.

I'm going to add some info after some checking.

-The first thing I can say is that the battery drains in front of my eyes. The voltage goes down about 0,1 volts every 20 sec. So there is a fault to ground somewhere.

-The voltages in U7 and U8 aren't the expected. And they show about 2,3V between legs 1-3.

-In U8 (main problem in my opinion), seem to be a shortage between legs 1-3, as they show continuity. After a short inspection of the schematic, the only way for this to happen is an internal shortage, or in the two following capacitors, C6 and C7.

Is this helpful for a diagnosys? Should I replace U8?

Regards.

[joeqsmith]

Sounds like you have made some progress.   On the bright side, this meter should provide countless hours of troubleshooting enjoyment!      

If the battery is draining that fast, I bet some parts are getting really warm to the touch. 

The 3 supply drives more than two caps according to the schematic you linked.   Lot's of ways to proceed.   You need to start breaking down the problem. 

Not knowing what equipment you have or your skill level, my only advice is  I would not use a dremel tool just yet!       

[PedroDaGr8]

Many thanks for your answers.

I'm going to add some info after some checking.

-The first thing I can say is that the battery drains in front of my eyes. The voltage goes down about 0,1 volts every 20 sec. So there is a fault to ground somewhere.

-The voltages in U7 and U8 aren't the expected. And they show about 2,3V between legs 1-3.

-In U8 (main problem in my opinion), seem to be a shortage between legs 1-3, as they show continuity. After a short inspection of the schematic, the only way for this to happen is an internal shortage, or in the two following capacitors, C6 and C7.

Is this helpful for a diagnosys? Should I replace U8?

Regards.

As Joe said, dumping that much current something is going to be getting very warm (if not hot). You should try feeling around on the board and seeing if any of the chips (or capacitors, resistors or diodes) are getting hot. Alternatively, you can start removing the suspect chips and seeing if the short circuit stops.

[Flyfish]

Sounds like you have made some progress.   On the bright side, this meter should provide countless hours of troubleshooting enjoyment!      

If the battery is draining that fast, I bet some parts are getting really warm to the touch. 

The 3 supply drives more than two caps according to the schematic you linked.   Lot's of ways to proceed.   You need to start breaking down the problem. 

Not knowing what equipment you have or your skill level, my only advice is  I would not use a dremel tool just yet!       

That "countless hours" sounds delicious!     My dremel is getting nervous, anyway...
But yes, I've made some progress. The problem, at least part of it, is in that area, so I've reduced the searching zone.

As Joe said, dumping that much current something is going to be getting very warm (if not hot). You should try feeling around on the board and seeing if any of the chips (or capacitors, resistors or diodes) are getting hot. Alternatively, you can start removing the suspect chips and seeing if the short circuit stops.

I did that "finger test", and, as you both said, the area around U7 and U8 gets really warm. But my finger is too big for that accuracy level, and I can't say for sure wich one is the guilty. If I had a DMM with ºC measuring like, for example, a UT71E, I'd be able to differenciate. Oh, wait, I own one! Oh, wait... 

So my next step, as Pedro adviced, will be to remove U8, wich in fact shows a short circuit, and see if it remains present.

Many thanks for your help. I'll keep you updated of my progress.

Regards!

[joeqsmith]

I have been branded by components more times than I care to remember....   

Once you check the part out of circuit along with the remainder of the board, post what you find. 

[Flyfish]

Not good news ahead, it seems.

I've removed several parts by now:

-U8, C4, C5, C6 and C7. The short circuit between V0-V3.5 remains.

-U10, wich has also connection with V0-V3.5, and also short circuit present.

And all this components are OK. No short circuit on any of them. Wich is to be expected, as far as the short remains without them on the board.

So now, and after a long research over the schematic, the only part that I guess that needs to be checked is U2, wich is one of the mains components, and is the manager of the measurements, I think.

What do you think should be my next step? Remove U2 and check it after? U2 seems to me a tough one, as far as its pads are really tiny. And moreover, I dont' know if it's posible to replace that component. I've seen that it's cheap (about 5$), but too dificult for me to replace. I have a hot air soldering station, but I'm not that skilled with it. Also I don't know if it comes ready to work out-of-the-box, or it needs some programming, wich should be impossible for me to do.

Anyway, I'm not afraid. I assume that my 71E is lost, so I'll be glad if any hope still stands.

Thanks for your help, and regards.

[Wytnucls]

The ICs are protected by crowbar transistors or diodes. One of those may have failed shorted. Might be worthwhile checking them.
Q3/4/5/6
Q13/15/17/18
TQ1/2

[joeqsmith]

No mention of U3.  Have you removed it and found nothing as well? 

Rather than removing parts, you could lift the supply pins and check it.   

I did say countless hours of troubleshooting enjoyment?!       No matter if I could get the parts or not, I would want to know.   If I bought a new 71F, at least it would tell me where I need to focus to improve the design.   For once that's not a typo.  No way I would get another E model after going through this! 

I would pull U2 and 3 or lift the pins but I would NOT resort to the Dremel just yet. 

Not good news ahead, it seems.

I've removed several parts by now:

-U8, C4, C5, C6 and C7. The short circuit between V0-V3.5 remains.

-U10, wich has also connection with V0-V3.5, and also short circuit present.

And all this components are OK. No short circuit on any of them. Wich is to be expected, as far as the short remains without them on the board.

So now, and after a long research over the schematic, the only part that I guess that needs to be checked is U2, wich is one of the mains components, and is the manager of the measurements, I think.

What do you think should be my next step? Remove U2 and check it after? U2 seems to me a tough one, as far as its pads are really tiny. And moreover, I dont' know if it's posible to replace that component. I've seen that it's cheap (about 5$), but too dificult for me to replace. I have a hot air soldering station, but I'm not that skilled with it. Also I don't know if it comes ready to work out-of-the-box, or it needs some programming, wich should be impossible for me to do.

Anyway, I'm not afraid. I assume that my 71E is lost, so I'll be glad if any hope still stands.

Thanks for your help, and regards.

[joeqsmith]

You know... there is always the other option....   With the parts you have removed, break out the current limited supply,  add some jumpers, set the voltage to something less than 3 and the current to 0.   Attach it and slowly increase the current until you feel what's getting warm...   

A much better way is they make a tool that will allow you to trace out a short out with supplying power to the board.   You may want to invest in something like this.    You may have been able to trace down the shorted component/s without pulling any of them!   

[Flyfish]

The ICs are protected by crowbar transistors or diodes. One of those may have failed shorted. Might be worthwhile checking them.
Q3/4/5/6
Q13/15/17/18
TQ1/2

That's a good idea. I'll check them. I'm not sure if I'll be able to do it without removing them, I'd rather in that way.

No mention of U3.  Have you removed it and found nothing as well? 

Rather than removing parts, you could lift the supply pins and check it.   

I did say countless hours of troubleshooting enjoyment?!       No matter if I could get the parts or not, I would want to know.   If I bought a new 71F, at least it would tell me where I need to focus to improve the design.   For once that's not a typo.  No way I would get another E model after going through this! 

I would pull U2 and 3 or lift the pins but I would NOT resort to the Dremel just yet. 

I haven't removed U3 because in a first look, it doesn't seem to be related to the V3.5 pin, so I priorized in checking first the related IC's.

And about U2, I think that the first thing I'm gonna do is to lift the suspect pins and see what happens with the short-circuit. If it keeps present, U2 isn't the guilty one, wich should be a relief. And if not... good news for my dremel.

You know... there is always the other option....   With the parts you have removed, break out the current limited supply,  add some jumpers, set the voltage to something less than 3 and the current to 0.   Attach it and slowly increase the current until you feel what's getting warm...   

A much better way is they make a tool that will allow you to trace out a short out with supplying power to the board.   You may want to invest in something like this.    You may have been able to trace down the shorted component/s without pulling any of them!

That test will be next, if the above ones doesn't work.
And yes, that should be a kickstarter project that could count on me for investing. 

Many thanks for your answers, I'll let you know with my progress as soon as posible.

Regards!

[Flyfish]

Dremel time, guys. Countless hours of enjoyement have ended. We have a guilty: U2.

Or at least, it seems that way. First of all, I lifted, with the solder tip, legs 62, 63 and 64 of U2 (V0/V3.5 related). The short-circuit in the board disapeared at once. And in the legs the short remained. So, as we use to say here in Spain, if it's white and bottled, it's milk. What do you think?

And now, some questions about the fixing:

- I've found the U2 IC in aliexpress. It's the same reference than mine, LSD0007. It doesn't match the upper number, but I guess it's manufacturing serial or something. Could I buy it with confidence (I ask about the part, not about the shop. I've bought many times in ali with no problem at all)?

http://www.aliexpress.com/item/LSD0007-patch-QFP64-MSP430V103IPMR/32663712602.html?spm=2114.30010308.3.49.BwXe3s&ws_ab_test=searchweb201556_10,searchweb201602_1_10037_507_10032_10020_10017_10021_10022_401_10009_10008_10018_101_10019,searchweb201603_6&btsid=f8e38bec-4427-4a3d-a896-581a727399b1

- Does it come ready to work out-of-box without any programming?

- The dremel joke is funny, but, is it really the best way to remove it? Becouse I noticed that with my JBC 14s I could lift the legs really easily, so that could be a safer mode to lift it without damaging something else in the board by nervous-dremel-accident.

The IC costs about 8$, so I'm buying it for sure, worth it. In the meantime, while it arrives, I'll solder back again the removed parts after check them.
I don't know if I'll be able to solder properly those U2 tiny legs and bring back from the death my 71E, but anyway, the travel has been exciting and learning productive.

Many thanks for your answers in advance.

Regards!

[user99]

It looks like the LSD0007 may be a TI MSP430F149.  If you're lucky the LSD0007 will come programmed with compatible firmware. 

https://sigrok.org/wiki/Voltcraft_VC-920
https://sigrok.org/wiki/Tenma_72-7730

[joeqsmith]

Sounds like you are making some good progress.  If you put the power supply back together does the 5 and 3 both seem correct?  Wonder what, if anything else was damaged.   

At home, I use a heat gun to solder parts like this.

[Flyfish]

It looks like the LSD0007 may be a TI MSP430F149.  If you're lucky the LSD0007 will come programmed with compatible firmware. 

https://sigrok.org/wiki/Voltcraft_VC-920
https://sigrok.org/wiki/Tenma_72-7730

Amazing links, lots of info. Yes, I hope I'll be lucky. But I'm afraid this device needs its own specific firmware, so it doesn't look very good...

Sounds like you are making some good progress.  If you put the power supply back together does the 5 and 3 both seem correct?  Wonder what, if anything else was damaged.   

At home, I use a heat gun to solder parts like this.

Yes, good progress, but the firmware issue looks like a dead end. I'm buying it anyway and I'll give it a try.
Next thing I'll do is wich you suggest. I'll see if anything else is damaged.

And I have a hot air soldering station, but I haven't many experience with IC's. Maybe I'll do some tests in old boards first. I've been watching some soldering tutorials in youtube, and it seems at least not impossible. We'll see...

Many thanks for your answers.

[joeqsmith]

Agree, it's a good idea to practice on some lead and lead free scrap boards.   Different thicknesses and surrounding parts.   I have done some pretty big parts by hand so it's possible.   Good luck.   Post what you find after putting the supplies back together.           

[joeqsmith]

What ever happened with your UT71E?   I figured you would have the thing running by now or you hit it with a hammer out of frustration. 

[PedroDaGr8]

I'm curious as well, just thought to check back and see

Sent from my LGLS992 using Tapatalk

[joeqsmith]

Hammer   

[joeqsmith]

It's a UNI-T isn't it?   

It's a totally different from the 181a I had damaged.  U8 is your 3, U7 your 5.  You will need to trace everything out in that are.   I'm going to guess U1 at a minimum.   Hope I am wrong.

Even a $350+ UT181A is still vulnerable to transients?