Dead UT71E? Help needed to repair or discard it. - Page 1
Forum section: electronics / repair

author: Flyfish
author_rank: Newbie
timestamp: May 24, 2016, 12:24:44 pm
content: 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.

author: PedroDaGr8
author_rank: Super Contributor
timestamp: May 24, 2016, 12:59:25 pm
content: 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.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 24, 2016, 04:46:51 pm
content: 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.

author: Flyfish
author_rank: Newbie
timestamp: May 24, 2016, 05:36:55 pm
content: 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=30953I'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.

author: retiredcaps
author_rank: Super Contributor
timestamp: May 24, 2016, 10:31:35 pm
content: Your regulator is probably U7 in the schematic. The ES51966 says it uses a 5V power supply as perhttp://www.cyrustek.com.tw/spec/ES51966.pdfA 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

author: joeqsmith
author_rank: Super Contributor
timestamp: May 24, 2016, 11:04:58 pm
content: 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.

author: Flyfish
author_rank: Newbie
timestamp: May 25, 2016, 07:56:56 am
content: 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.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 25, 2016, 11:31:34 am
content: 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!

author: PedroDaGr8
author_rank: Super Contributor
timestamp: May 25, 2016, 01:49:05 pm
content: Quote from: Flyfish on May 25, 2016, 07:56:56 amAs 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.

author: Flyfish
author_rank: Newbie
timestamp: May 25, 2016, 03:57:48 pm
content: Quote from: joeqsmith on May 25, 2016, 11:31:34 amThat "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.Quote from: PedroDaGr8 on May 25, 2016, 01:49:05 pmI 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!

author: joeqsmith
author_rank: Super Contributor
timestamp: May 25, 2016, 04:49:23 pm
content: 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.

author: Flyfish
author_rank: Newbie
timestamp: May 27, 2016, 08:14:49 am
content: 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.

author: Wytnucls
author_rank: Super Contributor
timestamp: May 27, 2016, 10:29:16 am
content: The ICs are protected by crowbar transistors or diodes. One of those may have failed shorted. Might be worthwhile checking them.Q3/4/5/6Q13/15/17/18TQ1/2

author: joeqsmith
author_rank: Super Contributor
timestamp: May 27, 2016, 11:35:13 am
content: 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.Quote from: Flyfish on May 27, 2016, 08:14:49 am

author: joeqsmith
author_rank: Super Contributor
timestamp: May 27, 2016, 11:44:31 am
content: 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!

author: Flyfish
author_rank: Newbie
timestamp: May 30, 2016, 02:52:41 pm
content: Quote from: Wytnucls on May 27, 2016, 10:29:16 amThat'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.Quote from: joeqsmith on May 27, 2016, 11:35:13 amI 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.Quote from: joeqsmith on May 27, 2016, 11:44:31 amThat 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!

author: Flyfish
author_rank: Newbie
timestamp: June 01, 2016, 08:03:02 am
content: 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!

author: user99
author_rank: Contributor
timestamp: June 01, 2016, 07:45:53 pm
content: 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-920https://sigrok.org/wiki/Tenma_72-7730

author: joeqsmith
author_rank: Super Contributor
timestamp: June 01, 2016, 10:54:52 pm
content: 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.

author: Flyfish
author_rank: Newbie
timestamp: June 02, 2016, 08:55:39 am
content: Quote from: user99 on June 01, 2016, 07:45:53 pmAmazing 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...Quote from: joeqsmith on June 01, 2016, 10:54:52 pmYes, 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.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 02, 2016, 11:53:42 am
content: 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.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 20, 2016, 11:28:40 pm
content: 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.

author: PedroDaGr8
author_rank: Super Contributor
timestamp: June 23, 2016, 12:50:31 am
content: I'm curious as well, just thought to check back and seeSent from my LGLS992 using Tapatalk

author: joeqsmith
author_rank: Super Contributor
timestamp: June 23, 2016, 02:54:09 am
content: Hammer

author: joeqsmith
author_rank: Super Contributor
timestamp: June 23, 2016, 10:26:13 pm
content: It's a UNI-T isn't it?Quote from: blueskull on June 23, 2016, 02:57:47 am

author: joeqsmith
author_rank: Super Contributor
timestamp: June 23, 2016, 11:24:54 pm
content: The Keysight I tested was far more robust than the 181A. Not suggesting yours would be. It's a different model and you would hope with the higher cost they would do a better job with it. But like the Fluke 87V vs their cheapest 101 model, you just never know until you try them.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 24, 2016, 12:39:29 am
content: I have seen my fair share of MOVs coming apart.You have the PTCs there to limit it. I did not take apart the 298 when I had it to compare with the 181. Are there two in series or in a "Y" and just one of them is bad or didn't you check?They are supposed to warranty for life, or at least that's what I keep seeing posted. Strange they would not just issue an RMA and get you a quote on repairs.The 181A used a coin cell battery. They had some fairly good protection in the meter (not just for UNI-T but in general) but the layout has a few problems which seems to cripple it. Not a bad meter but I would have gladly paid another $50 to see it certified to the EMC standards, a more scratch resistant LCD cover, AC adapters for the charger and a spare battery. Or even better, had used a standard set of batteries.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 24, 2016, 12:53:58 am
content: There is no local service center for Fluke in China? That's a little odd being most are made there now. I would have thought your local distributor would have handled it for you.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 24, 2016, 03:35:20 am
content: Not sure why it would be a problem if you were the original purchaser and had the POP. Are you using the new meter to check the old? Curious if you really ran into the problem with normal use or if you where playing with the new toy and needed something to try it on. If not, I am surprised you are using a meter like this on voltages that would damage the MOVs. Let's here the real story...

author: Flyfish
author_rank: Newbie
timestamp: August 03, 2016, 09:13:51 am
content: Quote from: joeqsmith on June 20, 2016, 11:28:40 pmQuote from: PedroDaGr8 on June 23, 2016, 12:50:31 amSorry guys for the delay. My hammer (the big one) is about to make presence. And it's not because I wasn't able to fix it, or definitely discard the UT thingy. I'll explain myself:I received the IC recently:http://es.aliexpress.com/item/LSD0007-patch-QFP64-MSP430V103IPMR/32661954515.html?spm=2114.13010608.0.93.fifAzGAfter that, I read carefully the datasheet in order to figure out if it might (in my best wet dreams) come ready-to-work out of the box. But the datasheet is really hard for my english and electronics level, and, so far now, I'm not sure. That's all. I don't even dare to solder it because, if it isn't programmed, I'd have to de-solder it and try to programe it somehow. Too much motherboard warming and eye-hurting work.Another sidedown is if, it's not programmed (as I really think), I don't even have the tiniest clue of how to do it, not to speak how to get that kind of firmware.I have also checked the voltage in 3.5 and 5.5 after I lifted the shortaged legs from the IC and I resoldered the removed componentes, and they aren't so consistent. I need to do further checking.So that's my thin advance so far. I'm stuck with the solder iron in one hand, and the hammer in the other.Thanks for checking the thread. Don't fear, I'll have you updated as soon as I get some significant update.Regards!

author: joeqsmith
author_rank: Super Contributor
timestamp: August 03, 2016, 10:24:59 am
content: Thanks for the update. Good to hear you were able to at least get a replacement part. I would have guessed it was blank as well. I also would have guessed UNI-T programs them in-circuit.QuoteThere may be more than one bad part.

author: iXod
author_rank: Frequent Contributor
timestamp: August 03, 2016, 05:38:47 pm
content: Quote from: joeqsmith on June 23, 2016, 11:24:54 pmor, "you just never know until you fry them."

author: Macbeth
author_rank: Super Contributor
timestamp: August 03, 2016, 07:58:56 pm
content: Quote from: Flyfish on August 03, 2016, 09:13:51 amYou would not have to desolder it to re-program it, there is an 8 pin programming port labelled IO1. This is known as a "Bootstrap Loader" or BSL port for the TI MSP430 chips.Of course if your replacement U2 hasn't been pre-flashed then the problem is getting the firmware to program it with! (and you would have to make a programmer too)Anyway you have nothing to lose replacing the chip. If you don't have a hot air rework station then careful use of a scalpel from the top of the QFP pins, taking care not to cut the PCB or force the legs with a blunt scalpel to rip pads from the board. Then pick up all the cut off legs using a gob of solder on the tip. Some solder wick and flux would be handy to clean up and you will probably need them when you solder on the replacement too.If you find yours is not pre-programmed then you need to find someone with a UT-71E and 'l33t cracking skillz then you can read from the BSL port of his meter!(or you could ask UNI-T if they would send a pre-programmed one to you. You would be surprised, I got a replacement programmed 8051 MCU for my Maynuo DC Load and fitted it yesterday - worked first time!)