UT71E Repair. Pretty much FIXED! - Page 1
Forum section: electronics / repair

author: OiD
author_rank: Regular Contributor
timestamp: October 22, 2016, 09:46:35 am
content: Hi all,Seeking a bit of help here! I'm trying to repair a damaged Uni-T UT71E DMM that went boom.I believe it was damaged in mV mode as the selector switch shows arcing and the pad fell off. I've fixed it with some copper tape and soldered it in place.All the buttons work, as does the selector switch and LCD / buzzer.The following modes status is;mV, damaged. OLresistance, damaged. Cycles though Ohms, K, M. OLdiode, damaged. Short.continuity, damaged.capacitance, untested.uA, damaged. OLmA, damaged. OLDC Volts works with one polarity, but shows lower values in the opposite polarity.I've checke the PTC, input resistors, all the diodes and they check out ok.A quick probe at the transistors and other sot-23 devices also appear ok.I'm tempted to buy the ES51966P IC. I can't find the ES51966A and no information on the last letter. So I'm guessing their the same?I'm open to ideas (I'ts already replaced by a Brymen) to see if it can be repaired and what else to check.Thanks to all!

author: MosherIV
author_rank: Super Contributor
timestamp: October 22, 2016, 07:05:47 pm
content: QuoteThe company does not sell the chips on the open market, only to manufacturers.The data sheets can be downloaded.

author: retiredcaps
author_rank: Super Contributor
timestamp: October 22, 2016, 07:15:32 pm
content: Quote from: MosherIV on October 22, 2016, 07:05:47 pmThe IC is available on aliexpress.https://www.aliexpress.com/w/wholesale-ES51966.htmlQuoteHaving a quick look through the datasheet, keeping in mind, I'm not an expert and everyone troubleshoots differently, I suggesting measuring pins 37 and 38 (VRH and VR).After that, I would hookup the UT71E to a DC power supply and measure the current draw. In shorted ICs, you might see current consumption as high as 80mA whereas normal current draw is probably under 5mA. I don't know the battery life expectation of the UT71E without looking at the manual.I'll have to find time to read through the datasheet.

author: OiD
author_rank: Regular Contributor
timestamp: October 22, 2016, 08:03:25 pm
content: Hi all!Quote from: MosherIV on October 22, 2016, 07:05:47 pmActually I've found some on eBay at a good price. If they are not rejects or anything strange that is...Quote from: retiredcaps on October 22, 2016, 07:15:32 pmThanks for the suggestion, both pins check out OK, -199mV and -1.25V (might be a bit low).VR1 has semi survived, 6Mohm resistor just before the IC to the input jack.I'm thinking of replacing the following;7x 1AM sot-23 MMBT3904 transistors2x 2A sot-23 MMBT3906 transistors10x Y1 sot-23 SS8050 transistors2x C3 sot-23 1SS226 Ultra High Speed Diodes1x ES51966 ICand if I can discover what the following sot-223 components are... i'll swapp them aswell;1x "55a a06" might be a PTZA06 NPN. I'm not 100% sure.1x "35a b16" that I don't have a clue what it is...The diodes all check "correct". I'f any of them are zeners then I haven't checked that.Need to check what the other IC's are and check at least the power rails...I've been comparing the suggested squematic for the main IC and poking around. I definetly think the IC is damaged.Thanks!

author: joeqsmith
author_rank: Super Contributor
timestamp: October 23, 2016, 01:51:16 am
content: QuotePost some pictures of your workmanship.What's up with VR5. Looks like the board was badly burned in this area and maybe a SOT tacked in there.

author: sanny
author_rank: Newbie
timestamp: October 24, 2016, 03:29:01 am
content: ES51966P IC ? I can offer you freely for 5 pcs. I have this original condition!

author: OiD
author_rank: Regular Contributor
timestamp: November 16, 2016, 11:26:34 pm
content: Quote from: joeqsmith on October 23, 2016, 01:51:16 amAttaching a pic of the shoddy repair. There is a via underneath and it is soldered to it. There is nothing wrong with VR5, there is a jumper at a 45º angle and an unpopulated componentQuote from: sanny on October 24, 2016, 03:29:01 amThanks Sanny, but I already bought itThe IC arrived today and I swapped it out with the damaged IC. Now the multimeter works again!Well, everything except Resistance/Diode/Continuity. The selector dial seems to work okay in every range except the resistance/diode/continuity mode. The dial is very touchy in the resistance mode, I guess I'll need to make the pad with something a bit thicker and wider.Havent tested the Wattmeter yet.I picked up a Youyue 858D+ as well for SMD rework. For 16€ it works very well, picked the express shipping and got it in 4 days.Thanks for all the help, in the end only the IC was damaged, and for a 6$ fix (wondefully packed IC in nice static clingfilm...) sounds good to me. Now I just need to sort the dial out, make the input a bit more robust and get a new fuse. Any sugestions for fixing the pad in the dial?

author: retiredcaps
author_rank: Super Contributor
timestamp: November 17, 2016, 01:59:13 am
content: Quote from: OiD on November 16, 2016, 11:26:34 pmModemhead just finished repairing a Klein MM2000 (Nov 14) with missing pads on the rotary dial. Right now, it is "behind the curtain", but I'm sure he can make it public to all on his website. Just give him a few days since he is on the road a lot.

author: OiD
author_rank: Regular Contributor
timestamp: November 17, 2016, 01:17:22 pm
content: Quote from: retiredcaps on November 17, 2016, 01:59:13 amFound ithttp://mrmodemhead.com/blog/gallery/curtain/klein-mm2000/Looks like I'll have to sand down the UNI-T pcb a bit as the red silkscreen is quite thick and then build up from there. I'll also see if I can find some thicker foil, the one I have is very thin and flexible. Modemhead's one looks quite a bit thicker.Thanks for the heads up. I'm suprised it actually works...

author: joeqsmith
author_rank: Super Contributor
timestamp: November 17, 2016, 01:24:32 pm
content: Quote from: OiD on November 16, 2016, 11:26:34 pmMy last attempt to repair the switch on the UT90A was not much better. Epoxied the foil in place and soldered. Has some solder bumps from the wires. Works good enough.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 17, 2016, 01:27:03 pm
content: Quote from: OiD on November 17, 2016, 01:17:22 pmWow, that looks real nice.

author: ModemHead
author_rank: Frequent Contributor
timestamp: November 17, 2016, 05:42:38 pm
content: Quote from: OiD on November 17, 2016, 01:17:22 pmHey there! I don't usually post stuff until I can get some explanatory text written, but that's OK.Would you believe that's the first time I ever had to do that? Usually I can patch around bad tracks with wire, but of course that won't work here. "Real" PCB repair materials seem to be on the expensive side, so I just used what I had lying around. The copper is 0.006" foil. It really is too thick, but it works out because it does not try to conform to the bumpy damaged surface below it. The glue I used is just regular hobby store CA glue, without an accelerator, I let it cure naturally. I did a test case on the corner of the PCB and it seemed to hold well.As a last step and to mitigate the extra thickness, I masked the whole area with kapton tape, then cut out an area around the two repaired places. (You can see the score mark from that operation in the last photo.) I used a small diamond file to remove some of the thickness of the copper. I later used a fiberglass scratch brush to smooth it further.The unit is still holding up fine, no quirks in the switch at all. I've been using it on the bench the last few days, even though I really don't like the meter much. I've decided not to sell, give away, or otherwise pass on this unit, because if that glue lets go at the wrong time something unexpected could happen.

author: OiD
author_rank: Regular Contributor
timestamp: November 21, 2016, 01:10:18 am
content: Well if it works how it should then it's fixedGotta say that it was a pretty nasty arc you had to fix there. In the end I re-cleaned mine and scraped away where the pad was to make its flatter and put in a bit of effor to get it as good as the other pads. Now its very flat and after flattening out the brush on the dial a bit (warped with the arc) it works flawlessly.Need to take it apart again though, must of got some dirt on the LCD contacts as a few digits on the bargraph don't show up in the LCD test anymore.I'm pretty sure it blew up when switching from Watts to offah well, back to having two DMMS again... the other two arent even worth looking at repairingTill thy next endeavor!

author: CharlotteSwiss
author_rank: Frequent Contributor
timestamp: December 12, 2020, 03:59:41 pm
content: but is it really possible to solder IC with such close pins? it will never be possible for me to be able to do such a thing. Maybe the trick is to use a heat gun?if I try it, all the pins would have continuity with the neighboring ones