UT805 repair - Page 1
Forum section: electronics / repair

author: mortos360
author_rank: Regular Contributor
timestamp: January 11, 2013, 10:42:04 am
content: After watching Dave's repair videos, I thought I try that too, since it looks like funSo I bought a broken UNI-T UT805 off ebay and trying to repair it now... and.. it is funbut I am stuck a bit.I also wanna mention at this point that I really don't know much about electronics.. its a hobby and I like to play with in my free time, but I never really learned it, and got hardly any experience.That said.. here is what I got so far...The unit itself seem to work fine, but the display shows only garbage. I have connected it over USB to my PC and I did a few test measurements like resistance and DC volts, and it seems to work fine.All the buttons on the front panel work too.Here is an picture of the front panel:The IC that drives the display seem to be an ES51986A and here is the data sheet:http://www.cyrustek.com.tw/spec/ES51986.pdfIt looks like this IC does also some of the measurement and since measurements (at least volt and resistance) works.. I guess the IC is not the problem, or?I looked for any visual problems like burn marks, lose connections or solder, but I cant see anything that looks wrong.Maybe some of you can point me in a direction?Thanks!

author: mortos360
author_rank: Regular Contributor
timestamp: January 11, 2013, 12:06:15 pm
content: Also another question.. where do you guys always get the service manuals?I tried to google it, but nothing.

author: ElektroQuark
author_rank: Unknown
timestamp: January 11, 2013, 02:16:12 pm
content: I would check first contact between display and drivers,it looks like a mechanical (solder, connector,...) problem.

author: mortos360
author_rank: Regular Contributor
timestamp: January 11, 2013, 03:22:09 pm
content: I checked them.. I also checked the driver... everything seems fine.There is a IC for the keyboard and display interface:http://pdf1.alldatasheet.com/datasheet-pdf/view/102634/NEC/UPD8279C-2/+0037W7VlDwaUShwNDGKpw+/datasheet.pdfHe seems to actually sends commands to the display.. not sure I can test it, but I am looking into that now.

author: Chipset
author_rank: Contributor
timestamp: January 11, 2013, 05:08:52 pm
content: Probably not applicable but I have an HP 3478A that acted much the same way, turns out the +-15V rails for the analog stuff where completely dead. Presumably the regulators popped but I didn't bother troubleshooting, since it was just using 3 pin regulating IC's it was all of like $5 worth of components to replace the entire regulating circuitry.As Dave says, the first rule of troubleshooting is check voltages.

author: PA4TIM
author_rank: Super Contributor
timestamp: January 11, 2013, 05:53:44 pm
content: Repairing can be fun. If you have a manual and schematics.The schematics are all over the internet on ites like Bama, KO4BB, Kurts, eservice ect.BUT, they have manuals of A-brands like HP, Keithley, Tektronix, Fluke ec. They allways had very good manuals including theory of operation, trouble shooting, schematics, board layouts, partlists ect. Service was very important. Agilent anfpd Tek still have pdf's from old equipment (like scopes from the 80's) Some manuals are over 100 pages. Tek even made books about repairing their gear in general.But even from those brands it is hard to become manuals from rather new equipment .A brand like Unit-T is only interested in low pice high volume. Not in service. And many Chinese brands have no problems violating patents so a publishing a schematic/manual is not always wise. I doubt if you will find a manual and schematics.If you want to learn reparing, get some 60's-70's transistorised equipment from a known brand and allway first look for a manual. ( at this time in your "carreer" . When you know more, and know more people the risk of starting with rare instruments without a manual is sometimes worth a try.And do not forget, if you are repairing instruments you most times need instruments that are a grade better then the one under repair.Then the general rules;1) check the powersupply rails on voltage and ripple2) check the powersupply rails on voltage and ripple3) check the powersupply rails on voltage and rippleAs long as they are not perfect you can not go on with other problems.This sometimes even leads you to the main problem that is pullimg the line down.Instruments needed for repair of instruments in generalAlways:- scope with 10X probe and 1X probe- multimeterFor many repairs- function generator and if you do RF a RF signal generator- counter- LCR meter- ESR meter ( if you only have a simple LCR meter, with a really good one you can measure D, loss angle and/or ESR)For multimeter repair also- logic analyser ( if it is much digital stuff)- standards ( volt, current, resistance)For transmitters/receivers and RF gear a spectrum and network analyser comes in handy.But do not let me scare you off, i started also with nothing ( 10 MHz scope and Fluke 77) and with some luck I most times still managed to get them going but the more gear I got, the more knowledge and experience the easier it became. But i started mainly with old tube equiped gear. That is rather bomb proof. If you cause a short or make a mistake most times no dammage occures. In this Unit T one slip of a probe can do much harm.

author: mortos360
author_rank: Regular Contributor
timestamp: January 11, 2013, 08:29:10 pm
content: All the power rails looks good (there are testpoint for different rails)... just a bit off for .5 - 1v here and there... but looks ok I guess.All the connections look ok.. the display shows even what he is supposed to show.. but it does it just wrong. Its like some segments cant be turn on.. but they can!For example when you start the unit it should write UT-805 I guess... but I see only Ut- and then garbage... but the segments that are necessary to display 805 are there.. some other functions an trigger it.Also there are some setting that req. the display to write "Err-1" I see "rr-1" and the first digit is dead... but this first digit can be turn on, by some other functions...Its like the data that comes in into the display get interpreted wrong.. and I dont even know how to test that :/

author: Psi
author_rank: Super Contributor
timestamp: January 11, 2013, 08:34:29 pm
content: Quote from: mortos360 on January 11, 2013, 08:29:10 pm1V low on a rail can be quite critical.

author: mortos360
author_rank: Regular Contributor
timestamp: January 11, 2013, 09:23:00 pm
content: Quote from: Psi on January 11, 2013, 08:34:29 pmHmm.. I didn't think that it would be.I got only 1 DMM here and I have to trust it... Ill check this rails later again and post exact numbers here.

author: PA4TIM
author_rank: Super Contributor
timestamp: January 11, 2013, 09:38:06 pm
content: QuoteWhy you think I wrote it 3 times.

author: mortos360
author_rank: Regular Contributor
timestamp: January 11, 2013, 11:48:38 pm
content: Thanks for all the feedbackI checked the rails again and they are all fine... only the +30 rail has a bit more, its 31.4VAlso there is a 2.4V + and - rail which I thoguth was 3V, so thats also in spec now.How posible is it that the display driver chip (which is definitely the UPD8279C and not the ES51986A as I thought at first) is actualy corrupted?Could that couse such problems or would the chip just stop working if there was something wrong in it?

author: mortos360
author_rank: Regular Contributor
timestamp: January 15, 2013, 01:55:14 pm
content: Here a small updateI finally managed to repair the unit. Problem was display driver IC which is not soldered on the board, but instead in a socket.Somehow the leads oxidised or something.. they had some dark coating on them, and after cleaning them up, the display was perfectly fine. I did some testing with it for the last few days.. like heating it up and cooling it down, pushing and bending the board lightly... but it stays stable.. so I guess the IC leads ware really the problem.I dont understand it really, since the first thing I did back then, was a continuity test on all IC leads to the display connector and they showed all as fine. How ever.. its working again

author: Bored@Work
author_rank: Super Contributor
timestamp: January 15, 2013, 05:52:44 pm
content: Do you have some images from the inside of the UT805? I don't think we ever saw the inside of a UT805 here.

author: mortos360
author_rank: Regular Contributor
timestamp: January 15, 2013, 06:03:02 pm
content: Quote from: Bored@Work on January 15, 2013, 05:52:44 pmI can make some tomorow and post here.

author: Bored@Work
author_rank: Super Contributor
timestamp: January 15, 2013, 06:20:51 pm
content: Quote from: mortos360 on January 15, 2013, 06:03:02 pmThanks.

author: mortos360
author_rank: Regular Contributor
timestamp: January 16, 2013, 10:49:39 am
content: Here are the pictures from the UT805.I apologise for the blur, but I have no tripod here.http://img21.imageshack.us/img21/2332/dsc3070c.jpghttp://img818.imageshack.us/img818/5889/dsc3071c.jpghttp://img827.imageshack.us/img827/3958/dsc3072c.jpghttp://img191.imageshack.us/img191/4109/dsc3073c.jpg

author: flolic
author_rank: Frequent Contributor
timestamp: January 16, 2013, 11:13:14 am
content: I'm glad that you repaired itIt looks awful inside

author: mortos360
author_rank: Regular Contributor
timestamp: January 16, 2013, 12:54:26 pm
content: Quote from: flolic on January 16, 2013, 11:13:14 amheh.. that was my first thought too when I opened itAt first I thought somebody has already try to repair it because of all the cables and ugly looking soldering on the cables, but I thinkits just produced that cheapBut it works fine.. I have an brand new Agilent U1272A that I used to compare different measurements and they both give samereadings. So not too bad for a €20 DMM