Uni-t ut105  multimeter repair  help needed  - Page 1
Forum section: electronics / repair

author: Jogas
author_rank: Contributor
timestamp: May 07, 2019, 04:41:22 pm
content: I just put battery 9v in wrong polarityJust replace fuse and still no luck.I was thinking when I put battery in.please any ideas about what I can do about.

author: mzacharias
author_rank: Frequent Contributor
timestamp: May 08, 2019, 02:03:25 pm
content: I would at least look for a shorted protection diode. Most smaller DC powered items would have such a diode to protect IC's etc downstream.

author: PKTKS
author_rank: Super Contributor
timestamp: May 08, 2019, 03:38:28 pm
content: Quote from: mzacharias on May 08, 2019, 02:03:25 pmI don't think so.. these cheapos go directly to a small LDO regulatorusually using a small input filter cap.In general terms with luck the LDO is goneand no voltage output.Replacing that LDO (3 pin IC direct from BAT) should be top listAnother lucky shot is to power the unit w/LAB PSU in theplace of the removed LDO... to see if damaged occurredPaul

author: Jogas
author_rank: Contributor
timestamp: May 08, 2019, 04:35:06 pm
content: I just add photo.

author: PKTKS
author_rank: Super Contributor
timestamp: May 09, 2019, 12:58:43 pm
content: PHOTO helps but the thing is to trace the BAT leadsdirectly to the regulator.I had problems w/a brand new VICTOR 1000A clamp meterand after extensive hacking I found that problems were justbad solder joints (lead free cold joints)a rework of an almost impossible to find LDO and other stuffsolved all issues - the LDO is indeed 3.3 and ive spent quite sometime with that crappy regulator not working.Now i know that just in case it fails it is indeed a 3.3 LDO.suitable match easy to find - in your case you need to locateand identify that front end.. all chepos are likely to have somesort..That V1PH can not be found anywhere... indeed 3.3 LDOyou may notice a fat LEADED solder blob ive added to that joints.since then no more issuesPaul

author: Jogas
author_rank: Contributor
timestamp: May 09, 2019, 01:38:12 pm
content: On my multimeter almost no componentscomponents.no idea what's happen whenvoltage coming to switch and going to resistor one track other to capacitor.under screen I see just some mold chip I add photo.

author: Jogas
author_rank: Contributor
timestamp: May 09, 2019, 01:59:17 pm
content: Some voltage I checked they around 9v batt voltage pic added

author: daedalux
author_rank: Contributor
timestamp: May 09, 2019, 08:55:03 pm
content: 1) Check any visible diode for 0,6 forward or less and maximun voltage drop in reverse with another multimeter in diode mode.2) Check continuity for any resistor that may be in the power path. Anything there should be just a few ohms.3) Install the battery correctly and assure it's good. Then check all ICs starting with the three pin ones for any kind of heating that you could possibly discriminate with your thumb. If it's hot it's bad, or could not be hot and be bad anyway. The regulator has lots of chances of being the culprit.

author: Jogas
author_rank: Contributor
timestamp: May 10, 2019, 08:13:59 am
content: Here what get hot

author: mikerj
author_rank: Super Contributor
timestamp: May 10, 2019, 11:49:16 am
content: SMD code 062C is a TL062 Op-amp (full part number TL062CD). Being warm could indicate the reverse polarity damaged it, but unless it's pulling the 9v rail down to a low value I wouldn't expect it to stop the main ASIC and display from working.