Uni-Trend UT70D dead (or similar) - Page 1
Forum section: electronics / testgear

author: Warhawk
author_rank: Frequent Contributor
timestamp: June 02, 2014, 10:00:55 pm
content: Hello Everyone, I tried to google some information without any success, so I give it a try here.MytrustyUniTrend 70D suddenly died when I pressed a range button (I think) and won't turn on again in 99.9% cases. When I turn a range switch to some measurement position, LCD segments go to permanent on or some of them shows random mess which remains stable or fades away after couple seconds. One of hundred tries is successful and meter turns on.I took the meter apart and could not find any obvious damage, cold soldering joint or other fault. Well, I must admit that I found some solder residues (balls) inside, but all of them were permanently stick to PCBs and could not cause a short circuit I think. I also tried to clean PCB board, especially range switch pads (apologies for terminology) and LCD conductive rubber pads, but without any sign of success.Is any of you guys familiar with this kind of problem in this multimeter series family ?There are not a too much parts to replace....PS: I bought this meter several years ago when I was a university freshman. I like the resolution and accuracy was not bad either and if there is any chance to keep this meter alive, It is worth the shoot :-)PPS: Battery should be OK (~8.8V) , I had to measure it with Agilent oscilloscopeThanksJiri

author: Lightages
author_rank: Unknown
timestamp: June 02, 2014, 10:20:43 pm
content: This might be of some help

author: Warhawk
author_rank: Frequent Contributor
timestamp: June 02, 2014, 10:32:10 pm
content: Sir, that's excellent !I don't even know why I did not try to google schematic first.

author: Warhawk
author_rank: Frequent Contributor
timestamp: June 02, 2014, 11:04:15 pm
content: By the way could you someone explain me what the configuration like Q1 and Q2 does (see circuit diagram posted before) ? I have never seen such a configuration of BJT.thanks !

author: Hydrawerk
author_rank: Super Contributor
timestamp: June 02, 2014, 11:22:32 pm
content: It must be some kind of overvoltage protection. Or a voltage stabilizer?

author: Lightages
author_rank: Unknown
timestamp: June 02, 2014, 11:32:12 pm
content: Yes, you got it right. They are voltage clamps to prevent spikes getting into the measurement chip should the fuses blow.

author: Warhawk
author_rank: Frequent Contributor
timestamp: June 03, 2014, 10:55:20 am
content: Quote from: Lightages on June 02, 2014, 11:32:12 pmWhat is a principle of operation please ? Avalanche breakdown ? Why not TVS ?

author: Lightages
author_rank: Unknown
timestamp: June 03, 2014, 08:07:59 pm
content: Yes, they basically work with junction breakdown voltage. Why this is cheaper and/or better than zeners I am not sure. Perhaps they react faster.

author: Warhawk
author_rank: Frequent Contributor
timestamp: July 12, 2014, 02:35:18 pm
content: Thank you guys for your support. The DMM works again now. The problem was in a soldering joint on battery leads together with a poor battery itself. When UT-70D is turned on, it draws more current for a while. A high resistance on power net did not allow a proper "boot" of DMM although battery still had nearly 9V.