Products > Test Equipment
EEVblog 121GW Multimeter Issues
Andrew McNamara:
--- Quote from: PA0PBZ on May 23, 2021, 06:37:22 pm ---Looks like the marked part of the shielding should not be there - manufacturing error?
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I'd say possible assembly error, but also a design shortcoming - there is a layer of plastic over the shielding, but it's too tight in that area and it appears the shielding hit the top of the PTC, nicked it and crumpled up slightly instead of going down beside it - and there's no way to tell if this has happened as the meter is (re)assembled.
J-R:
Just checked the ones I have and the shield is firmly attached to the back cover and quite a bit away from PTC3.
EEVblog:
--- Quote from: vyruz on May 23, 2021, 04:49:00 pm ---Hi everyone.
It seems my 121GW just broke, and I have no idea how or why :S
I wanted to measure a simple 230V AC mains socket, so plugged my leads in to the COM and V sockets, set the DMM to V, and the mode to AC, but when I plugged the leads into the mains socket, there was loud POOF and scorch marks on one of the lead tips. Strangly the DMM display kept working, but displaying no voltage. I tried another DMM (Uni-T UT61E) and that one just correctly displayed 230V AC.
So I opened up the 121GW, and to my horror noticed the typicall smell, strangly, both fuses were still intact. Further disassembly showed burned/exploded components at the U17, C61, R131, C69, C68, C80 and PTC3 positions.
So 2 questions here: What have I done wrong? I'm 100% sure I connected the leads to COM and V, the DMM was set to correct mode, I can't think of a reason why something like this would happen.
Secondly, have I got any change at repairing this DMM? Replacing those 7 components at the earlier listed positions (given I can find the correct parts/values somewhere) is something I'd feel confident enough to do, but how high is the chance other components will have failed invisibly, and would the DMM still be thrusworthy calibration-wise afterwards?
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When did you buy this meter?
Serial number?
EEVblog:
--- Quote from: J-R on May 24, 2021, 02:03:16 am ---It appears the edge of the shield was cutting into the heat shrink around PTC3. Assembly or re-assembly error?
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The metal shield side would have to have been folded over somehow and touching or very close to touching a terminal of the PTC.
Like the outer PTC3 was actually behind the plastic shield and now on the metal side.
Can't see any other way this could have happened. Just cutting into the top of the heatshrink wouldn't have been enough for something as basic as 240V mains. Unless fo course there was a combination of this plus a sizeable tranisent to breakdown the clearance/creepage.
J-R:
Looks like PTC3 was pulled away from PTC4. On mine they are still stuck together, which keeps PTC3 out of the way. So it seems plausible the metal shield had been doing some cutting into PTC3.
Maybe vyruz can tell us if the DMM had been previously disassembled? Or they may not want to admit that...
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