Extech LCR Meter help with blown fuse detection - Page 1
Forum section: electronics / testgear

author: mlewus
author_rank: Newbie
timestamp: April 06, 2020, 01:04:27 am
content: Do any of you know how blown fuse detection works on multi meters, or have a schematic for any Extech LCR or for that matter any multimeter that has blown fuse detection? Here’s the story:I accidentally brushed 30V across the input lead to my Extech LCR meter model 380193 while in capacitor measurement mode. The source was a CC/CV power supply set to 30V/0.5A. The unit still works and correctly measures capacitance (at least), but when you power it on and it does it’s initialization the FUSE warning comes on and it refuses to proceed any further. The only way to bypass the warning is to turn the power off, connect the DUT and power on. This apparently prevents it from doing its normal power up checks. in this mode it does work so clearly the front end, microcontroller and ADC circuitry is working.The fuse measures OK but I replaced it anyway, no effect. I checked all of the passives and discrete semis and I found one bad sot-23 NPN transistor used in a collector coupled pair as an avalanche voltage limiter strung between the 5V rail and the negative meter terminal. I replaced it and checked the 5v rail which is fine. All of the other resistors capacitors diodes bipolars and fets check out. The only thing that seems questionable is a PTC on the capacitor measure input which reads about 2.5 K. Does anyone know if that’s normal? All of the voltage rails are reasonable for a 9v battery powered device: +8.1v, +-5v(analog),+3,3v(mcu) +1.2v(lm385 ref). I also did diode checks of all of the analog ICs, no shorts. This device uses a single ADC with switched inputs. I have a suspicion that one of the analog switches is blown but I really need a schematic to get much further.I tried to get a schematic but so far no response from Extech. The meter is five years old but a new one is over 200 US bucks so I really don’t want to replace it. Any help would be much appreciated.Thanks

author: mlewus
author_rank: Newbie
timestamp: April 08, 2020, 04:10:03 am
content: It gets better... No luck getting a schematic so I did more digging. The Extech measures both before and after the fuse, separately, I guess this is to check fuse integrity. It uses a couple of CD4052 CMOS dual 4 to 1 analog muxes, with one CD4052 connected to various points from the pos input and another from the neg input. One of those mux lines is connected directly to the positive meter jack upstream of the fuse through a 10k resistor. With a cap applied to be measured there is around 1v p-p across the meter jack. But if I measure at the CD4052 side of the resistor I measure 0 volts. Looks like the voltage spike released the magic smoke and the CD4052’s input line is shorted to ground. We all know how those old CMOS chips just love overvolted inputsI ordered replacements, hoping that will work. Failing that I’m thinking it’s time for a shiny new B&K 880.

author: radistx
author_rank: Newbie
timestamp: January 06, 2025, 11:06:47 pm
content: Hello, I have the same problem with 380193 meter showing Fuse error. I used 2 NPN low voltage transistors for replacement of Q20 and Q21 but I put in TO-92 package. Could it be the problem?

author: Tjuurko
author_rank: Regular Contributor
timestamp: January 09, 2025, 11:03:09 am
content: CHY-41R = 380193

author: radistx
author_rank: Newbie
timestamp: January 14, 2025, 12:02:11 am
content: Wow. Thank you so much!!!! I really appreciate you support with a schematic!