Fried 121GW meter- Repairable?? - Page 1
Forum section: electronics / repair

author: zrobotics
author_rank: Newbie
timestamp: March 07, 2025, 06:58:48 pm
content: So pulled a really dumb move today - I was measuring near the output stage of an ultrasonic welder, supposed to be a 475V 15KhZ signal. I was looking at the 24VDC control side, but due to the high voltage had been clipping my leads and energizing the device remotely since I don't really want my fingers near that. I misread my notes and clipped the meter in autoranging voltage mode to the output stage then triggered the welder. The meter screen instantly went blank and wouldn't power back up. Took the meter apart, I don't see any charring around the MOV's bu it does smell toasty. However, the real carnage is on the switch contacts. The red arrows point at scorched contacts, the yellow arrow points at a contact that was blown completely off the board (found in the bottom of the case).My main question is, I realize I can repair the fried MOVs and the switch contacts, although they'll never be 100%. However, given that the device doesn't power on, I'm afraid the control circuitry is fried. Does anyone have any ideas on a quick/easy way to test this so I don't waste a bunch of time if it'll be unrepairable?And I know this meter isn't rated for this type of use at all, but my fluke 27 doesn't have capacitance measuring. Or is the real solution here to just make work buy me a new meter?e: fix typo

author: joeqsmith
author_rank: Super Contributor
timestamp: March 07, 2025, 07:14:12 pm
content: Another member damaged their 121GW doing something similar.Can you post a few photos of what the switch contacts look like?QuoteJust from the photo you provided, I wouldn't waste any time with it.See here:https://www.eevblog.com/forum/testgear/eevblog-121gw-discussion-thread/msg1590877/#msg1590877

author: coromonadalix
author_rank: Super Contributor
timestamp: March 07, 2025, 07:37:31 pm
content: sadly R.I.P busted pretty bad

author: bdunham7
author_rank: Super Contributor
timestamp: March 07, 2025, 08:32:23 pm
content: Quote from: zrobotics on March 07, 2025, 06:58:48 pmDo you have any reason to think that the output was far above that? TIG welders are notorious for destroying measuring instruments because the arc makes the output very noisy with high frequencies and that has an effect similar to RF burn. An ultrasonic welder has no arc and shouldn't be that noisy, so if the meter burned up on that it seems pretty weak. The manual only specifies a max frequency for AC of 5kHz, but no mention of a Volt-Hz rating or that going above 5kHz results in damage. The way that it toasted seems like there was more than 475V.I wouldn't attempt some heroic fix on that. You might open the schematic and trace through the switch contacts to see if you can power up the chipset just to confirm it is dead, but I can't see that whole mess turning out well at all.

author: zrobotics
author_rank: Newbie
timestamp: March 07, 2025, 08:44:28 pm
content: Quote from: joeqsmith on March 07, 2025, 07:14:12 pmThanks for the link, from reading that thread it doesn't look like it got investigated much further since the meter continued to work after cleaning the contacts.The picture taken at an angle shows the damage to the contacts, charring on the outer one and a break in the lower center one. The other side of these contacts appear undamaged, the top-down shot shows the other side and the trace that was blown off the board.TBH kinda surprised at the amount of damage to the board and contacts, it's not like the meter has zero input protection or anything. Yes it's not an industrial meter but I'm still surprised there was enough energy to blow traces off the board like that.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 07, 2025, 09:00:40 pm
content: That's a bit of damage. Looks like a bit of copper laying inside the upper case.Can you provide the manufacture and PN# of the welder/head that it was connected to at the time? Let's see if the manufacture provides any data for the one you have. Google AI:QuoteAnother member also took out theirs with an ultrasonic welder. See below:https://www.eevblog.com/forum/testgear/uni-t-ut61e-first-problem/msg1377433/#msg1377433

author: zrobotics
author_rank: Newbie
timestamp: March 07, 2025, 10:39:29 pm
content: Welder in question:https://www.danrel.com/DANREL-PLC-20KHz-2000W-Ultrasonic-Plastic-Welder-Price-China-15KHz-2600W-Ultrasonic-Welding-Machine-Manufacturer-pd45550745.htmlThis is the DR-1526-PLC, 15Khz at 2600W.From thisstackexchange linkto double check what I though the voltage ballpark would be:20% worst-case derating for a cheap offshore machine->13Kw input power. I don't have a manufacturer spec for the transducer impedence, but from looking onlinesimilar transducershave an impedence range of 15-20Ω. So let's go more worst case and say 25Ω, which gives V=sqrt(13kW*25Ω)=570V RMS.TBH, I'd gotten the ~475V number because I'd earlier measured these same points using my Fluke 27 set to AC volts. I know that number isn't super-accurate, I was wanting a ballpark to determine if an x100 oscilloscope probe would be safe or if I'd need to buy a differential probe. I've attached a photo of the back of the connector of the welder's output stage, I really don't think this would be adequate insulation for 3kv. The output connector is the one with the brn/red wires.My best guess is that it isn't so much that the voltage was super high, but that it's effectively a 15khz audio signal at 600V. Perhaps the impedance of the multimeter went down due to the high frequency? I'm definitely not an expert on this though.Edit: Apparently people just love blowing meters up with ultrasonic welders, huh? Wouldn't have expected that to be quite this common

author: MadTux
author_rank: Frequent Contributor
timestamp: March 07, 2025, 10:53:32 pm
content: This is the reason why there are $3 DT830/850 or whatnot meters around, you you don't bust your expensive stuff.Or get a tube voltmeter ;-)And the reason the voltage was that high, is because the ultrasonic thingy is an oscillator with quite good Q factor, only little excitation needed to push voltages bad levels, for your meter.

author: tom66
author_rank: Super Contributor
timestamp: March 07, 2025, 11:13:08 pm
content: If the meter screen went blank I'd be suspecting the main meter ASIC (multimeter-in-a-chip, feeds data to the MCU) bit the dust, it's probably clamping the 3.3V supply due to failed ESD diodes.First test would be for resistance across a few of the decoupling capacitors for various ICs, there seems to be a schematic here:https://www.welectron.com/mediafiles/manuals/eevblog/EEVBlog-121GW-Schematic.pdf

author: thm_w
author_rank: Super Contributor
timestamp: March 07, 2025, 11:43:14 pm
content: Those connectors look like nothing special so I would definitely not expect more than 1kV on them.At the same time, the photos look like arcing in the 3kV+ range?Can't see much on the frontend that 15kHz would make a difference to:https://github.com/tpwrules/121gw-re/blob/master/pdfs/121GW%20EEVBlog%20Circuit%20diagram.pdf

author: bdunham7
author_rank: Super Contributor
timestamp: March 08, 2025, 12:30:06 am
content: Quote from: zrobotics on March 07, 2025, 10:39:29 pmEvery meter (and everything else, mostly) has a dV/dt limit above which things start smoking. For meters this is typically expressed in Volt-Hz, simply the product of the RMS voltage and the frequency. Assuming a sine wave, this corresponds to dV/dt, but it's more complicated for non-sinusoids. Some meters list this limit in their specs but the ones with low-frequency only AC ranges usually don't, even though they're susceptible to HF AC. Interestingly enough, transient protection methods may lower this limit and make meters more susceptible to this issue. The 600V @ 5kHz maximum specified in the 121GW manual sort of implies a very low 3 x 106V*Hz limit. Other handheld meters like your Fluke 27 may have V*Hz ratings of 1 x 107V*Hz and the top-of-the-line 289 is 2 x 107, but modern bench meters like the Fluke 8846A and HP 34401A state 8*107V*Hz. I still wouldn't connect them to any sort of welder or igniter output....It is possible that you simply exceeded the actual capabilities of the meter without exceeding the 600V limit. Or, since your transducer impedance is 25R (or whatever) only at a certain resonant peak and is much higher if you deviate from that, the actual voltage might be higher than you think. Can you carefully inspect the PTCs to see if they look burnt or arced over at the bottom? Once those short out it's typically curtains for the meter.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 08, 2025, 03:52:47 pm
content: Quote from: zrobotics on March 07, 2025, 10:39:29 pmDid you remove that hot glue insulation to connect the meter, or did you remove the head and connect to the outside of the box?I do have some version of a non-functioning 121GW. It was purchased a couple of years after the release but I know Dave had newer revisions available at the time they sold me these meters. You can see that it does still display ACV. I would need to pull it apart to check for any damage to the board but we could try and replicate your 15kHz 600V signal and just see if it causes the meter to breakdown. I kept this meter for further experimenting anyway and have no intention to attempt to repair it. So it is no big loss if we have further damage. Just keep in mind that it may not be the same hardware that you damaged and with it already have been damaged with ESD, it may breakdown at a lower level.***I tried a few different setups to see if I could achieve the voltage/frequency with what I have on-hand but no luck.

author: zrobotics
author_rank: Newbie
timestamp: March 08, 2025, 05:40:08 pm
content: Quote from: joeqsmith on March 08, 2025, 03:52:47 pmNope, I had some minigrabber clips and clipped to the output signal on the main driver board instead of the 24V control terminals. The only thing in the welder signal chain between those terminals and the ultrasonic transducer is a very chunky inductor.Went and looked at this under the microscope, none of the MOVs look damaged and test open circuit. That's not definitive on them being undamaged, it seems that HV can cause them to fail open instead of short. However, the corner of PTC1 has been blown off, see photo. Apologies for the terrible photo, I don't have a microSD adapter handy to export a photo from the microscope. I did have to cut off the heat shrink, so that's the scratch on the side but the bottom corner is chipped away and isn't knife damage. I'm not 100% sure, but it also looks like PTC4 has a brown streak on it. Testing in-circuit they appear to be slightly low, PTC4 measured 1.1K and the other 3 measured 1.3K at room temp.Quote from: joeqsmith on March 08, 2025, 03:52:47 pmYeah, even a well-equipped lab wouldn't likely have test equipment to replicate that type of signal. I know one person whomightbe able to replicate that signal in a controlled manner at work, but he works on radar systems. Otherwise, HV at that frequency is getting fairly unusual. Thanks for checking though.Quote from: MadTux on March 07, 2025, 10:53:32 pmI mean, Idefinitely, absolutelywouldn't trust one of those to not actually explode in this scenario. I'm not pissed off that the meter blew up, I definitely shouldn't have been probing that signal with this meter. This is fully classIII territory, and the 121GW is very clearly targeted at electronics work not industrial repair. It did fail safely though, I'm pissed at myself for doing a dumb.I guess I'll be off to the multimeter thread to start shopping for a replacement and figure out if there's a better option since I purchased this in 2018 or if I just want to pick up a replacement and not be such an idiot this time. And I still need to fix the welder power supply, which is what I was trying to do originally. For anyone that's curious, that appears to be some blown BJTs on the main switching/driver board, I have replacements on order from Digikey and hopefully that solves my problem.

author: bdunham7
author_rank: Super Contributor
timestamp: March 08, 2025, 06:09:42 pm
content: Quote from: zrobotics on March 08, 2025, 05:40:08 pmActually any well-equipped cal lab will have an AC amplifier that will put out that signal, although perhaps not with the energy behind it that your welder has. For example, one of the specified calibration check points on an HP 34401A is 750VAC @ 50kHz. I have an old Fluke 5205A with a low-impedance transformerless output that will provide as much as 1200VAC @ 120kHz and up to 100mA. Most labs now will have something like a Fluke 5725A which is less powerful but still does 750VAC @ 100kHz.I agree with an earlier comment that there doesn't look like there's anything in the front end that would appear that vulnerable to 15kHz. So this may not be a dV/dt issue after all and that PTC is sort of telling us that it was a significant overvoltage, perhaps a peak but not RMS overvoltage. You would need to put an oscilloscope on there to see what is going on and that will take a really special probe to do safely.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 08, 2025, 06:40:20 pm
content: Quote from: bdunham7 on March 08, 2025, 06:09:42 pmNow we just need you to run a few tests for us.

author: bdunham7
author_rank: Super Contributor
timestamp: March 08, 2025, 07:26:11 pm
content: Quote from: joeqsmith on March 08, 2025, 06:40:20 pmThe 5205A is sitting in my garage awaiting an overhaul. If there were meters to be incinerated for amusement, I could prioritize it to next-on-bench status. It powers on with no faults, so long as the tubes are OK I should be able to fix it up easily enough.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 08, 2025, 07:30:56 pm
content: Do you have a 121GW you could test?

author: bdunham7
author_rank: Super Contributor
timestamp: March 08, 2025, 08:18:41 pm
content: Quote from: joeqsmith on March 08, 2025, 07:30:56 pmNo, nothing like that. My handhelds are either Fluke or random junk from Harbor Freight.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 08, 2025, 08:24:57 pm
content: Quote from: bdunham7 on March 08, 2025, 08:18:41 pmMaybe Dave would consider sending you a return or other to run such a test.

author: zrobotics
author_rank: Newbie
timestamp: March 19, 2025, 07:24:05 pm
content: So while I was finishing repairing the welder, I thought I'd get some actual measurements of the output signal since I was curious and it seems that that info isn't readily available anywhere.I dug out my HV probe for the multimeter, that measured 380V although with the high-ish frequency I'd take that with a grain of salt.Using a USB oscilloscope with a x100 probe, I measured it at 331V at 14.03kHz. So well under the 600V rating of the meter. It definitely isn't the most stable, I was seeing variations of 50V between captures with as low as 270Vpp and as high as 350Vpp. Ignore the markers in the image, I didn't have them set and didn't realize they were captured in the output file.Not going to lie, I'm kinda disappointed in this meter, I definitely won't be replacing it like-for-like. The logging features and open firmware are nice, but I guess it really should have survived this a bit better. The manual states:QuoteWelder ended up frying 1/2 of the 2SC3998 NPN transistors on the output switching board, so thankfully a fairly cheap and easy repair. Just thought I'd update this post with measurements of the signal. Thanks for everyone that chipped in with helpful info.

author: bdunham7
author_rank: Super Contributor
timestamp: March 20, 2025, 02:46:05 am
content: Quote from: zrobotics on March 19, 2025, 07:24:05 pmTypical 80K-6 will be down about 30% at 15kHz. So you likely have 500-ish volts or so, just like you originally said it should be. You can measure that directly with the Fluke 27 if you like, it shouldn't flinch. Unless there was a large surge or something during the original measurements, I have to agree that the meter breaking down seems unreasonably weak. I'm not even sure what would have given out--the PTC could fail at that level but I don't see what would have caused the large fault current internally. What's really odd to me is that the damage doesn't seem to be to the MOVs or MOV-related PTCs. You said it was in the ACV range but the it looks like the damage is in the Low-Z circuit.

author: J-R
author_rank: Super Contributor
timestamp: March 20, 2025, 05:40:22 am
content: Some here may be aware that while in calibration mode, the 121GW can display voltages well past the 600V rated limit. For example, I've given it 1,600V DC continuously without incident and it displays a valid reading. I can generate 3kV with the current setup, but haven't been sufficiently motivated to possibly damage things. Of course AC is a different scenario, but I don't think the 121GW will just automatically break if you exceed its rated voltage.

author: zrobotics
author_rank: Newbie
timestamp: March 20, 2025, 04:28:50 pm
content: Quote from: bdunham7 on March 20, 2025, 02:46:05 amYeah, I'd originally gotten the 475V measurement with the 27 directly, I used the HV probe to see if I'd get a different measurement. From what I was seeing on the scope, there can be +/-75V on the signal, and I certainly wouldn't rule out a power surge.I took another look at the switch contacts, the damage is on the low-z and mV contacts, the V range contacts appear undamaged.From looking at theschematic, the blown switch traces are 1,3, and 15. I will admit it is possible I had the meter in a different range, that could be possible and I can't say with 100% certainty but it does seem a bit strange that the switch contacts saw enough power to blow traces off the board without obviously blowing a MOV.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 24, 2025, 12:27:02 pm
content: With you being the third person now who had ran into problems with this type of welder, it would be interesting to see if the higher frequency had anything to do with the failure. Did you by chance capture the waveform before the repairs?QuoteAssuming what you have shown is correct, the voltage levels are far below where the MOVs would conduct. Still, as I mentioned even with the input limiting the current, you can still get more than enough to do this level of damage.https://www.eevblog.com/forum/testgear/eevblog-121gw-discussion-thread/msg1598122/#msg1598122Quote from: bdunham7 on March 20, 2025, 02:46:05 amAre you volunteering to start testing some of these meters? If you do, consider making a video showing it.