Stick welder repair - Page 1
Forum section: electronics / repair

author: dog80
author_rank: Regular Contributor
timestamp: May 17, 2013, 09:50:30 am
content: They brought me a Miller GoldSeal 160i that had two blown diodes.The board is this:The blown diodes are on the other side, somewhere between the two big heatsinksAs you can see, part of the board is burned out.The diodes were already removed when I got the board on my hands so I don't know what type they were. I installed a couple of 1N4007 that I had handy, turned the welder on and BOOOM! the diodes blew again in milliseconds.The traces on the board that lead to the diodes are thin, so I don't think these diodes handle too much current. So either there's higher voltage there that the 1000V the 1N4007 are rated for, or there is a short somewhere else on the welder. Also, since part of the board was missing, I am not sure about the direction of the diodes. I installed both in the same direction as the schematic on the board. Could the other diode be in the opposite direction?

author: digsys
author_rank: Unknown
timestamp: May 17, 2013, 11:24:03 am
content: Turn the board over and see if the Cathodes are connected (or use a multimeter if that's not possible).ALL diodes seem to face the same way, which was a* convention in older days ie ICs, diodes, colour codes etc all orient the same.Most likely there is a short elsewhere, but if that is too difficult, you'll HAVE to trace the circuit.

author: Psi
author_rank: Super Contributor
timestamp: May 17, 2013, 12:01:46 pm
content: I would check all the fets/igbts for shorts. That's what normally fails when stuff like this blows up.If they're blown make sure the gate drivers are ok before replacing with new ones, or they may smoke again.

author: dog80
author_rank: Regular Contributor
timestamp: May 17, 2013, 01:23:39 pm
content: That's weird. I removed the diodes and tested them with a multimeter and they seem ok. Also there are no burn marks on them or anything. But when I turned the damn thing on there was a loud bang and some electrical flash coming from the diode area.I did some probing here and there and the lowest resistance I measured was something like 20 KOhmdigsys, the two diodes do not share anything.

author: digsys
author_rank: Unknown
timestamp: May 17, 2013, 01:30:16 pm
content: In that case you may have introduced a NEW fault. The burnt area is now conductive, carbon deposits from theflash / burn. So we may be able to explain the 2nd incident, but not the 1st. You'll need to grind away ALL black / burnt bits.You'll definitely need to trace where each end of both diodes goes to.

author: G7PSK
author_rank: Super Contributor
timestamp: May 17, 2013, 02:10:35 pm
content: Check the big capacitors, they run at full welding current and often go short. I have had that happen, blew the diodes and half the tracks off the board, mind it was a bigger welder, at 360 amps.

author: dog80
author_rank: Regular Contributor
timestamp: May 17, 2013, 04:09:53 pm
content: I tested those big electrolytics and some of the polymer capacitors with an Atlas ESR meter. Didn't find anything weird.

author: RCMR
author_rank: Frequent Contributor
timestamp: May 17, 2013, 10:50:28 pm
content: Ah... welders!I'm about to rip one of my AC/DC TIG welders apart and see what I can find.It started going intermittent on my some time ago and then one day, when I went to use it -- only the HT was working.A quick look inside made me cringe a little -- quite a bit of "bodge" on the PCBs.Might be an interesting repair, although without a schematic and given the price of the IGBTs I don't know that I'll spend too much time on it. I already have a replacement TIG so I'm not going to blow a lot of money trying to fix the old one.I'll watch your progress with great interest -- even though stick welders tend to be a lot simpler than AC/DC pulse TIG ones.

author: Troy
author_rank: Newbie
timestamp: May 18, 2013, 12:28:00 am
content: Millers are notorious for blowing up. We have had one release the magical miller blue smoke, and my high school has had ~8 blow up on them.

author: dog80
author_rank: Regular Contributor
timestamp: May 18, 2013, 11:35:42 am
content: I had a better look at the board (access is difficult because the traces are right between those huge heatsinks).Part of a trace is gone, but it looks like the terminal of one of the diodes is connected on traces on both sides of the board. I don't think I've ever seen this. Does it sound logical?Also I have a nagging suspicion that the other diode should point to the opposite direction. I measured the voltage on the supposed cathode and it was minus 300 Volts. I couldn't measure at the anode because it is difficult with the board installed, but later when I will have more time I will solder a piece of wire to get a measurement.

author: dog80
author_rank: Regular Contributor
timestamp: May 18, 2013, 04:24:28 pm
content: Found the culprit. I powered the board again without the diodes and there was more arcing and flashing. It was the capacitor right next to the diodes.The first time I checked the board, this cap felt a bit swollen to my touch, but I tested it in-circuit with my ESR meter and it read OK. After the last arcing incident I re-tested the cap with the ESR meter and again it seemed ok.I took it off the board and it reads like open circuit. I guess the ESR meter was measuring some other cap in parallel.

author: digsys
author_rank: Unknown
timestamp: May 18, 2013, 11:43:03 pm
content: Great news. DON'T forget to clean up all the carbon burns !!! It WILL cause you problems later or sooner.

author: G7PSK
author_rank: Super Contributor
timestamp: May 19, 2013, 08:20:18 am
content: Might be worthwhile checking the other caps that look to be the same type on the board as they could be close to failure as well.Also make sure that all the metal fumes have been removed from the board, The biggest cause of failure on most welders is they draw in the weld fumes and grinding dust, I made it a sackable offence for anyone to point the grinder in the direction of a welder, or another person.

author: dog80
author_rank: Regular Contributor
timestamp: May 20, 2013, 02:00:18 pm
content: I found another swollen capacitor of this type. They are just 6 of them so I took all of them off and will replace them as a precaution.What's puzzling is that when I test them off circuit with the Atlas ESR meter they all seem like open circuit. The UT71E though measures the capacitance correctly. I know that the Atlas cannot detect the capacitance if the cap is smaller than 1uf, but at least I hoped it could give me the ESR.I also found signs of arcing between some of the high voltage traces, near the bad cap. Maybe something else is wrong with the board and the cap blew because of this other fault.

author: dog80
author_rank: Regular Contributor
timestamp: May 20, 2013, 02:41:27 pm
content: THere's lots of arcing between the traces where the bad cap was. The cap is rated at just 250V and there's a good creepage beetween the traces, so there must be quite higher voltage than 250. Also it is quite possible that the cap was damaged by the high voltageAny ideas what could cause the voltage to increase?

author: SeanB
author_rank: Super Contributor
timestamp: May 20, 2013, 03:47:09 pm
content: As it is part of a LC circuit as C decreases the voltage increases, as power in is still pretty constant.

author: dog80
author_rank: Regular Contributor
timestamp: May 20, 2013, 05:12:54 pm
content: Thanks!I will replace all the caps and try again.

author: G7PSK
author_rank: Super Contributor
timestamp: May 20, 2013, 05:22:43 pm
content: Signs of arcing over could also be due to grinding dust and metal fumes having been in there in the past, highly likely whoever opened it up previously cleaned it up before selling it.