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  • #31
    Previously, on Adventures In Welder Repair, we had just gotten welder #1 working... (continuation due to post length limit)



    Might as well go into the second xmt304's troubleshooting as well, since this post needs to be even longer... (and the stereo has now advanced to the next cd, flesh is the law)

    Got this one knowing it didn't work, and the guy I got it from told me it showed a help 7, but he got it that way and had no idea of its history. I never powered it up to test.

    Did the same initial process as the first one. No dust in this one, though - nice and clean! Went through the ohm/diode readings. Input rectifier module is entirely toasted in every way. Like, of the six diodes, two are good, and the scr has all three terminals shorted together. One of the IGBTs measures shorted. Hrmm, one of the capacitors does too! Visually inspect capacitor. Has a boob (sorry, someone in another thread called them that, and it's laughably appropriate) on the side and a puddle of electrolyte under it. Ah. Pull out the screws holding the pcb to the capacitors and slip a thin piece of plastic (I used two tags from potted plants, because they were handy in my driveway) between the back of the board and the terminals on the capacitors, so I can measure them separately. IGBTs now measure correctly. Woohoo! Not as many blown parts as it could be! The earlier bad reading was due to the shorted capacitor across them. Capacitor still measures shorted. Repeat the whole checklist, and everything else measures good, including the bleeder resistors often blamed for capacitor failures. Notice the plastic insulator around the power switch shows signs that the power switch likely let its magic smoke out, but the switch itself is clean. Likely failure sequence is the capacitor failed, this blew out the input rectifier module, this blew out the power switch, someone replaced the charred blob of a power switch and it didn't fix it, and up for sale it went. Ordered two new capacitors and a new rectifier module. Installed and put everything back together. I used the thermal compound that came with the new input module, but it kinda sucked - I would use Aavid Thermalcote (my favorite white goop) if I did it again.

    Now, time for the big test... Is it going to work, or are all my new parts going to instantly blow because of some other failure that I didn't detect? Used the old TV repairman's trick, and stuck a light bulb in the power line. A 500W halogen in this case. If something is shorted, rather than blowing all my expensive semiconductors, the light would just turn on. Flipped the switch, and it powers up perfectly, but switches to a help-6 a few seconds after completing the power-up sequence. Makes sense, there's a light bulb in series with the power. Re-wire for normal power, and it seems to work perfectly. Grab the leads, give it a test. Shuts down if I turn the current over 100A, but that turned out to be a legitimate issue with my power to it - rather than using my oscilloscope to monitor the various voltages while trying to weld, I just took it to someone else's house and verified it worked on their power correctly. Easy tests first!


    So, no magic... Just plodding through repairs one step at a time.


    Ok, this post is long enough. Stereo is now playing Terrorvision. Somehow it's dark out now, and I didn't get anything done I was going to do. Oh well. Hopefully this helps!

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    • #32
      Man, this is almost to a T my experience with a known bad eBay. Except yours was much better written. And... well, I didn't replace the switch until after testing the arc and finding I needed a switch... I haven't seen a boob on a capacitor yet, only signs of leakage and stinkage...

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      • #33
        Originally posted by jjohn76 View Post
        Man, this is almost to a T my experience with a known bad eBay. Except yours was much better written. And... well, I didn't replace the switch until after testing the arc and finding I needed a switch... I haven't seen a boob on a capacitor yet, only signs of leakage and stinkage...
        Here's the thread that I was thinking of: https://weldingweb.com/showthread.ph...MT-304-Problem

        I would imagine the boob shape indicates failure with internal arcing, and is a sure sign of a bad capacitor.

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        • #34
          Originally posted by Bushytails View Post

          Here's the thread that I was thinking of: https://weldingweb.com/showthread.ph...MT-304-Problem

          I would imagine the boob shape indicates failure with internal arcing, and is a sure sign of a bad capacitor.
          I remember reading that thread and most everything ccawg posted. I have seen a few capacitors with an air blister on the top where gas has vented, never anything on the side of the case.

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          • #35
            I replace the buss capacitors on any machine I keep because of ccawg's recommendation. It's better than replacing caps, the input bridge, and the switch at a later date. The older Sanrex Thermal Arcs (outside the crappy plastic case, they're awesome) use 8 each 470uF caps instead of the two big ones like the Millers. I haven't actually compared the ripple current capacities, but would guess the thermal arc's design can handle more ripple current.

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            • #36
              My cable provider sucks, I want a revolution to. Yea!
              That's a five minute song and I just finished reading the post as it went into the second song on the album play list, Vampire Lover.
              Thanks BT for the lengthy reply. WOW! And theres a part two? Let me shake your hand.
              Truly, that's a WOW! I'm surprised they don't have a diagnostic port of some kind to plug a system function tool into.
              I'm saving this to my sticky file on welder repairs for future reference. Admitedly, I'm dazed in reading it. Listening to Industrial rock probably didn't help that but It was playing in the back ground. You have me thinging maybe I can. Once again, thanks.

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              • #37
                It took me the entire Blackheart Revolution album and half of the Flesh Is The Law album to write that.

                Part two was the part about the second welder with the bad capacitors.

                Listening to industrial rock helps most repair projects!

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                • #38
                  Originally posted by jjohn76 View Post
                  I replace the buss capacitors on any machine I keep because of ccawg's recommendation. It's better than replacing caps, the input bridge, and the switch at a later date. The older Sanrex Thermal Arcs (outside the crappy plastic case, they're awesome) use 8 each 470uF caps instead of the two big ones like the Millers. I haven't actually compared the ripple current capacities, but would guess the thermal arc's design can handle more ripple current.
                  Heh, I was kinda hoping to see new capacitors when I opened this one up... but nope. one of the few parts they didn't throw at it.

                  Unfortunately, I really don't have the money to drop $250 on replacing working parts... and I've already spent way too much on these welders. Now that I have two, I'll probably need to sell one. Somehow the bills keep going up while the paycheck keeps going down...

                  The CDE 550C series seems to be low-ESR high-ripple inverter capacitors, with fairly good ratings. The actual miller part number is a special order with no spec sheet online, but you can read about the series at http://www.cde.com/resources/catalogs/550C.pdf . I didn't look up the UCC ones, since my unit had the CDE ones, but I'd imagine they're something similar.


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                  • #39
                    Originally posted by Bushytails View Post

                    Heh, I was kinda hoping to see new capacitors when I opened this one up... but nope. one of the few parts they didn't throw at it.

                    Unfortunately, I really don't have the money to drop $250 on replacing working parts... and I've already spent way too much on these welders. Now that I have two, I'll probably need to sell one. Somehow the bills keep going up while the paycheck keeps going down...

                    The CDE 550C series seems to be low-ESR high-ripple inverter capacitors, with fairly good ratings. The actual miller part number is a special order with no spec sheet online, but you can read about the series at http://www.cde.com/resources/catalogs/550C.pdf . I didn't look up the UCC ones, since my unit had the CDE ones, but I'd imagine they're something similar.

                    Yeah, the only issue I had was finding the right size cans. The 550C's and 550CE's I could only find decently priced in a 3" diameter can. They may be able to fit, I just never tried them.

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                    • #40
                      Originally posted by jjohn76 View Post

                      Yeah, the only issue I had was finding the right size cans. The 550C's and 550CE's I could only find decently priced in a 3" diameter can. They may be able to fit, I just never tried them.
                      I found some of those too, but they had 1.25" terminal spacing instead of 1.125" (if I remember right) to go with the larger can, so you'd have to slot the holes in the pcb as well as deal with other mounting issues, like some way to support the back without the stud to mount it by. I finally decided that I needed the welder working now, and spent the money on the miller parts. A lot more money than I wanted to spend on a pair of capacitors.

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                      • #41
                        Originally posted by Bushytails View Post

                        I found some of those too, but they had 1.25" terminal spacing instead of 1.125" (if I remember right) to go with the larger can, so you'd have to slot the holes in the pcb as well as deal with other mounting issues, like some way to support the back without the stud to mount it by. I finally decided that I needed the welder working now, and spent the money on the miller parts. A lot more money than I wanted to spend on a pair of capacitors.
                        Yup, there is a direct replacement Nichicon I found too, but it's only saving $40 on the pair...

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                        • #42
                          I seen the group of 304 XMTs up on ebay a few months ago and thought about it myself. I ended up finding a fairly new one up there for around $1200 that looked like new. Got lucky it worked fine and has little use on it. I have took the ebay gamble and came out ahead 8 out of 10 times. I got a multimatic 200 (the one in the portable pelican case) and paid near nothing for it and found out just the power cord push on terminal had came off. Slid it back on and it worked fine. Some times it takes a bit more though. A lot of the time you can make one good one out of two if you can get them cheep enough to make it worth while or just order a few parts. Its a gamble tho,

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