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Miller Maxstar 280 Dead, PC1 Troubleshooting Help

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  • Miller Maxstar 280 Dead, PC1 Troubleshooting Help

    The Maxstar 280 I picked bought used weeks ago gave up the ghost. I put about 20 minutes of arc time on it over the course of 2 days. Went to power it up from standby mode in the middle of a practice Tig session yesterday; pushed the power button on the front, and instead of the fan coming on and the display activating, I just heard "snap" and the digital display went blank. Sounded just like when a breaker pops in the house.

    I got the machine used with 111 arc hours on it, looks to have only been used in stick mode its whole life.

    Anyway, a comparable unit new is $5000 machine only. I paid $1300 machine only, the previous owner said it was a 2015, so no warranty.
    • serial number is MF500383L

    I'd like to fix it without getting fleeced by some "repair shop," as the miller service center in my area (NY) has a horrible reputation, regardless I called them and once I told the owner of the shop that the machine was out of warranty he shifted his tone, and made it seem like it was going to cost an arm and a leg to fix.

    My options are to buy a new machine or try and resurrect this one myself, I've no interest in dealing with the repair shop.


    At present I've opened the machine up and there are no obvious signs of a southern bbq occurring.

    I also removed the PC1 board and found no obvious signs of failure.

    If it is the PC1 board I can stomach $1350 for a new board, just would rather not pay some else to install it.


    I called Miller, and after some nagging the technician told me that if it failed as I was turning it on from standby mode rather than in the middle of an arc, it is more likely something on the "primary" side that failed, and more specifically PC1 or PC7.

    Anyone here have experience with these machines, and willing to give advice?

    Tempted just buy the mainboard so I can get back to welding, but seems my lucks run out for now so, if putting a $1350 board into a 5 year old inverter machine is a fools errand then I'll have to send the unit off to its final resting place in some African landfill.

  • #2
    If there was a pop, it was likely the input power transistors, which attach to heatsink 5. Can you post pictures? You should be able to see evidence of smoke on the board or cracked transistor module housings on the back side of PC1. I haven't worked on the Maxstar/Dynasty 280s, but they use a boost pre-regulator before the voltage, and it likely keeps 800+VDC in those capacitors, so it's worth being careful. PC1 is self contained with the buss capacitors and transistors, so you may be able to get away with just replacing PC1. PC7 looks to be the auxiliary power supply and may have been affected as well. I would definitely recommend you pick up the technical manual if you go forward, and work through the pre-power/troubleshooting checks if you decide to repair it. When you power it up, having two light bulbs in series with the input power is a good way to make sure you don't torch the components. You probably will get a low voltage error on the machine, but as long as the lights went out, you should be good to go when you remove the

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    • #3
      The more and more i read these horror stories is reason enough not to buy one. I hope its something simple for you...Bob
      Bob Wright

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      • #4
        Originally posted by jjohn76 View Post
        If there was a pop, it was likely the input power transistors, which attach to heatsink 5. Can you post pictures? You should be able to see evidence of smoke on the board or cracked transistor module housings on the back side of PC1. I haven't worked on the Maxstar/Dynasty 280s, but they use a boost pre-regulator before the voltage, and it likely keeps 800+VDC in those capacitors, so it's worth being careful. PC1 is self contained with the buss capacitors and transistors, so you may be able to get away with just replacing PC1. PC7 looks to be the auxiliary power supply and may have been affected as well. I would definitely recommend you pick up the technical manual if you go forward, and work through the pre-power/troubleshooting checks if you decide to repair it. When you power it up, having two light bulbs in series with the input power is a good way to make sure you don't torch the components. You probably will get a low voltage error on the machine, but as long as the lights went out, you should be good to go when you remove the
        Thank you for your help!! Below are pics of PC1, I am a graduate student in computer science but know little about hardware. I saw no obvious signs of failure, but then again I don't know what to look for.

        Normally when I powered the machine down to standby mode, the cooling fan would run like a raped ape for a few seconds, which I assumed was the unit purging the capacitors. Fortunately, the machine shut down properly before it failed, and then I assume failed once it tried to recharge when I turned it back on? Otherwise, as you mentioned I might have gotten whacked removing the board..
        Click image for larger version  Name:	IMG_2827.JPG Views:	0 Size:	2.89 MB ID:	608498Click image for larger version  Name:	IMG_2828.JPG Views:	0 Size:	2.19 MB ID:	608500Click image for larger version  Name:	IMG_2830.JPG Views:	0 Size:	1.54 MB ID:	608501Click image for larger version  Name:	IMG_2829.JPG Views:	0 Size:	2.20 MB ID:	608503
        Attached Files
        Last edited by LittleBird; 05-22-2020, 03:31 PM.

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        • #5
          Please make sure you check for voltage across bus common and bus + before working on this. We're already past that in this case, but that will tell you if there is any dangerous voltages in the machine.

          You can get an idea on what went bad on that board. Usually, when things go, whatever failed usually takes out whatever is before it in the input chain (this could be a little different on this one if Miller designed some fault protection like the XMT350). TP1 and TP3 look like the rectified + and - voltage from lines 1,2,3. The right module is your input pre-regulator module, which has the three phase bridge rectifier as well as the boost diode, doost IGBT, and boost snubber IGBT/diode.
          To check the bridge rectifier, use the diode tester on your multimeter and check diode drop both ways from Line1, Line2, Line 3 each to TP1 and TP3. If transistors failed, at least two of these diodes will show a short.

          Check the boost and snubber diodes checking from L1/L2 to the Bus +, as well as from. L2 to Bus +. Check the boost IGBT diode drops from L1/L2 to Bus common. Check the boost snubber IGBT with diode test from L2 to Bus common.

          I need to take care of a couple things, but will try to walk through the half bridge checks next.

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          • #6
            Where does the transformer connect to the board? Is there a black plug near the four big electrolytic capacitors at the top?

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            • #7
              One last thing. I have had the input switch go on XMTs, but they tend to pull much more amperage. I have also seen that switch go in Dynasty 200s, but they were pulling excessive current. When you heard the pop, did the input switch trip? Does it feel much weaker now (easier to switch on/off) or are there any cracks in the side?

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              • #8
                Also, check resistance across the large capacitors- positive lead on BUS +, negative lead on BUS Common. There looks to be a connector for the transformer along the bottom of the board, right by the big electrolytics in your picture of the back side of the board. I think the left one goes to the center leg of you Half Bridge, but can't tell. The right one looks like it connects to the DC blocking capacitor right above it (white metal film capacitor that prevents the transformer from saturating due to DC offset). Check the resistance of the left terminal to the BUS+ and BUS Common points. If there is a short in any of the resistance checks, your inverter bridge is shot too and there could be another issue. While you're there, check around the four big capacitors for any bulging/swelling/leakage.
                Last edited by jjohn76; 05-23-2020, 10:33 AM.

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                • #9
                  jjohn, thank you!
                  • Input switch feels normal, and I don't recall it tripping (I would definitely have noticed).
                  • There is a black plug just as you described next to the capacitors. Picture is at the very bottom, below.

                  Bridge Rectifier

                  TP1 to Line 1: ~1 & 948 (Good?)
                  TP1 to Line 2: ~1 & 907 (Good?)
                  TP1 to Line 3: ~1 & 895 (Good?)

                  TP3 to Line 1: ~1 & ~1 (Bad?)
                  TP3 to Line 2: ~1 & ~1 (Bad?)
                  TP3 to Line 3: ~1 & ~1 (Bad?)

                  Boost and Snubber Diodes

                  Bus + to Line 1: ~1 & ~1 (Bad?)
                  Bus + to Line 2: ~1 & ~1 (Bad?)

                  Boost IGBT Diode Drops

                  Bus Common to L1: ~1 & 619 (Good?)
                  Bus Common to L2: ~1 & 616 (Good?)

                  Resistance Across Large Capacitors
                  (Pos. lead on on BUS + Neg. Lead on Bus Common)
                  -No resistance

                  Resistance of Left Terminal to BUS +:
                  Click image for larger version  Name:	IMG_2844.jpg Views:	0 Size:	2.51 MB ID:	608525

                  Resistance of Left Terminal to Bus Common:
                  Click image for larger version  Name:	IMG_2845.jpg Views:	0 Size:	2.48 MB ID:	608527

                  Plug in Question & No bulging of capacitors:
                  Click image for larger version  Name:	IMG_2843.jpg Views:	6 Size:	1.39 MB ID:	608522
                  Attached Files
                  Last edited by LittleBird; 05-23-2020, 12:35 PM.

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                  • #10
                    Click image for larger version

Name:	1590264319821816514109.jpg
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ID:	608529 I think I typed too fast up above, and the test points might have been off. Here is what I see as the general circuit, minus the H-bridge inverter snubber network (ran out of room on the right), a possible pre-charge relay/SCR (didn't see these on the board), and RC340A and RC340B might be backwards (can't tell which one is which from the pictures). Hopefully this helps you confirm where to check. Check both directions diode test from Lines 1,2,3 to Bus Common. Check both directions diode test from Lines 1,2,3 to L1 Boost. Diode test L1/L2 Tap to Bus+. Diode test Snub L2 to Bus+. Diode test L1/L2 to Bus Common. Diode test Snub L2 to Bus Common. It looks like your multimeter diode test voltage goes up to 1V, so anything showing 1V consider it open loop (there is likely a snubber capacitor across it, which is why it doesn't show open). Anything not showing zero is ok. Let me know if there are any diode tests showing open in both directions. That indicates an error in the schematic below, either because I didn't account for a pre-charge relay/SCR or some other issue.

                    It's worth checking continuity across your main switch when it is in the on position, just to rule it out.

                    It looks like your electrolytic capacitors were a bit hot looking at the conformal coating on those solder points. They could be the issue, which is common on older machines, but yours is a bit too new for that (hard life?).

                    Another source of that snap could be from your auxiliary power supply on board PC7. Look for any damage to the transistors on the heatsinks.


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                    • #11
                      Originally posted by jjohn76 View Post
                      [ATTACH=JSON]{"data-align":"none","data-size":"medium","data-attachmentid":608529**[/ATTACH] I think I typed too fast up above, and the test points might have been off. Here is what I see as the general circuit, minus the H-bridge inverter snubber network (ran out of room on the right), a possible pre-charge relay/SCR (didn't see these on the board), and RC340A and RC340B might be backwards (can't tell which one is which from the pictures). Hopefully this helps you confirm where to check. Check both directions diode test from Lines 1,2,3 to Bus Common. Check both directions diode test from Lines 1,2,3 to L1 Boost. Diode test L1/L2 Tap to Bus+. Diode test Snub L2 to Bus+. Diode test L1/L2 to Bus Common. Diode test Snub L2 to Bus Common. It looks like your multimeter diode test voltage goes up to 1V, so anything showing 1V consider it open loop (there is likely a snubber capacitor across it, which is why it doesn't show open). Anything not showing zero is ok. Let me know if there are any diode tests showing open in both directions. That indicates an error in the schematic below, either because I didn't account for a pre-charge relay/SCR or some other issue.

                      It's worth checking continuity across your main switch when it is in the on position, just to rule it out.

                      It looks like your electrolytic capacitors were a bit hot looking at the conformal coating on those solder points. They could be the issue, which is common on older machines, but yours is a bit too new for that (hard life?).

                      Another source of that snap could be from your auxiliary power supply on board PC7. Look for any damage to the transistors on the heatsinks.

                      The previous PC1 diode tests came back good. Switch shows high resistance in "off" position and none in "on" position with my multimeter set to 200k Ohms.

                      Can't see anything weird on PC7, except for some yellow paint on top of one of the capacitors if that's what that item is in the top right (picture below).

                      The snap I heard was pretty loud, so as unscientific as this sounds I find it hard to imagine it having come from one of these boards. However, when I went to shut the machine down just prior to it breaking, when the fan purged the capacitors (if that's what it's doing) the fan ran for a much shorter duration than it usually does.

                      My guess at the moment is that the machine was losing its ability to hold a charge (boost?) and then when I turned it on that last time, it tried t build one but failed.



                      ‚Äč

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                      • #12
                        I am probably at about a 10% success rate helping troubleshoot board problems, even less with boards I haven't traced to find the schematics. There is usually clear evidence of transistor failure, though the auxiliary power transistors can make a pretty loud bang. If your bridge rectifier is showing appropriate diode drops and the boost/H-bridge IGBTs are showing a diode drop in only one direction (it's their free wheeling diodes with them), then if it were me, I would look at replacing the electrolytic capacitors for probably around$60 (3900uf 250V with 4-terminals to key the polarity). They're a pain to remove - I use a 260W soldering iron and slowly rock the capacitor out with each time I heat up one of the terminals. It's not a fast process, but worth going slow to keep from lifting one of the solder pads.
                        Jon
                        Last edited by jjohn76; 05-24-2020, 09:37 PM.

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                        • #13
                          Looks like I have been "had" by all the inverter propaganda. Not sure what course I will take yet. Thank you very very much for your time and effort John.

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                          • #14
                            its a 3 layer board, other than the trouble shooting pre power inspection, there is not much you can do. Likely one of the mods are shot. have to replace the assembly. shoot us a pic of the on/off switch.

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                            • #15
                              Originally posted by cruizer View Post
                              its a 3 layer board, other than the trouble shooting pre power inspection, there is not much you can do. Likely one of the mods are shot. have to replace the assembly. shoot us a pic of the on/off switch.
                              Mods as in PC7 or PC1?

                              Click image for larger version

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ID:	608686
                              Attached Files

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