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Dynasty 200SD low buss voltage

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  • Dynasty 200SD low buss voltage

    Hi, a buddy brought his Dynasty 200SD, SN LE151928, because the front panel wouldn't light up. After going through the pre-power checks, I powered it up and found the buss voltage is only 325VDC. So I checked out the boost board, PC10, Assembly 200841.

    The only issue I found on the board is -12V instead of -15V (tech manual ref value) on Pin 8 of RC1. It is EXACTLY -12V which makes me wonder if it should be -12V. It's also a clean voltage, with very little ripple. Has anyone else seen this on their board? It powers the boost circuit current sense (HD1 on andPC2) and negative bias for off switching on boost IGBT, so I don't really want to power it up without verifying this voltage first.

    I also found the input relay on PC2 was fried (back left corner, picture below) which wasn't linking the boost inductor circuit to the main lines. For those who have a buss voltage of only 325V, this could also be the issue. To check it, I pulled PC10 and put 24V across Pin 5 and Pin 6 on connector RC10, making sure I heard the relay click. I checked the resistance across Pin 12,RC1 and L_Input on PC2. Mine read 200 ohms, which means the relay didn't close. Hopefully this saves someone some time down the road...
    Attached Files

  • #2
    A follow-up, I worked out most of the buck-boost circuit on the attached schematic. It looks like the reference voltage comes from D8, which is a 1N4742A (12V zener diode). I think I am missing something or the technical manual should show -12V for Pin 8 on RC1. Someday I will get a schematic program loaded to make these easier to follow.


    • #3
      First, let me say I'm impressed with your patience and diligence! I don't know if I would have been patient enough to trace all that out! I've done it in years gone by (many years gone by actually) but WOW!

      Hopefully one of the Miller techs will see this thread and be able to help. You might also call them with this info and see what they say--probably "Take it to an authorized service center" but it never hurts to ask. In my experience they have been very helpful. Pretty cool that you at least have a tech manual--I have a blue lightning 200DX and have been looking for a tech manual for several years.


      • #4
        Thank you Aeronica. It took a little while to trace it out, but I only did a small part of that board. I came across a thread on an website where a member rebuilt the entire board, schematic, PCB and all. Now that guy has some serious patience! The buddy that brought the Dynasty over has helped me out on a lot of things, so I didn't mind putting in the effort on an obsolete board. Radwell advertises to rebuild them in case I can't get an answer. The guy took it to a Miller tech shop up here in Washington who gave it a clean bill before it wouldn't power up, so he's looking for another tech shop for his other Miller welders. I think I need to recheck my schematic, something about the inputs to that NAND gate Schmidt trigger at the bottom doesn't make sense to me. That Schmidt trigger seems to me what drives the gate signals for the buck-boost IGBT.


        • #5
          Yes--possible something is missing there. I looked up the element14 site--that guy is astounding! Makes me want to go in and haywire a fuse into the VR4 circuit on mine! It won't be in the next couple of days, but I will also measure R86 and send the numbers to him.


          • #6
            Yes, I know my wife won't allow me to commit that kind of time to this project, not even going to ask... I forgot to mention I get my technical manuals from eBay, and the seller just happened to have them for this model and my Dynasty DX. He only sells printed copies, it would be nice to get them in PDF files though. I think I am going to replace those zener diodes. 11.4V and 9.8V are a little too far off spec for 12V and 10V zener diodes, though it may lead me to the real problem if it doesn't work. The board isn't pulling less than 100mA from a 30V input, so I don't expect a component drawing heavy current anywhere.
            Thank you!


            • #7
              Getting PC2 ready to replace the relay, I found a couple hot spots on the board (resistors that look to be a part of the control transformer switching circuit). Does this happen often? I need to draw out that circuit, but believe the resistors and capacitors are a snubber circuit for that IGBT on the heatsink in the center of the second photo.


              • #8
                I just talked with Dan at Miller Tech, who confirmed that 12V is correct for the negative rail for the hall device and the boost IGBT (Pin 8 on RC1 for PC2 and PC10). I am definitely writing this down in my manual on the PC2 and PC10 checks.


                • #9
                  Good to know. Thanks for sharing it with everyone! Might help someone else in the future.


                  • #10
                    Unfortunately, there turned out to be many more problems with the Dynasty 200. All resistors across the full DC bus voltage (not looking at it now, but they are the orange transistors in pairs or triplets on PC2 are showing signs of too much heat (mostly discolored traces), the two buss capacitors on PC2 seem like they vented a little into the pocket on the top of the cap (is this normal in the attached picture?), and the power module PM1 seems to have let out smoke (crack on two sides, evidence of smoke venting on bottom of PM1 and the heatsink). Strange though, I check the diodes and IGBTs directly on the board (IGBT tester to check IGBT function), and all work, so I don't know what component in there failed. Has anyone ever seen that before? Either way, I am assuming PM1 got hot enough that it will have a short life.
                    Attached Files


                    • #11
                      I spent a little more time on the Dynasty 200 today, and discovered the Snubber S1 wouldn't turn on. The pre-power checks for this require the test point between C12 and C13, and outside of pulling PC2, I wouldn't have found the issue. The pre-power checks guide through checking the input rectifier and ensuring the inverter and boost IGBTs are not shorted and their free wheeling diodes are good. Attached is a picture of the PM1 with it's circuit connections. I believe the Snubber S1 and S2 connect XFRM2 to the common point between C12 and C13, but don't know for sure. Here is how I checked the PM1 IGBTs with an IGBT tester:
                      Boost IGBT
                      E - RC1-3
                      G - RC1-4
                      C - L_Tap
                      High Side Inverter IGBT:
                      E - RC4-2
                      G - RC4-1
                      C - RC1-1
                      Low Side Inverter IGBT:
                      E- RC4-12
                      G - RC4-11
                      C - XFMR2
                      ​​​​Snubber S1:
                      E - RC4-8
                      G - RC4-7
                      C - TP between C12 and C13
                      Snubber S2:
                      E - RC4-6
                      G - RC4-5
                      C - XFMR2

                      Please let me know if I have the Collector for Snubber S1 wrong. I believe because S1 failed, there was no voltage clamp across the inverter, which is probably why all of the components across the 810V DC bus overheated.

                      Attached Files
                      Last edited by jjohn76; Yesterday, 09:24 PM.


                      • #12
                        So, after replacing the relay and putting it on a current limiting circuit (a box of four light bulbs in parallel before the plug), the machine powered up. It clearly sounds like it is missing a transistor- it has a very low pitched buzz sound, somewhere below mains frequency. This thing is going on Craigslist for parts...

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                        • #13
                          So after selling the last one and helping the buyer swap the PM mod and PC2 from another DOA Dynasty, my buddy drops off two more welders, a Dynasty 200SD and Maxstar 200SD, to figure if I can get them running. The Maxstar has a weak input switch (apparently the weldors were holding the switch closed to keep it powered) and the buss capacitors have definitely overheated (signs of gas venting). The inrush bypass relay on PC2 (bad one shown in a picture of a different welder in first post above) is still good and doesn't look like it was previously replaced (white relay on the Maxstar, black relay on the Dynasty) so that to me is a good sign the boost PFC circuit is still ok. However, it seems to me that the 30A inrush bypass relay would go before the 45A input switch.

                          The Dynasty has more problems, with a short somewhere in the 30V power supply chain causing the buck IGBT (Q11 on PC2) and current sense resistor (R6 on PC2) to go. I haven't checked the buck diode (D4 on PC2), but will do that soon to see if it took excessive current. The 30V supply feeds pretty much every board in the Dynasty, directly or indirectly, so all of the boards need testing separately. We'll see how that goes. Hopefully at the end of this thread, there will be at least a bit more troubleshooting information than what is in the Technical Manual. My initial process is to check the remaining components of the 30V buck circuit (PC2 and PC10 have these), power the 30V from a current regulated power supply to PC1 and PC10 after first checking for shorts, then working my way through each board the same way.

                          Edit: I don't know how I missed the thread below when last researching the Dynasty problems, but the failed snubber in the semikron module (PM1) is kind of common but tough to check. Without the snubber, the buss capacitors could likely suffer from overvoltage (810V bus across two 450V capacitors with a total surge voltage limit of 1000V). The PM check above should identify any failed component in semikron PM before anyone starts replacing buss capacitors, bleed resistors, or PC2 entirely.

                          Last edited by jjohn76; 09-18-2019, 06:56 AM.