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  • #31
    Good-ish news for board PC10. The output of the 15V regulator VR4 now powers up to 15V without overcurrent. I found and removed components that were shorted, which includes Q5 and Q6 (these are the totem pole gate driver for the flyback mosfet Q1 on board PC2 that provides the power to board PC1), U3 (signals Q5 and Q6 to switch), VR4 (15V voltage regulator that powers the boost and flyback control circuits), D19 and D20 (I still need to run down what these do in the boost circuit, but my guess is a reference voltage), and D26 (clamps the output gate drive to the boost IGBT at 18V so it doesn't fry the IGBT), and C34 looks like it was knocked around - it is part of a low pass filter from the input current sense circuit that signals through optocoupler OC3 for the inverter board power supply (flyback converter, will cover it next) to shut down if input current exceeds 35 amps.

    Next thing to do is look at what those components either source or sink voltage to make sure nothing up or down the power chain will cause them to fail again. So I need to check the output from Q5/Q6, which is the gate drive circuit on PC2 for Q1. Then I will put 15V on the outputs from U4 (same CD4093BCN as U1) by putting 15V on pins 3,4, 10 and 11 to check for overcurrent. More to follow on the diodes...
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    Last edited by jjohn76; 10-07-2019, 10:39 AM.

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    • #32
      I received most of the parts today, just before I realized I already had but couldn't find those parts... Thanks again Franz...

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      • #33
        Originally posted by Aeronca41 View Post
        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.
        Aero, I am pretty sure Voltage regulator VR4 (takes the incoming regulated 30V and drops it down to 15V for the inverter board power supply controller and the boost PFC controller) is what failed in this Dynasty 200SD. It is toast, which likely let 30V into the 15V circuits and killed the components identified above. Like you mentioned and the poster in element14.com mentioned, this voltage regulator has no heatsink or fuse. Bad. Thermal. Management. It doesn't look like it needs to provide 1A of current, but still looks like a cost savings at the expense of some reliability. I am still thinking whether or not to put a heatsink on it while waiting for a new one to arrive (every voltage regulator on hand but a 15V one, thanks again Franz...)

        After removing the leads from the board, I applied power across voltage regulator VR4 on board PC10 - it's toast not even getting to 5V before pulling more than 200mA.
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        There is no additional cooling for VR4, not even the ground plane extends to the regulator body (top middle shows the circular boss for the rivet attachment, no other cooling)...
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        ​​​​​​​

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        • #34
          I always have problems desoldering and removing chips from the board, so I typically cut the chips from the pins using a cutoff wheel on a dremel, then solder, heat and remove the pins individually. The middle of the photo shows how I cut it.
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          This usually works well, except when it doesn't... Sometimes I cut too deep... You can see the severed PCB trace in probably the worst place for a trace repair...
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          • #35
            Originally posted by jjohn76 View Post

            Aero, I am pretty sure Voltage regulator VR4 (takes the incoming regulated 30V and drops it down to 15V for the inverter board power supply controller and the boost PFC controller) is what failed in this Dynasty 200SD. It is toast, which likely let 30V into the 15V circuits and killed the components identified above. Like you mentioned and the poster in element14.com mentioned, this voltage regulator has no heatsink or fuse. Bad. Thermal. Management. It doesn't look like it needs to provide 1A of current, but still looks like a cost savings at the expense of some reliability. I am still thinking whether or not to put a heatsink on it while waiting for a new one to arrive (every voltage regulator on hand but a 15V one, thanks again Franz...)
            [ATTACH=JSON]{"data-align":"none","data-size":"small","data-tempid":"temp_39186_1570807586545_838"**[/ATTACH][IMG2=JSON]{"data-align":"none","data-size":"full","src":"https:\/\/forum.millerwelds.com\/image\/gif;base64,R0lGODlhAQABAPABAP\/\/\/wAAACH5BAEKAAAALAAAAAABAAEAAAICRAEAOw=="**[/IMG2]​
            After removing the leads from the board, I applied power across voltage regulator VR4 on board PC10 - it's toast not even getting to 5V before pulling more than 200mA.
            [ATTACH=JSON]{"data-align":"none","data-size":"small","data-attachmentid":603275**[/ATTACH]
            There is no additional cooling for VR4, not even the ground plane extends to the regulator body (top middle shows the circular boss for the rivet attachment, no other cooling)...
            [ATTACH=JSON]{"data-align":"none","data-size":"small","data-attachmentid":603276**[/ATTACH]

            ​​​​​​​
            Thanks for posting this. I had forgotten about measuring that resistance value for him--been working on my roof, and traveling a good bit to settle my mom's estate 300+ miles away--haven't been around this forum much as a result. Hoping to have all of that wrapped up in a few weeks.

            With a several-year stint as a reliability engineer in a former life, stuff like you're discussing here makes me cringe. I fix old Tektronix oscilloscopes as sort of a hobby (but not very much in the past few years--quite a pile in the basement awaiting attention), and this is just like the U800 horizontal output chip in the 24xx series of scopes. There is a long and sordid history associated with its thermal issues, even in a company as top-notch as Tektronix, made worse by unqualified techs working on them out of the case without setting up a fan to blow air on the chip.

            30v on the 15 v buss is ugly. Makes me want to make yet another mod to mine, and add a crowbar over-voltage protection circuit in there somewhere. Probably another one of those things I'll never get to until I wish I had!

            I like your dremel tool approach when you can use it, but that is a nasty place to have to fix a trace on a board. Happens in a millisecond! If you're anywhere near my age, time to get it under the microscope--I had to buy one, even though the doc says my vision is absolutely normal with glasses. Maybe normal for 70+, but sure not the normal I had when I was fixing a lot of PC boards 40 years ago.

            I have to say again I'm impressed by your logic, patience, and stick-to-it-iveness! I don't seem to have quite as much of those things as I used to in my younger days, but it's really great to see how you approach this stuff. Nice work!

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            • #36
              Thank you Aero. I cheated on that one trace and just soldered in a jumper on the back side.

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              Those two ICs didn't want to solder to the board. I think it had to do with the ground and power planes needing too much heat, but couldn't explain the other leads.

              I think those three 18V zener diodes I replaced (at least the first one) were acting as a crowbar, and in retrospect, I probably moved through troubleshooting too fast. I removed the first 18V zener and the standard body diode (1N4148), and believe when I powered the board up again at 30V, because the voltage regulator VR4 already failed, I killed the other 18V zener (it clamped the boost gate driver circuit) and the controller circuit for the flyback (totem pole transistor and the quad nand chip). Multi-tasking mistakes... at least it was less than $10 in components.

              All but the voltage regulator are back together, and so far, no signs of overcurrent on the 15V circuits or the 30V circuit.

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              Last edited by jjohn76; 10-12-2019, 08:49 AM.

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              • #37
                More progress today and after going through all pre-power checks again, I then powered up the boards, individually first, then as a unit.


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                The conformal coating is drying tonight, and we'll see how it does tomorrow...

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                • #38
                  Originally posted by Aeronca41 View Post
                  I have to say again I'm impressed by your logic, patience, and stick-to-it-iveness! I don't seem to have quite as much of those things as I used to in my younger days, but it's really great to see how you approach this stuff. Nice work!
                  In agreement I second that. Reading the talk between you two has me dazzled, dazed, confused and challenged to understand. Makes me want to poke it and ask, what's that do? And that thingy...what about it?

                  Speaking for myself, it still looks complicated? But it's been great reading.

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                  • #39
                    Thank you Noel. I wish I had more time to retrace all of the board sections, but am running out of it very quickly. We'll see how the welder does today. If it needs further fixing, then the thread will probably get a little longer, but hopefully not.
                    Jon

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                    • #40
                      You are quite welcome friend. That's good stuff. Has me wondering however, what do you think the possibility is of adding frequency adjustment to an old Syncrowave 250? I'm guessing it's more then adding a board, cut and splice a few wires and a hole for the knob to adjust?

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                      • #41
                        Thanks, Noel. I really like doing stuff like this, but unfortunately have not been able to do much for many years due to my wife's health issues, death, mom's health issues, death, estate responsibilities--It just goes on, and as I often say, I find myself to be incredibly inefficient in retirement! And back when my knees worked, things all happened a lot faster. :-) Really hoping to get my electronics bench re-established and dig into some of this stuff more over the winter. Hoping to weld up some bench-top and floor-standing equipment racks--the cost of buying them is just out of sight for what you get compared to materials cost and not that much effort to make them. Time will tell.

                        jjohn is a real do-er!

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                        • #42
                          2019 seemed like a year of dealing with troubles. Hopefully 2020 will be the year to pause and reflect on them? As you mention, time will tell.

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                          • #43
                            The good news is this machine now has 810V on the DC buss, which means PC10 is mostly functioning - the buck converter that makes 30V DC from the DC buss is working, VR4 is regulating the 30V to 15V as designed, and the buck/boost circuit is putting out the negative 12V used for the LEM/input current and the boost PFC IGBT controller (it switches the IGBT gates between +15V and -12V to make sure that transistors completely and quickly turns on and off).
                            The bad-ish news is the flyback converter that provided +15V/-15V to the PC1 inverter board (PC1 regulates and provides power to the display boards and the arc starter board). Here is the circuit up to the primary side of the transformer. I didn't re-draw the circuit using LM339 U2 that was in my much earlier post, because it seems to be working.



                            I will hopefully get a chance to consolidate all of these schematics in a schematic capture software at some point. The flyback transformer at the bottom, which is connected to the mosfet Q1 Heatsink HS2 on the PC2 interconnect board. The flyback control circuit on PC10 switches Q1 on long enough to build enough energy (magnetic field) in the T1 primary, so that when it turns Q1 off, the primary magnetic field collapses, which changes the magnetic fields in the secondary and transfers energy to the five isolated secondary circuits (+15V and -15V for the PC1 board, and three 20V circuits for the output inverter). Here's a general flyback circuit that shows one secondary output. All five outputs look the same as on schematic.

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                            The Dynasty's flyback controller isn't switching at all right now, which is the reason for the schematic. The controller uses Q5 and Q6 as a totem pole driver. U3D is one of the NAND gates on chip U3, and because both of its inputs are tied together, inverts whatever is on the input. So it turns the mosfet Q1 on whenever it's inputs are low and turns Q1 off whenever the inputs are high. It's used because it's output is either 15V or 0V, where its input can be somewhere in the middle. The Q1 mosfet needs to receive +15V or 0V (or negative) to survive.

                            The top right of the schematic shows the inputs to the U3D nand gate.

                            Along the top of the schematic is the voltage feedback circuit. It checks to see whether or not there is a litte over 30V between the +15V and -15V flyback secondary circuits (these two have a common ground reference). Zener diode D9 is a 30V zener, and will basically stay open until 30V is across it. When it starts to conduct, optocoupler OC2 drives it's output to the -BUS (ground reference for PC10's separate 15V circuit), which turns on Q7, bringing the input of nand U3D high, which turns off mosfet Q1. So basically, when there is 30V plus the diode drop voltage in OC2's input between the +15V and -15V secondary circuits, the controller turns Q1 off.

                            Along the right edge of the schematics are the flyback current feedback and the DC buss feedback circuit. This circuit isn't complete here, but was on the original schematic. Basically, output U2B is 0V when the DC buss voltage is 810V (indicates the boost input circuit is functioning properly) or 15V otherwise (the DC buss isn't at the proper voltage). When output U2B is 15V, Q9 is turned on, which turns on Q11, drives the inputs to U3D high, and turns off the Q1 mosfet.

                            The bottom right is the flyback current feedback/limit circuit, which senses the current through the flyback transformer primary using R8 and R9. When the voltage across R8/R9 is higher than the threshold voltage for Q8, Q8 turns on, which turns on Q11, driving U3D inputs high, and ultimately turns off mosfet Q1.

                            Testing yesterday, I need to work through two issues. The current feedback circuit is constantly showing 17V, which I think is because Q2 is not working properly. R14 and R13 seem out of spec, which is new to me - I have either seen them test in spec or open. The 20k R14 is testing at ~10.5k and the 10k R13 is testing at around 7.8k. It's making me think there is something wrong with Q11, which is leaking current through to PC10's 15V and -BUS references. Does this make sense?
                            Last edited by jjohn76; 10-20-2019, 12:17 PM.

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                            • #44
                              Noel, based on the way the AC output works in the Syncrowave, I think the only way to make a variable AC output is to build an output inverter from the DC output.

                              Here is a guy that documented his way to AC tig.from his DC Cyber Tig. I don't know if I have seen him in this site, but he was on Practical Machinist for a while

                              http://igor.chudov.com/projects/Home...o-AC-Inverter/

                              ​​​​​​​Here is another one I found a while ago. This guy used an AC buzzbox, so he first rectified everything to make DC, then made an output plus arc starter and gas. That's a serious project!

                              http://www3.telus.net/public/a5a26316/TIG_Welder.html

                              I have somewhere in my project list to make an output inverter with gas for my Thermal Arc 400 GMS (close to pin compatible with the Miller XMT 304) and completely rebuild my 460V Hobart Portatig with a PFC boost input circuit and AC output. I have picked up pretty much all of the parts from eBay for cheap over a very long time, the things delaying these projects are the Dynasty DX I rebuilt and the garage finds buddies keep dropping off (add a POWCON 200SM and two 400SMTs to the list...).

                              The only reasons for still wanting to do them after finding the Dynasty DX is to see if it's doable and to overcome the duty cycle issues with the AC inverters. The Dynasty DX is an XMT 304 that took up some of the original room for the capacitors (shorter lifetime?), added a clamp board on top the recycles the switching spike energy back into the DC buss, and just adds the output IGBTs to the "output" heatsink. It can do 300 amps, but not for long and doesn't have any of the new trendy waveshaping (not something I personally need, but WTH). I found a really good deal on a bunch of 800 amp IGBTs, a couple 600 amp three phase IGBTs (1800 amp parallel), and some massive heatsinks, which should be good for half their rated current at 60% duty cycle. Just crunching the numbers, this would make the XMT 304 good for ~285 amps @ 60% DC Tig single phase and be limited by the machine's three phase (voltage drop on the output inverter is between 3 and 4 volts at that amperage). If/when I start those, I will put them on a different thread...

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                              • #45
                                That's why I work on cars for fun. You just do it because you can. Sounds harder then it needs to be this electronics? While that I'm sure this displays my ignorance, it may also be a testimony to making things more complicated then they need to be?

                                Seriously, I figured wire here, wire there, magic out the other end? Don't I have a lot to learn.

                                I have this spot stitch panel...I didn't keep it around all there years thinking I couldn't do something with it, I did however think it would be easier to put it to use? I thought a frequency adjustment could be almost as easy? Little do I know eh?

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                                While my attention is however drawn else where, I'm learning from your guy's exploits and answers.

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