Dynasty 200SD low buss voltage

[jjohn76]

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...

[jjohn76]

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.

[Aeronca41]

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.

[jjohn76]

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 element14.com 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.

[Aeronca41]

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.

[jjohn76]

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!
Jon

[jjohn76]

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.

[jjohn76]

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.

[Aeronca41]

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

[jjohn76]

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.

[jjohn76]

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 the C12/C13 midpoint, 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

Edit - note in my picture that I had the common connection between C12 and C13 the same as XFMR2. This SHOULD NOT be the case - the pair of Snubber IGBTs are in series, emitter to emitter, between these two points. Mine showed a short between the two. The Power Module pre-power checks catches this if you have a later model (after LK050198L), but earlier models don't have direct access to that test point, so if you have a problem on PC2 and PM passes all other power checks, you can do a diode test between RC4-6/8 and the C12/C13 point directly on the module (see attached picture, left side middle). With a diode tester: red lead on RC4-6, black on C12/C13 midpoint should be ~0.6V; black lead on RC4-6 and red on C12/C13 midpoint should give open loop.

Jon

[jjohn76]

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 has a very low pitched buzz sound, somewhere below mains frequency. This thing is going on Craigslist for parts...

[jjohn76]

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) shows signs of the solder connections overheating, though it still functions properly (test is in a post above).

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). Visual/Audible clues are a low pitched buzzing sound when powered on, or signs of burnt PCB/discolored traces at the bleeder resistor arrays (two of them, they each have three orange 150k ohm resistors side by side). The PM1 check above should identify any failed component in semikron PM before anyone starts replacing buss capacitors, bleed resistors, or PC2 entirely.

https://forum.millerwelds.com/forum/...ding-the-parts

[jjohn76]

Updates to this thread, the Maxstar PM1 passes all checks above, which includes the Snubber IGBTs. I am pretty sure at this point the capacitors have aged to the point they are drawing too much current and overheating. The photo below hopefully shows how some gas has expanded in the tops of the capacitors (the cases themselves are not bulging, just the plastic cover is blistering). With PM1 functional, I am just planning to replace the two buss capacitors - these are 50mm diameter by 80mm tall Panasonic T-UPs, which have a CDE direct replacement - 382LX152M450B082VS. Total cost for two plus shipping and taxes is $52. I will also replace the input bypass relay on PC2. The only concern I still have are the hot marks on PC2 from the R1,R2,R36,R37 resistor network on LC2. These 4 ea 3W 150k ohm resistors basically make an equivalent 150k resistor with around a 12W capacity. These are not the capacitor bleed resistors as thought in a different thread - these basically limit current in the 15V bootstrap power supply. It connects directly from the DC buss (163V to 325V at power up depending on 120V or 240V AC input, then 810V once the boost PFC circuit is engaged) to a 15V zener and filter capacitor to provide 15V to the buck circuit controller (left side of PC10) that controls the 30V supply linked to every other board. I noticed these are soldered much closer to the board on the Maxstar than the Dynasty (about 3/32" space between resistors and board shown in pictures). The buss capacitors are in series, and the bleed resistors are two each triplets of the same orange 150k resistors. These are not showing signs of overheating (like they were on the previous Dynasty), so my guess is the DC buss voltage wasn't excessive. This leads me to believe the hot spots are common on early designs and Miller incorporated the standoff on later designs.

blistered top of cap, the lower right shows the bleed resistor network
top side of PC2, orange resistors in contact with board on Maxstar heat marks on bottom of Maxstar PC2
Dynasty orange resistors have a bit of standoff from PC2

[Noel]

I have a buddy that does cross word puzzles...What you have there is the New York Times edition. I'm thinking you both are learning new words as a result of your interests? Well done! I'm cheering for you.