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  1. #1
    Join Date
    Oct 2008
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    97

    China ACDC TIG, Mitech 160 malfunction

    I have been reading about the China ACDC machines around here and decided to pick one up that was non-functioning. When in MMA (stick) mode the unit turns on, the amp pot works and the digital display adjusts from 1 - 153 amps. The internal fans are on.
    The OC (over-current) light is also on and the + and - weld terminals read 0 volts. It seems like there is a short causing the OC LED to come on. I pulled the cover and the left side PCB (viewed facing the front of the machine) has the green LED on and the red LED on going to the OC LED on the front panel.
    I diconnected the large steel lead going to the right side of the machine and ohmed the two steel leads on that side. It read infinite so no short on the right side PCBS/alum_block. The left side with the transformers/coils reads almost zero ohms but I think it is because of the input tramsformers in parallel with the switching MOSFETS. The lower PCB MOSFETS on that side read almost zero ohms across the drain and source.
    None of the MOSFETS look blown and nothing has a burned smell to it. The machine just stopped working and the previous weld bead was only 1 inch long at 80 amps.
    Anyone have a schematic for this thing or have seen this problem? I am going to start disassebling the left side (PCB with green and red LEDs) and check the PCB at the bottom of the machine with two big clear cased relays (24vdc).

    For those that dislike the far east machines......yeah I know.......I should throw it away. I like messing with these things. The USA units hardly ever break so they are no fun to play with.
    Thanks,
    Junk

  2. #2
    Join Date
    Oct 2008
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    97
    Progress.....
    It looks like the left side top PCB with the MOSFETs that creates the first stage of AC from the two diode rectifiers is fine.
    The lower left side PCB shows a short between the two main lugs that go to the right side H bridge to make the output AC and DC to the front of the machine.
    Chasing that back showed that the lower left side PCB output diodes have a short. Just need to find where the short is. Maybe a bad diode or bad solder joint...maybe some solder got hot and formed a bridge.

    Overall, top left PCB is the first stage of creating AC from the diode rectifiers that output DC from the main wall input of 220VAC.
    Lower left PCB turns that first stage of AC to DC using transformers and diodes (that is where I have a short).
    The output from the lower left PCB goes to the right side upper PCB to make the welding Freq. AC (what you use with the tig torch) and the lower left PCB creates DC if in DC mode at the front of the machine.

  3. #3
    Join Date
    Oct 2008
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    97
    Had some time to take apart the machine today. I had to remove the left side PCBs attached to the large aluminum heatsink. I then pulled the lower PCB from the aluminum heatsink.

    After some poking around with the continuity meter it looked like something was up with the diodes going from the large toroid inductor output back to the transformer center taps. All the solder joints looked very good like a USA quality product. The large weld output diode data sheet shows they are good for up to 240Amps continuous. So they are sized correctly for a 160 Amp machine.

    I unsoldered the small diodes from the output inductors(large toroids) to the input transformer center taps. I then ohmed them and each (two in paralle) was shorted both ways (blown). Then I checked the larger one they were connected to. It was shorted both ways. I pulled that one and the part number 1.5KE150CA shows it is a bi-direction TVS (transient voltage suppressor). Overall, it looks like the TVS and two diodes are blown. The short at the PCB was gone after removing them. The two small diodes in parallel were labeled HF 104. So it looks like they are 400v, 1Amp devices. The package size shows they are only good for 30Amps of surge current each. The TVS connected in series to them is good for 200Amps of surge current.

    I am going to replace the TVS with the same size and upgrade the two diodes to either 50Amp or 100Amp units. The 100 Amp units may be to big to fit in the space. Overall, it appears to be a flaw in the design so far.

    Also, after looking at Riland, Chiry, and Rijui (spell) pics on the web of internals I have come to realize that they are all pretty much the same circuit design when looking at the board layouts. Some machines have two PCBs built into one and are located in different areas internally. But overall the components pretty much look the same.

    The HF start PCB is interesting in this Mitech. It uses a HV coil, some caps, and copper contacts for the spark gap.

    Hopefully I can get the box back up and running with $2 worth of parts from Digikey (and probably $10 for shipping).

  4. #4
    Join Date
    Jan 2007
    Location
    Deltaville, VA
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    885
    This poster really likes talking to himself.

    First it was a lengthy discussion about how to rebuild a Harbor Fright tig machine and now it's another piece of junk.

    If he spent half as much time working on his "welding skills" (oh yeah, he already knows all about that) as he does tinkering with junk, he'd be ahead of the game.

    Still hasn't figured out that, "you can't make a silk purse out of a sow's ear".
    SundownIII

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  5. #5
    Join Date
    Oct 2008
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    97
    It is about putting the info. out there so others can learn from it and possible fix their own machine. There is more to welding than just taking classes, reading about the right rod or tungsten, or spending hours running beads. If you understand how a machine works you can use it to the fullest.
    I don't know why you bother post to the threads if you can't be open to learning new things. Your post adds nothing to this thread other than your nonsense saying which is kind of old fashion.
    Last edited by junkweld; 02-22-2009 at 06:35 PM.

  6. #6
    Join Date
    Dec 2007
    Location
    Buffalo, NY
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    7
    Quote Originally Posted by SundownIII View Post
    This poster really likes talking to himself.
    Original poster isn't talking to just himself, he got YOU to read his posts didn't he? And look at the number of views for this thread, seems like he is talking to a bunch of other people too.

  7. #7
    Join Date
    Jul 2006
    Posts
    16
    This thread is no different than the hundreds if not thousands of posts where the OP provides blow by blow documentation of some project, whether it be building a welding cart, lengthening a truck bed, restoring a vintage though obsolete buzz box, or in this case trouble shooting a tig machine. Keep on posting JW, many others are interested. Jeff
    LINCOLN IdealArc Tig 300/300
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  8. #8
    Join Date
    Dec 2008
    Location
    Australia
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    118
    Dont know the machine, but check the mosfets and the gate resistors, from what you say the output diodes are shot, wouldnt worry too much about increasing the rating as they might be rated to suit, so that if they blow they wont take out anything else, maybe!!!!
    Cheers
    Jake

  9. #9
    Join Date
    Oct 2008
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    97
    Quote Originally Posted by mendoje1 View Post
    This thread is no different than the hundreds if not thousands of posts where the OP provides blow by blow documentation of some project, whether it be building a welding cart, lengthening a truck bed, restoring a vintage though obsolete buzz box, or in this case trouble shooting a tig machine. Keep on posting JW, many others are interested. Jeff
    That is exactly what this thread is about. It is interesting the projects you mention. One of my rigs is a chevy truck cab and bed on a Blazer frame. I cut the bed down to 5 feet long in order to fit on the short Blazer frame. My welding cart for the MM130 MIG is an old dumpster dive gas grill frame I cut and modified.

  10. #10
    Join Date
    Oct 2008
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    97
    Quote Originally Posted by Jake_SS View Post
    Dont know the machine, but check the mosfets and the gate resistors, from what you say the output diodes are shot, wouldnt worry too much about increasing the rating as they might be rated to suit, so that if they blow they wont take out anything else, maybe!!!!
    Cheers
    Jake
    That is exactly the first thing I thought when I saw a short at the output of the lower left side PCB output. After pulling the board I saw the TVS and small diodes coming from the output back to the transformer center taps. Since all the large output diodes (12 of the FML 33S) would be a lot of work to remove and check for shorts, I decided to remove the small diodes (little round ones DO-41 case) and check them first. It was the easiest to check and due to size it looked like the easiest to fry. Once I unsoldered one side of the diodes from the PCB I then ohmed the both ways and they were shorted both ways (blown). I then checked the 1.5K TVS and it was also blown.

    With the TVS and two small diodes removed I then ohmed the output. I read infinity one way and a diode drop the other way. That is, all of the 12 welding FML 33S (20 Amp max. each) diodes are good. None are shorted. Since each bank of FML 33S diodes can handle up to 120 amps at 50% duty cycle (20KHz AC freq square wave) then replacing the TVS with the same unit and the small diodes in series with 50 Amp surge peak current will blow the small diodes before the welding diodes.

    It seems like I may have had a voltage transient on the input wall 220Vac and caused the TVS and small diodes to blow. I will try and get a digital camera and take a picture of where they are. I could also label the parts on the board and what they do.

    The board actually looks like it was designed and built well. All of the solder joints look good. The board is encapsulated / potted (has a plastic type coating) to keep small metal particles / dust from shorting out things. If it wasn't for the 1.5KE part having a "ML" as the manufacturer which checks out as China origin and the transformer didn't have a chinese name, I would think it was a USA fabbed board. The only downfall looks to be the TVS and diode circuit. With any design using a TVS.......it is there to protect the other circuits and it appeared to do it's job.

    Maybe I can snap a digital picture of the board and post where the parts were and what the other parts on the board do. I am going to order the parts this week and maybe put it back together this weekend.
    Last edited by junkweld; 02-23-2009 at 01:16 PM.

  11. #11
    Join Date
    May 2007
    Location
    Medford, MA
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    456
    Quote Originally Posted by junkweld View Post
    The USA units hardly ever break so they are no fun to play with.
    That comment is just too weird for words

    Seriously - Nice thread on diagnosing & troubleshooting problems.

    Frank

  12. #12
    Join Date
    Sep 2003
    Location
    SF Bay Area, CA
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    199
    Unfortunately the Chinese manufacturers will use the absolute lowest bid components, so the quality of the design or pcb construction is somewhat moot.

    I have even heard of Chinese manufactured electronics where they INSIST on no component final testing from the manufacturer in order to save a few cents on the price of each device (in this case Op-amps).
    HH210
    2 Smith Airline outfits, 2 Harris 50s,2 W200s ,J27, Meco Midget and Dillon
    Thermal Arc 185

  13. #13
    Join Date
    Oct 2008
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    97
    Quote Originally Posted by kenc View Post
    Unfortunately the Chinese manufacturers will use the absolute lowest bid components, so the quality of the design or pcb construction is somewhat moot.

    I have even heard of Chinese manufactured electronics where they INSIST on no component final testing from the manufacturer in order to save a few cents on the price of each device (in this case Op-amps).
    Lots of companies use lowest bid components for a speced part. That is how things are done. It is not country dependent. It is company dependent. The quality of the design does matter. That is one of the reasons people claim these machines are junk. This PCB does appear to be decent quality.

    I have heard a lot of things about a lot of things. Have any actual proof and company names?

    All I am trying to do here is document fixing a REAL IN MY HAND machine and giving my opinion (just mine...may be worth nothing to anyone else) of the design and build quality. Maybe others can learn from it.
    Yes, a real machine in my hand......not a "I heard of" machine. Not trying to give you a dig. If you've heard of companies not testing to save then I would like to have the names of them so I can steer clear or buy their DOA machines at a cheap price because they would have a fair amount of DOA and I could fix and sell them as a part time hobby job.

    The parts should be here later this week. I will work on getting a digital pic of the ckt board.
    Last edited by junkweld; 02-24-2009 at 11:53 AM.

  14. #14
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    Oct 2008
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    97
    Off topic post:

    When my miller MM130 MIG had a problem of sparking ever time you touched the wire to the work piece I opened up the box. The two resistors across the large output capacitor were blown (open). It would not discharge the cap after running a bead. So the wire would spark before pulling the trigger just for a second. It was annoying. Miller wanted $20 - $25 for the two resistors. I bought them new retail from an electronics supplier for $5. Wow, nice mark up. I also put in larger watt ones (same resistance) that would not blow out as fast and laster longer. for 4x-5x less than the original part. I am mentioning this because a lot of the USA stuff is good product, but over priced.
    Last edited by junkweld; 02-24-2009 at 12:07 PM.

  15. #15
    Join Date
    Jun 2008
    Location
    Las Vegas
    Posts
    56
    Quote Originally Posted by junkweld View Post
    Lots of companies use lowest bid components for a speced part. That is how things are done. It is not country dependent. It is company dependent. The quality of the design does matter. That is one of the reasons people claim these machines are junk. This PCB does appear to be decent quality.

    I have heard a lot of things about a lot of things. Have any actual proof and company names?

    All I am trying to do here is document fixing a REAL IN MY HAND machine and giving my opinion (just mine...may be worth nothing to anyone else) of the design and build quality. Maybe others can learn from it.
    Yes, a real machine in my hand......not a "I heard of" machine. Not trying to give you a dig. If you've heard of companies not testing to save then I would like to have the names of them so I can steer clear or buy their DOA machines at a cheap price because they would have a fair amount of DOA and I could fix and sell them as a part time hobby job.

    The parts should be here later this week. I will work on getting a digital pic of the ckt board.
    I have chinese Super200P machine (TIG, Stick, Plasma.) I don't use its plasma capabilities because switching from TIG to plasma and back requires switching gas for compressed air, disconnecting the TIG torch, and connecting the plasma one and go through the entire PITA again to switch back. That is why I bought a dedicated plasma cutter, Cut60D (also chinese.)

    I can not say any of those machines is any worse or better than comparable US made ones. May be I'm lucky but the components are decent, PCBs are OK, and nothing's wrong with their design. I'm an electronic engineer and I do know what to look for. They lack some capabilities of the latest US made machines (e.g. Super200P has only 60Hz AC TIG frequency) but otherwise they are fine machines.

    I have not looked into many of them so it might be I'm just lucky There is also a nice touch in them -- all transformers and coils have clearly marked turns number for each and every winding.

    The only thing that will eventually fail is their HF unit because they use a pair of relay contacts for a spark gap.

    One more thing -- those who just purchased such machines should open them first and check for loose wired and disconnected connectors. I had those in both machines. That is not and indication of poor quality or some particular deficiencies. Those machines simply made a very long trip across half the planet enduring rough handling, vibration, etc. so I would be surprised if something didn't get loose. I'm pretty sure US made machine would not have arrived in better shape if it had endured such a long journey.

    And remember, you can but FOUR such machines for a price of one Dynasty 200DX or similar one.

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