Troubleshooting dead Maxstar 150 STL (with pics)
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  1. #1
    Join Date
    Jul 2011
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    42

    Troubleshooting dead Maxstar 150 STL (with pics)

    Troubleshooting a dead Maxstar 150 STL for a friend.

    Plug it into wall power, flip the switch, and nothing happens. Welder acts as if it has no power... front panel is completely unresponsive. This is an old model (SN LE050152M) that doesn't have an inline fuse.

    I get the cover off and do a visual inspection, no obvious signs of damage.

    I get the cover off and power it up. Mains switch is working fine. With the cover off I can hear an audible squeal which is either the switching circuit, or a dying component.

    Looked at the welder with the thermal camera. Only signs of life is two of three (what I believe to be) power resistors and the heat sinked IC adjacent to them getting warm... see images

    The leftmost white box I can read the printing... says RCD K3-361 on the top line, 10W 15K on the bottom line. Sure enough, I ohm it out and it's 15.1 kohm. I check the other two, and they're not giving a solid reading... moving around like when you put an ohm meter across a capacitor. Google search on the part number turns up nothing. The three parts are jammed so close together, I can't read what values the two rearmost are supposed to be.

    The adjacent IC that seems to give off heat is the switching IC... PN 1M0680R... datasheet here: https://www.mouser.com/datasheet/2/3...RB-1120775.pdf

    The main board has two large caps on it... values check out fine. The positive rail on one is tied to the negative on the other. The next largest cap on the board, quite a bit smaller, also checked out fine.

    With power on, I measured the following:
    Rear-most cap: 85 VDC
    Front-most cap: -79.5 VDC (my positive meter lead was on the negative terminal of the cap, since this is the common terminal between the two caps)
    Rear-most Power resistor: 72.5 VDC across the leads
    Middle Power resistor: 79.7 VDC across the leads
    Front-most power resistor: -6.8 VDC across the leads
    Smaller cap (shows orange in thermal image... but wasn't warm, metal face was actually reflecting my IR signature): measured 2.3 VDC

    I didn't have the pinout for the switcher at the time, so I didn't check VCC on it. Can't get to it right now to see if it's in spec.

    Anyone have any experience troubleshooting this model, or have any suggestions based on the info I've provided so far?


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  2. #2
    Join Date
    Jul 2011
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    Re: Troubleshooting dead Maxstar 150 STL (with pics)

    Thinking about this for a few minutes, I definitely need to get back in there and check VCC on the switcher and whatever else that I can safely probe.
    The total voltage across the caps, about 165V, makes sense. The power resistors, I have no idea what they're for, or why the two are behaving as they are. I hope someone here can give me some insight.

  3. #3
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    Re: Troubleshooting dead Maxstar 150 STL (with pics)

    Did you check the board close to see if there were some fuses blown, or anything else. These aren't the obvious fuses they are soldered into the board.
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  4. #4
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    Re: Troubleshooting dead Maxstar 150 STL (with pics)

    Quote Originally Posted by blawless View Post
    Did you check the board close to see if there were some fuses blown, or anything else. These aren't the obvious fuses they are soldered into the board.
    When I looked over the board for damage, I didn't see any soldered fuses. But when I go back to measure VCC on the chips, I will check again.

    Sent from my Moto G (4) using Tapatalk

  5. #5
    Join Date
    Mar 2010
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    Ceres, California
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    3,261

    Re: Troubleshooting dead Maxstar 150 STL (with pics)

    If you have power to the pcb either 115 or 230 volts. Follow the wires from the switch.
    And welder does not light up. no blue light. Power pcb is dead. the whole power pcb will have to be replaced.
    Trying to repair is near impossible. There are no published diagrams and miller will not give any out.
    I would have a Miller authorized shop install the power pcb. that way you get some warranty.
    Self installed Miller pcbs will void any warranty on the new pcb.

  6. #6
    Join Date
    Jul 2011
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    42

    Re: Troubleshooting dead Maxstar 150 STL (with pics)

    Quote Originally Posted by ccawgc View Post
    If you have power to the pcb either 115 or 230 volts. Follow the wires from the switch.
    And welder does not light up. no blue light. Power pcb is dead. the whole power pcb will have to be replaced.
    Trying to repair is near impossible. There are no published diagrams and miller will not give any out.
    I would have a Miller authorized shop install the power pcb. that way you get some warranty.
    Self installed Miller pcbs will void any warranty on the new pcb.
    Yeah... that sucks, as a new main board for this welder runs almost what a used welder runs for. So it's not really worth fixing, imo. Not at least without trying to hack up the main board first.
    I was hoping that between here and the Miller forum, I might find someone that's already been through this. Looks like I'm not going to get that lucky though.

    Once I get back to working on this, if I make any progress, I will be sure to post up what I did, to pass along the knowledge.

  7. #7
    Join Date
    May 2009
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    Wyoming US
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    3,155

    Re: Troubleshooting dead Maxstar 150 STL (with pics)

    Odd that you can hear the switcher, unless you are hearing a harmonic.
    Those type power supplies are nasty to trouble shoot,too many feedback circuits.
    They tend to use simple ways to power switcher chips, might have a shorted zener or a shorted cap.
    Some of the older welders had a low power circuit as the first thing that gets power.
    It provided plus/minus power to the logic boards and the relays.
    If it went out nothing would work or light up.
    I suppose check the power running the switching chip (VCC) and see if a feedback circuit is messing with the chips control lines.
    Last edited by Bluewelders; Yesterday at 03:17 PM.

  8. #8
    Join Date
    Jul 2018
    Location
    Bay Area, CA
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    14

    Re: Troubleshooting dead Maxstar 150 STL (with pics)

    Working through something similar right now. the board is real similar to my Passport Plus, I think those 3 big resistors are all the same value, there is a 0 ohm resistor used as a fuse on mine. get the magnifier out and start lookin around the middle of the board, but I think it may only protect one of the ICs. Ive learned a lot from my repair attempt so far, like my shop teacher used to say: "Dont destroy the evidence" dont start desoldering and poking too hard til youre sure youre on the right track. Youll end up buying a bunch of stuff from Mouser or Digikey that wasnt broken before. Id check all the tiny stuff first, get tech sheets for everything i can get a part number on and look for vcc and gnd on all of it. Theres multiple control voltages depending on the IC in there and way more on the switching section. Just be careful!!!

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