Miller Dynasty 200 SD Fan Problems
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  1. #1

    Miller Dynasty 200 SD Fan Problems

    Just bought a used but "new to me" Dynasty 200 SD. It is an older machine, but all the welding functions work perfect. Only hiccup is that the cooling fan never turns on. I have not gotten it to overheat, but I assume it would if I welded for a while. When the machine starts it displays "-7" on the display, which I can't find any reference too in any of the manuals. I assume this is some sort of warning code.

    I've done a bit of troubleshooting already:
    1. Hooked up the fan to a power supply - it runs fine.
    2. Checked the resistance on all the thermistors - All within spec.


    That is all that is in the factory troubleshooting for the fan. I'm not too keen on replacing PCBs here. Anyone have tips on repairing this? Anyone know what the "-7" code means? I'll start tracing through the fan drive circuits on the boards but was hoping this would be an easy fix.

  2. #2
    Join Date
    Mar 2014
    Posts
    164

    Re: Miller Dynasty 200 SD Fan Problems

    It'd help if you can post the serial #

  3. #3

    Re: Miller Dynasty 200 SD Fan Problems

    Serial number is LE186332, Stock No is 907099

    Thanks!

  4. #4

    Re: Miller Dynasty 200 SD Fan Problems

    I checked pin 10 of RC1 on PC1 which looks like it is the fan control pin:
    * 10 Input Fan enable; 0 volts dc=on, +5 volts dc = off

    This line is at 5v and appears to stay at 5v. I laid down about 5" of bead on alum at 200A, which I assume should be enough to start the fan.

    I pulled that line down to ground with a 500ohm resistor and the fan turned on. The signal dropped to about 2.5v when I did this (guessing there is a 500ohm pullup to 5v somewhere on PC3).


    I guess this means the problem is in PC3?

    Looking at PC3: The fan control pin comes in on PLG1. There is a small tantalum filter cap right at the pin (C30), and then there is a 681ohm resistor(R18) to the Atmega16 micro pin. Presumably the Atmega16 is driving this at 5v so when I pull it down with the 500ohm it goes to about 2.5v. So unless the micro pin is burned out it isn't reading the thermistor properly (even though it reads correct resistance).

    Anyone know if the fan is based solely off the thermistor hooked to PC3 at RC2, or is it based on the second one as well? Thermistor at RC2 comes in, goes through a resistor(R38) then has a diode D6 (D6, 1N4146 from ground to the signal, I guess to control ground bounce) then another resistor(R14) to microcontroller pin. Diode tests fine as do both resistors seems fishy that the microcontroller pins would go bad.
    Last edited by arsenix; 10-07-2018 at 08:02 PM.

  5. #5

    Re: Miller Dynasty 200 SD Fan Problems

    Ok continuing troubleshooting here... the "supply feed" for the top of all the thermistors is supplied by an OpAmp A1. This is fed by a 10v voltage reference chip (LT1460D), which is putting out 10v. A1 cuts it in half down to 5v. That all looks like. The thermistors are all fed by 5v from that circuit. So that seems fine.

    For RC2 thermistor input the voltage into the ATMEGA is about 1.1v at room temp, goes up to 1.5v after I weld a bit. All the discretes on the board seem to be fine. Maybe the Atmega micro has some burned out pins?

    When I pull RC2 on PC3 out display switches to HELP-2. Goes away when I plug it back in. It isn't easy to unplug the "Module Thermistor" that goes in through PLG1 since it is on a ribbon cable. Maybe the issue lies there. I haven't traced that back to the microcontroller.


    I'm guessing the "-7" on boot means something here that will lead to some insight.
    Last edited by arsenix; 10-07-2018 at 10:53 PM.

  6. #6

    Re: Miller Dynasty 200 SD Fan Problems

    The "module thermistor" input to the microcontroller doesn't seem to change when I weld... it is at around 2.4v. Not sure what this should be. Not even sure where this other thermistor is. Somewhere on PC1?

  7. #7

    Re: Miller Dynasty 200 SD Fan Problems

    Any one have any idea what the "-7" on start means? Maybe that is a clue.

  8. #8
    Join Date
    Jul 2008
    Posts
    1,770

    Re: Miller Dynasty 200 SD Fan Problems

    Quote Originally Posted by arsenix View Post
    Any one have any idea what the "-7" on start means? Maybe that is a clue.
    Should be detailed in your owner's manual, I would expect.

  9. #9
    Join Date
    Mar 2014
    Posts
    164

    Re: Miller Dynasty 200 SD Fan Problems

    Quote Originally Posted by arsenix View Post
    Any one have any idea what the "-7" on start means? Maybe that is a clue.
    Might should call or email Miller tech support on that. I checked the tech manual and it doesn't even show that help code.

    https://www.millerwelds.com/support/contact-us-form

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