Please Help with XMT 350 CC/CV Auto-Line low voltage displayed, no welding output

[Torchplasmasales]

Good Morning,

I have a Miller XMT 350 CC/CV Auto-Line S/N MB29012A. Its connected to single phase power which is at around 245 volts AC phase to phase.

The machine starts up and shows data on the display. Any setting with a voltage display reads between 0v and 0.3v. The amperage displays according to what I select on the A/V pot. When I change the selector to another setting the voltage will read at around 30v then quickly ramp down until it reads 0.3. If I connect the work clamp to metal and connect try to spark an electrode on stick mode The voltage display reads 0. I measured the voltage between the studs with a multimeter and saw similar values. A clamp meter on the work cable showed 0 amps while attempting to strike an arc.

I have continuity <0.5ohm between the work and either lug from inside the case.

Can anyone please recommend some things I can look into to try to identify the problem? I am trained to work inside of high voltage systems and am comfortable with a multimeter if that helps.

Thanks in advance for your help.

[Bushytails]

Download the service manual (look for one for a red-d-arc ex360) and follow the pre-power inspection checklist. It finds a large portion of problems on these machines.

[Torchplasmasales]

Thank you for the tip. I'm running through it now.

It was going very well until I ran into some anomalies. I am hoping that the brains here can help shed some light on what I'm seeing.

I don't want to incorrectly influence anyone's thought process but I already had observed a few things that lead me to suspect that PC1 has been replaced recently. I unfortunately do not have access to the history of this machine.

Everything tested perfectly in spec until the PC1 - Preregulator Control Circuit Test (Page. 28/29)

What I am seeing is that D47 and D53 appear to be omitted from my PC1 board.
D50 and D52 are on the far end of the board from where the check docs said they would be located and both of them tested outside of spec (.136 < .20) and (.138 < 0.20) respectively.
U6P6 -> U6P7 (Buck IGBT Gate Driver ) Tested out of spec as well. (.166 < .20)


Thanks again for your help. I look forward to learning more about this.

[Torchplasmasales]

One detail that I left out from my original post is that when the selector switch is on VSense the voltmeter cycles between whatever value is selected on the V/A pot then .4, then .3 then repeats. Connecting the electrode to the work on this setting has no effect on the the voltmeter displayed value.

[Bushytails]

Anything >0 is likely fine when doing the diode checks, especially if it's only in one direction.

I'd start with the parts closest to the output, like the output diodes, and work back from there. The earlier checks for input circuitry will probably be good, since the unit powers up.

[Torchplasmasales]

Am I understanding correctly that the output diodes are configured as a rectifier? And if so should I be able to read an AC voltage feeding into the diodes? Does anyone know which nodes I should expect to find the AC voltage on?

[Bushytails]

Being an inverter machine, the AC is very high frequency, and can't be read with most meters. You'll want to use an oscilloscope. I can't remember if there's sample waveforms in the manual or not - work has me too busy to check.

[Torchplasmasales]

Thanks. I have a stack of oscilloscopes both analog and DSO models that I can use. Do you know at what location I can measure the AC the voltage from?

[blue_tech]

The frequency of the AC in the T1 transformer is 25k Hz. There is about 330 VAC on the primary side where the HF Transformer connects to the PC2 interconnect board (right side) and it steps down to about 55 VAC on the secondary side where the two yellow leads come out to connect to the output diodes on the left side of the unit.

[Torchplasmasales]

Thanks I will take a look at that today. I’m kind of at a dead end with this welder. Usually it’s obvious where the problems are but on this one everything I test is turning up good. I just have 0 voltage at the lugs. I’m starting to wonder if someone screwed around in it before I got it. It looks like the PC1 board has been replaced and someone had done a remarkable internal cleaning job.

[Torchplasmasales]

New info.

The fan is not running, although I read 120 V at the fan plug. When I plug the fan into external power it runs fine. When I plug it back into the board the voltage drops to 12VAC but I cannot see any current drawn by the fan. Weird.

[blue_tech]

There will only be 120 vac at the fan when it really needs to turn on. It is a very long story, but the triac that is gated to send the 120 vac to the fan motor is "leaky" and will show voltage with a meter. However, there is not really the voltage present to the fan motor until the internal temperature is high enough.

If you have the 470 VDC on each bus capacitor, the next step is the PC1 gating the IGBT's in mod2 to create the AC to the T! transformer. If you don't get the AC coming into the T1, the board is not gating the IGBTs, or the IGBTs are bad. Only other random item that comes up from time to time is if the Hall Device is bad. It can be unplugged just to see if you get OCV output back, but NEVER load the unit with this unplugged.