XMT 300 cc/cv welder problems

[Glen Leininger]

I purchased a XMT 300 cc/cv and MIG assy. I pluged it in and it didn't work. After playing with it I found the connector to W1 loose in the socket. It still didn't work. It had sat in a cargo container for mant years. The serial number is KD502780. I checked the switches for operation, the bottom switch was not working the same as the middle one. I disassembled it and put it back together twice, wrong operation first time, now it functions the same as the middle switch.

With all of the switches pushed to the right, it still doesn work. Sometimes it will charge to 8 or 10 volts at the output, current not at zero and voltage set to 20. So I got the scope out, and watched the output. I get occasional pulses in both + and -. If I try to load the output the voltage goes to zero.

I started checking the regulators and I shorted the right 2 pins, followed by resistor getting hot on the +v regulator, it is just to the right of the pulse transformers. What should this voltage be? Have clipped the center lead on regulator and it want to 10 vdc with no load. I probably shorted a filter cap, + voltage went to +32vdc and caps are probably rated for less. There is a test connector on the board. The artwork is 159978. What are the names of the pins in this connector. My initial goal was to see if the pulse generator was trying to drive the igfets. The bridge appears to be in good condition, the bus caps charge and the series caps look good.

So now to my questions:
1) what are the test point names?
2) What is used to remove the coating?
3) How can I check the Bridge IGFETs?
4) Can I obtain a schematic for the artwork #159978?

This was working before it was set on the shelf then into a cargo container. So I think that with all of the inital problems it had brfore I blew the board, it would have worked.

[jjohn76]

Hi Glen,

To Home and #4, it's probably very unlikely that you will find information on the test points or board schematics, unless you can track down the tech manual. Have you already downloaded the operator manual? These are fairly old machines. But all is not lost if you want to relate different controls to different welders. You can readily find the XMT304 manual (Red d Arc website, search for EX300 or Extreme 300 tech manual) that has board schematics to give you an idea how the XMTs work. It's been a while for the XMT300, but I believe the current and voltage references are derived the same way with analog components. I believe the biggest difference between the 300 and 304 was control of the resonant tank capacitor voltage, but it's been a while.
For #2, I tend to use acetone on q-tips and a lot of patience. Those coatings definitely do their job.
For #3, it's best to test anything on the primary/mains side, including the IGBTs with just a multimeter unless you have HV differential probes for your scope. There are other ways, but best to research and practice those before trying. Definitely start all testing with the machine powered off. IGBTs tend to fail closed, so if your bus caps aren't reading short, the IGBTs are probably ok.

Based on how it was stored, there's a good chance one of the connectors and/or one of the board/component leads is corroded. Either that or one of your electrolytics failed somewhere.

Your board is powered by the AC secondary that connects across pin RC5-2 and RC5-3. My guess is it's probably 34V across those two pins, giving you ~+/- 24VDC at your voltage regulator inputs. What's written on your regulators? 7815 and 7915?

Hope this helps,
Jon