250ex arc problems welding on ac but not dc ?????

Started to weld sum aluminum the other week and my machine seems to be stuck in pulse mode but it’s not on pulse. The arc cuts in and out I’ve messed with the balance frequency and doesn’t seem to change anything using the foot control or torch buttons same thing. It welds fine in dc pulse spot timer all work fine I haven’t been able to get in touch with support recently ray emailed me back once and left a message I called back and left a message also seems like it’s trying to ball the tungsten like the inverter is switching to negative but the positive side is cutting out in the cycle i really don’t know how else to explain it. I work in the power industry as a millwright/ power plant operator , I know my way around electronics pretty well for the most part and have a pretty good understanding how the inverter based welding machines work and I’m decent welding aluminum. Just wondering if anyone else has had any problems or where to start to look as far as troubleshooting thanks.

You should be able to hear a difference in the sound of the arc as you change the frequency control. If you say that does nothing, then you have a problem. Obviously, the first thing would be an overall visual inspection.

Are you trying to set it as a percent of positive or negative? It should be set somewhere around 30%. It's possible that if you can't hear the AC arc, that a plug on the main board may be loose...or somewhere in between the main board and the drive side of the unit.

Yea, sounds like a plug is loose or off. Before opening the unit. Make sure you are in AC mode, pulse off for now, and the two yellow knobs are (left) at 2 o'clock and (right) at 11 o'clock. Should not sounds like DC at all and should not pulse. If still doing it I would unplug and pop the cover and check the plugs on the main (top) board and front.

http://youtu.be/rFi9kHr5v4M
here's a link i uploaded a video if it works sent it or tried to send it to tec support. but checked all the plugs they looked good. The start and stop of the arc you really cant tell if the frequency is changing ive welded quit a few aluminum pieces with it before just seems odd for it to start doing this out of the blue

wasn't really trying to weld in the video just trying to show what i was talking about with the arc starting and stopping but thanks for the replies

All you are getting is DC with blips of AC. You need to check the top board inside (after you unplug it and wait 15-20 minutes), and then check for loose wires. There's an wire block on the front of the board, that if it comes loose or not fully seated can create an issue similar to this. Check for any other loose wires or crossed connections.

checked all the wires and board connections nothing was loose and haven't noticed anything that looked burnt. I was thinking if I'm correct there's two sets of inverters one after the rectifier to switch to a high frequency signal then through the transformer then it gets rectified back to dc and the secondary inverter that makes the sign wave or square wave for ac and controls frequency balance pulse through the control board for that inverter ???? if so which set of igbt's and what board controls the ac side of the machine so I can have a better idea what area to focus on. Also I have sent the machine in once about a year ago there was a problem with the chip that controlled the foot pedal I guess its a little older of a machine and they had to solder a new board for the chip.

There's a plug on the front top side of the board...(or used to be) near the middle, with 4 wire or so, maybe more I think. It could be labeled "AC" or something. The igbt's for the AC side are on the right hand side I think facing the front panel as you normally would. Its right behind the main panel. I wish I had one out to show you...It's been a while since worked with it though. It's not too hard to trace. Could be a bank of igbt's that were buying their time after getting over amped to act up?? Mike might have a better assesment.

alright thanks i will look a little closer into it. I really appreciate the help if I don't figure it out i will give you or mike a call

ok I looked over everything a few times and nothing is loose how should i go about troubleshooting the igbt's and the control board ?

That's something you need to do on the phone with a tech person.