So I took the gamble of spending $40 on fuel to go and look at the welder, and flippin heck it was bigger and heavier than I thought! 60kgs heavy to be precise, he'd had it weighed to get a freight quote.
I made a silly offer which he accepted, burled it in the back of the car and headed home! It was almost new, apparently all it had ever done was make a bullbar for a 4WD and that's all, looks like it too, the insides were still clean even! He reckons the original owner had paid $1,600 for it brand new not too many years ago.
Fired it up and sure enough the wire feeder was going full speed. Took the sidecover off, flip that's a big rectifier and transformer! It's no chinese cheapie that's for sure.
Measured the speed control pot and it was OK, so must be in this board as the feed motor is driven from here and the pot as well:-
Traced the board out and tested everything and hey presto the MJ2955 (big silver thing in the middle) is shorted out between collector and emitter, even when removed. Makes sense as it's the driving transistor for the motor, being shorted it's holding the motor on full speed. Scrabbled around my spares bin and couldn't find one so a quick trip to Jaycar tomorrow should see the $2.95 part in and working hopefully. I'll replace it's base driver transistor as well just to be certain although it measures OK. Then I can have a play hopefully
So looks like I've scored one mean mutha of a welder for little bucks