I think SIP realised (early 80's) that they could ramp down the build quality and give Practical Classics a bung to rate the new, inferior models?
It's a nice old thing. I'd like to get it working properly, good for another 40 years service.
I cleaned up all the bolted connections earlier. Trans to choke was a bit loose but not burnt, like the Clarke last week. I think even I could refurb the PCB...
So perhaps the fault is electrical, rather than mechanical. Rectifier diodes breaking down when warm? Maybe a voltmeter - an analog one, even - might show some fluctuation whilst welding, or after gentle warming of the rectifier with a hairdryer.after a minute's welding the wire feed slows and the arc is erratic