Welder SCR heat controller issues

This document is also relevant: **broken link removed**

Sorry to re-ignite such an old thread, but my question seemed to flow on from much of the discussion here. It's obvious many who participated will be familiar with the circuit I am having issues understanding. here is my question:

After 12 years of sterling low current usage my trusty and dry stored Migatronic 220 MTE Dialog has thrown a wobbler. I run it on UK single phase mains, on a 80 amp supply. I very rarely run it at all hard, I am a race car engineer and most jobs are thin ali work, and thin stainless. The AC "over voltage on the torch socket" warning light is on all the time, even when idle and just powered up in AC mode. On AC the current is uncontrolled at the torch when the torch button is pressed. If I turn it to DC with the unit powered up it instantly draws MASSIVE current off the single phase mains, and emits an alarming low hertz buzzing. This is without the torch switch being activated. I just have to switch it to the DC setting. I took the covers off (I am a licenced radio ham, so have some electronics knowledge, and understand good high voltage and high current safety techniques), and had a quick poke about. R1, a 10 ohm wire wound over a grooved ceramic former resistor with a slidable tap, sitting on top of the cooling fan, has heated to failure. The thing is about a foot long and an inch in diameter, but fed with very thin wires. The section from M3 pin 3 to M3 pin 10 is the section that's had extreme heat, and the ceramic former is shattered. The other "half" shows no sign of distress. The overheated section has failed open, and is dangling into the fan blades. I am trying to understand the schematic and work out what would have caused one section of R1 to heat so badly. I suspect a triac driver issue or one of the 2 thyristors in the secondary has failed, but I am unsure. . Anyone familiar with the workings of these things? All the (Type I) control panel functions seem alive and well, the current digital display ramps up and down, gas control, blah blah still responds. It seems to be a control switch / board or rectification issue, (I think....). Thanks! The manual with schematic is at https://www.gatesgarth.com/manual.pdf if it's too big as an attachment to this posting. Thanks for any insight.