Any ideas? I was going to treat it to a new torch/liner anyway as it seems like it hasn’t been changed for a while. The wire feed feels strong when I press the trigger.
It’s a replacement for my R-Tech which just went bang for some unknown reason.
That is good news that you have been able to solve the problem. I'm pleased that your welder is working again.
From the diagram above, the spot timer circuit is disabled by the switch being open-circuit, so it looks like the switch on the back of the pot has failed permanently "on". A new switched pot of the correct value should fix that - or you could add a separate small on/off switch for that circuit.
R-Tech are known for excellent warranty and after-sales support. Have you asked them to fix it?