Aren't most MiG machines fairly modular? In that you have the feed section, the motherboard (more properly the main board I suppose, I was an IT technician so I think in terms of Motherboards), a cooling section and then stuff like a plug in torch and earth lead. So I'm looking for a machine that has good backup in the way of spares. Again, I ask I am looking for suggestions
Transformer based MIG machines are simple and modular. Even the control boards tend to be fairly simple and are either available as spares or could be reverse engineered. With some of the older machines there may be special-to-model parts and no easy substitute.
However, the CWS machine being discussed is a low priced inverter based machine. Essentially, they are much like the switch mode power supply in a PC, and who bothers to repair them? It is possible to repair an inverter based welder and there companies which do it, but prices are likely to start at £100 and in view of the new price of a non-industrial grade inverter welder (£200-£450), getting it fixed with some sort of guarantee on the repair - but not on other things which could fail soon after - isn't an obviously good idea. Inverter MIGs are modular, but the part which is most likely to fail, the main PCB, is likely to cost a significant proportion of a new welder.
Some makers' service manuals for inverter welders can be found on the WWW, and they describe things such as changing defective IGBTs and output diodes. They give circuit diagrams and a description of how the circuit works, scope waveforms and so on. However, they usually suggest that their agents don't attempt component level repairs on the boards and do a board swap instead.
Inverter MIGs have all sorts of advantages; they're lighter and more compact, they may have an MMS function, better duty cycle at the top end (where transformer machines can have a pitiful duty cycle), you can get more welding amps out of a 13A socket, but a lot of people prefer traditional transformer based MIGs - at least in the amateur market - because they are believed to be more reliable and usually more easily fixed if they do go wrong.