Right, new state of play. I've got a new multimeter as the Vac settings on my old one were knackered. I can now comprehensively rule out the contactor and the rectifier. The rectifier is only being supplied 9Vac on the lowest power setting and turns it into 11Vdc, so it looks like the rectifier is doing as it should, just that it is being under supplied. I've got a good 240Vac supply to the main power selector switch when the contactor is closed, so I need to look at the diagram to see how it is wired and what I am expecting it to output.
I've checked the continuity of the output side of the transformer (that supplies the rectifier) too and it appears good.
As you are probable aware the selector switch will have a common connection in and the switch position will determine which part of the primary winding is used. If the switch contacts are poor/fried then they wont pass much current = poor input to transformer = poor output! As the transformer is not (?) humming VERY loudly I guess, then would assume it's not shorted internally. Try disconnecting the common onto the switch and meter between it and one of the selected outputs. Should be <5 ohms (ish). As you seam to know your way round electronics you could pick a switch position and bypass it to the common and see if it then welds or ups the voltage indicating a duff switch.
Thanks, always helpful to have someone to bounce ideas off of/sanity check.
The machine is a 240/400V machine, so the switch wiring doesn't appear quite as trivial as common in and 'selected setting' out. There are two common ins and two rows of outputs. Same theory applies, but just makes it a bit more necessary to get my head around the diagram before I tinker.
Could it be that the machine is 3 phase? Even though it has 240 in could be 240 3phase as well as 400 3phase. Hench the lack of power!!
I have looked at the diagrams for the 185 and there are quite a few variations of input including one version that is 240/110. Dose the data plate indicate that it is both single and 3 phase ?
I hadn't given that any thought tbh... as it had a standard 3pin plug on it when I bought it and it was sold as single phase. I started to have a look at the power switch and it may well be faulty, I only took a couple of the connections off to meter them and spray a bit of contact cleaner on them and I think that the switch has gone a little wrong inside whilst switching between the settings, it now doesn't switch smoothly between settings. As I haven't taken it apart this might indicate that it needs some work.
I'll try and bypass the switch by direct wiring the respective input to the correct output and see if I get the right voltage out of the transformer.
The plate on the front indicates that it is single phase (I assume I'm correctly interpreting: '1~' to mean single phase). It shows single phase by both the 240v and 400v sections, which I assume means the machine is 2 phase and uses 2 phases of a 3phase supply to supply it if set to 400v settings?
Update, I bypassed the switch and got the same voltage to the input side of the rectifier ~9v which indicates that the problem lies in either the wiring from the switch to the transformer or the transformer itself. I've noticed a definite oddity which may be the cause of the issue... whilst the main 230/400V jumper is correctly set, there is a much more 'busy' jumper board of connections in the bottom of the welder that convert the 8 outputs from the power selector switch to the numerous tappings of the primary side of the transformer. Upon closer inspection this jumper board has indications for either 220V or 380V. I hadn't noticed it before as it has loads of wires going to it which hid the writing. It would appear that the settings are all set to 380V. I will swap them over to 220V and see what happens.
Update: Houston we have voltage! - Need to test it (can't in the garage... too much flammable stuff) but the voltage at the rectifier and the torch tip looks good and I can audibly hear the wire feed motor up its speed as I flick between the settings which it didn't do before. The only remaining issue is the need for a nice big replacement resistor to sort out the issue of capacitors not draining.
It's looking like my punt on a cheap 'decent' welder was worth it, as I always knew there was risk but bought at a price where it was economically fixable. (paid £150... identical one sold on ebay the other day for £500!) - even better is the smug feeling of knowing that someone sold it to me knowingly fleecing me and actually they could have doubled their money if they'd have bothered to have a bit more of a look.