[Radio Wrangler]

This is easy.

It's a DC welder.

The whole of the welding side is isolated from mains and is isolated from ground. This is how it should be.

Some welding processes are done with the electrode positive and the work negative - relative to each other. The electrode is insulated and is treated as 'live', the work probably isn't and is treated as ground. The work isn't grounded by the welding machine, but may be via its environment. The reason for this polarity is that electrons are real, holes are lacks of electrons where one might have been. Heat dissipation is uneven, electron bombardment heats the electrode strongly in addition to the heat from current flow. THe work gets current flow heating but misses out on electron bombardment, in fact electron emission consumes energy. The electrons emitted by a cathode cool it a little. If you're doing coated electrode 'stick' welding the heat on the electrode is needed to melt the flux, turn it reactive and release a cloud of CO2 to shield the molten metal from atmospheric oxygen. The spray of molten metal from the electrode conveys heat to the work. So electrode positive is normal for the most common electric welding process of the past century.

However, some welding processes are used with the electrode negative, even some coated stick electrodes are made that need this polarity. In this case the work gets the lion's share of the heat and this helps the weld pool penetrate more deeply into the work, making a much stronger joint. The electrode has to have a coating that works with the (relatively) lower heat input.

So, welders are made with floating outputs so the guy doing the welding can choose to connect his electrode holder and his work clamp to whichever terminal suits the rod clamped in the holder. and either could go to planet earth, though it makes more sense for the work to be earthy.

So! is you want to go scoping around on the secondary sides of this machine, you can put the scope ground clip anywhere you want, in theory, just so long as you don't make any other ground anywhere else.

I said in theory. If the place you want to ground is in the midst of an inverter (this unit is not an inverter) it could have a strong high frequency content, and the stray capacitance of things like the transformer could give you a problem with current in your scope probe ground clip. It should be OK with this unit.

This welder has a big iron transformer and does current control by controlling the phase of triggering those big thyristors in the rectifier bridge. That's what the opamp-ery is for.

it senses welding current to decide when to net fire the SCRs. So to see the thing working, you'll need some welding current. This is going to be difficult. You want a dummy load that will dissipate something like 1kW at 50 amps if the welder is not wound up much. As you're fixing the current control, it might wind up flat out and the dummy load would need to handle several kW.

Sorting out a suitable dummy load is the hardest part of fixing these things, and for scoping around, the dummy load has to float.

Plan B is to check the transformer alone, try the two big diodes, and check the thyristors aren't shorted on. After that, all the parts are cheap... you could just replace the lot.

So, no, putting the scope probe ground clip on the chassis is pointless. It is isolated from the circuitry you are going to look at.

David