In the first photo, the quarter size blue disk to the right, next to the diodes is an MOV or varistor of some sort. It is intended to protect the machine from lightning strikes or similar events.
You might have to unsolder it to test it correctly. It should show a high resistance when you test it with your ohmmeter.
If the MOV is shorted, or defective, you would not be getting 100% of your welding voltage or current.
You might want to make an effort and try to check this one part first.
If it is in fact defective, it would be easy to get a new one.
Typically, MOVs fail if there has been a lightning strike, while the machine was plugged in.
Other things to check:
At the lower left of the same picture is some sort of fuse element ... blue .... on a daughter board next to the switch assembly. I can't tell if it is soldered in place or not. The fuse should measure a low resistance with your ohmeter, or nearly zero ohms.
...Unplug the machine from the wall before you start either of these tests.
.... The diodes, to the right of the picture are the individual bolt on kind, not a block, like I had guessed previously. It looks like there should be two heavy gauge work leads coming from the diodes. These would be your positive and negative DC voltage points.
The data plate in photo two gives a rated value for the maximum open circuit voltage, and also the welding voltage. These numbers would give you a general range to compare your DC voltage measurement to.
What do you think about trying to measure the DC voltage here? This would require the machine to be plugged in and turned on, probably with the trigger switch pressed momentarily, while you read the voltmeter.