Ignoramus12834 wrote:
It is a AC/DC welder. Look at FETs T21 and T22, you can see that they
can be used to supply either +, or -, or +/- to the line labeled
+. Whereas line marked - is the center tapped point of the secondary.

OH, heck, you are absolutely right! I was viewing the thing in Mozilla,
which just gives me a peephole view of the file. Yes, T21 and T22
are between the bridge rectifier and the output terminals. (Dumb
labeling of those, they should be electrode and work, not + and -.)

in AC mode, I think, only one half of the secondary winding is
working. I may be wrong, I slept only 3-4 hours for the last 2 days
and am barely thinking, .
No, only one half of the bridge rectifier is working. Presumably
the work terminal is connected to the center tap, and the bridge
always produces a + and a - voltage referenced to that. The two
transistors select from the two polarities. Certainly a minimum
solution to the problem.

Jon