Lincoln welder

Here's my "funny" welder story. Decades ago I worked in a local, large, parts store. We had a separate welding store, sold Miller, bottles, repairs, etc. Someone had brought in an engine driven Miller they had bought at an auction, and it had come from one of the deep rock mines, flooded. They had rebuilt the engine, cleaned it up, looked great and ran good. But it would not weld and sounded as if it was loading the engine with certain switch settings. this was not a DC generator like yours, this was a 3 phase alternator, using 6 huge diodes, an oversized car alternator.

The "mechanic" wanted me to help him, he could not figure it out. We ended up pulling all the diodes and I had a big load bank to test alternators. I tested the diodes under load. BUT I HAD NOT NOTICED an important point. Yeh, they all tested good, all right..................Turns out that 4 of them were one polarity, and two were opposite---they are supposed to be 3 and 3!!!! Someone had replaced one and got the wrong polarity diode!!!