Old abused Miller Blue Star

[duaneb55]

Resistor R4 with the tap is to fine tune field voltage so if you can't adjust it past the open winding area, field voltage may too high resulting in your high output.

Also, engine speed has a direct affect on output voltage so check to make sure the POWER speed is correct at 1800RPM.

[workaholic53]

I acquired a surplus Blue Star welder in the early 90's from a timber company. It neither welded nor produced power. I did some rudimentary troubleshooting at the time and found that the C3 cap was bad so I replaced it and used it to power my homestead for about 9 months until it browned out and quit producing power. The power company was to energize service the next day so I didn't work on it. Fast forward from then through three moves and living outside for 25 years to today. I began to ts the unit and found the coils were fine and the rotating field was good. I polished the commutator, checked the brushes and removed the pc board and re-soldered the connections that I could reach and since rectifier 1 tested erratically I replaced that and the C4 cap was suspect so that was replaced also. Still not producing power or welding voltage. I cleaned bridge rectifier 2 contacts and the rotary selector switch contacts and noticed that the fine adjustment rheostat was cracked across the terminal connector metal so I took it off welded it and checked the resistance. Got rated so reinstalled. Still nothing. I ordered a couple 20 microfarad 600 VAC caps and installed one still nothing. I re-installed the old C3 cap (which tested good) and nothing. I was about to call a halt to the festivities when on a whim installed the other 20 microfarad 600VAC cap I bought just to try. The adjustable resistor fried instantly; since obviously there was an energized circuit somewhere in the machine, I shut it down and headed in to get my meter. Finding that the area of resistor wire between the fixed connection and the movable clamp on the adjustable resistor no longer had continuity I turned it end for end and got a fresh resistor to work with. I set it up with the same relative distance that the other end had and tested it and got 5.75 ohms. I unplugged the ground from the fine current regulator rheostat and fired it up. Got 174 VAC at the receptacle and got power to the welding circuit and welded with it. Strangely I had used that machine on a service truck that my partner and I used in the late eighties and it welded just the same as I remember it. My dilemma is the over current flowing through the adjustable resistor. I could live with it getting hot, but this melted the resistor wire in half. This will not do. Thanks in advance for any input you might have.