old Hobart Welder is giving us a headache

[Breadfruit_]

my dad and I are welders, and we've been trying to bring this old Hobart Champion 8500 back to life. It's an early one, and it was supposedly working great before it got put in storage ten years ago.

We got the engine running solid, but we're barely getting any AC voltage, it's just 3 VAC at the outlets.
We figured out the rectifier on the excite circuit was cracked, so we swapped in a 40amp one. That got us up to 8 VAC, but still not enough.
We've already pulled the brushes, cleaned the slip rings, and they look fine, but if we jump 12 volts DC to the excite circuit, it puts out about 35 VAC, so the machine can definitely produce power. The problem is we're only getting 3.5 VAC going into the rectifier.

We're stumped on what to check next. Any ideas on what's killing the voltage before it even gets to the rectifier? We're open to anything

 

[t0mmy j]

We figured out the rectifier on the excite circuit was cracked, so we swapped in a 40amp one.

You replaced the main rectifier, which is great, but sometimes there's a smaller, single direction exciter diode (or sometimes a few of them) tucked away somewhere that feeds the circuit.
If that diode is partially shorted or damaged, it could be bleeding off the little bit of AC power the exciter coil is producing, causing your low 3.5 VAC reading. These can be tiny and easy to miss, often hidden near the main bridge rectifier or bolted to the frame

 

[welder B0b]

@Breadfruit_ bcs the exciter coil itself is producing a super low 3.5 VAC, you might have a problem one step before that power leaves the main stator/rotor assembly.
did you check the continuity or resistance of the exciter coil windings themselves? if a winding has rubbed and is partially shorted to the frame, it would bleed off power and drop your VAC dramatically

 

[Dillydally]

Check for broken or corroded connections, burned wires, or a faulty field resistor that might be dropping voltage before it hits the rectifier and also make sure any fuses or switches in the excite circuit aren’t compromised from sitting idle, cos they can look fine but still have high resistance and kill the AC input