Lincoln sae died. Rheostat burned up. Diagnosis?

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The other day my Lincoln SAE generator stopped welding with no warning. The main generator only produced 2v from residual magnetism.I turned the job selector and it came back to life but then (I am told) produced a shower of sparks and a glow from the control box. I wasn't there but the guy helping me was acting as a remote control and shut it down quickly.Inspection revealed that the flat metal ribbon making up the rheostat between the exciter and stator winding of the main generator (the "job selector") had burned up and was melting in one spot.I can replace the rheostat but I want to know why it burned out in the first place .What wattage rating should it have? The current one is rated 300W for an SAE-600.At this age (like 60 years old) are shorts in a main generator stator winding common? For that matter, is it heard of for this rheostat to randomly quit in an SAE generator of whatever size?FYI the exciter tested goodLast edited by Iron Beaver; 2 Hours Ago at 12:53 AM.
Reply:The news you buy from electronics suppliers. The bad news is it will do major damage to your bank account. Dave

Originally Posted by Iron Beaver

The other day my Lincoln SAE generator stopped welding with no warning. The main generator only produced 2v from residual magnetism.I turned the job selector and it came back to life but then (I am told) produced a shower of sparks and a glow from the control box. I wasn't there but the guy helping me was acting as a remote control and shut it down quickly.Inspection revealed that the flat metal ribbon making up the rheostat between the exciter and stator winding of the main generator (the "job selector") had burned up and was melting in one spot.I can replace the rheostat but I want to know why it burned out in the first place .What wattage rating should it have? The current one is rated 300W for an SAE-600.At this age (like 60 years old) are shorts in a main generator stator winding common? For that matter, is it heard of for this rheostat to randomly quit in an SAE generator of whatever size?FYI the exciter tested good
Reply:I'm fine with spending some amount of money on this machine,  I just want to make sure I fix this problem once instead of burning up $1000 rheostats for troubleshooting.So I'm highly motivated to find the real problem. The one that made the old rheostat burn out in the first place.