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1965 portable welder model 400037 300Amps 40Volts rheostat burned out. What is there for a replacement

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  • Dave Carlberg
    Dave Carlberg Apr 06, 2015

    The welder is a Stick welder with the serial# 12CW39551 spec 4296. This is a large portable welder used at a construction site



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  • Welding Tools Master
  • 45,301 Answers

Contact the mfg only they will know .

Posted on Apr 06, 2015


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No power to weld or use A/c power outlets

You don't say what model welder you have, some Miller welders have 3 slip rings and brushes some have just 2. In either case the next step is to use an ohm meter to check the resistance of the rotor and brush assembly. If this is a 2 slip ring model, take one of the wires off of one of the brushes then measure the resistance between the 2 brush terminals - it should be something between 10 and 100 ohms. If you get an open circuit measure the resistance from one slip ring to the other right at the slip rings - if you now get something other than an open you've got bad brushes (about $10 each to replace),

The 3 slip ring models are similar but have 2 rotor windings - take off the wire from the middle brush and measure from it to each of the ends.

If your rotor and brushes checked out OK you should next check the fuse in the exciter circuit, this will be on the block where the other end of the brush wires go.

If the fuse was good you need to do an excitation test. This is a bit dangerous so be careful. Look at the schematic pasted inside the case and figure out which brush is the positive (+) one. The wires are numbered and so is the schematic so this isn't as hard as it might seem.
Disconnect the wires from both brushes (all 4 wires in the 3 slip ring model) and tape them off so you won't get shocked. Get a 9V battery or make up jumpers that will let you connect the welder battery to the brushes. Start the welder and connect your temporary power supply, measure the AC at the outlets (it should be around 60VAC with a 9V battery) and the DC voltage at the weld terminals (should be above 10V), lastly measure the voltage at the wires you disconnected from the brushes (should be around 70V DC). Disconnect the temporary power supply and shut down the welder.
If you didn't get AC at the outlets and DC at the weld terminals above, something is wrong with the stator in your welder. If you didn't measure any DC on the wires that go to the brushes (most likely thing to happen), you've got something wrong in the exciter circuit. Check the rectifier, the capacitor, the exciter winding in the stator, and lastly the current control board. Older welders will also have the fine current control rheostat in this circuit (look for burned windings on it).

That's about all of the general advice I can give you without knowing more about which welder you have.


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