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Been a while since I messed with a lincoln but here goes anyway. The first thing to look for is a fuse or circuit breaker on the wire feed drive motor or circuit card. If this is popped, reset the breaker or replace the fuse with the exact same size and type as you removed or as is marked on the unit. If the fuse or breaker opens again, you may have a shorted wire or drive motor. The next thing to check is if the drive motor has brushes, if so, remove the caps and check these. Then check the wire connections on the motor for breaks or shorts. If there is no problem you find in this area and the fuse or breaker opens again, stop and get it to a repair shop. I think the drive motor gets it's power from the secondary transformer which also gives you your weld output. I don't recall if Linc. uses diodes or SCR's for rectification of the AC to DC current. Either way, odds are one of these has opened. Make sure all the power is disconnected from the machine. Open the case so you have access to the guts. Look for a large heat sink, usually a piece of bar stock aluminum, copper or brass. On this you will see two or four goofy looking studs with large wires on one or both sides, if they are SCR's, you should also see a small wire attached to each along with the bigger wires. (the big wires may actually flat copper bar with a thin brown/gold colored coating on them) Take note of the wire marker tags and where the wire is connected. Remove all wires from these devices so there is no connections attached to them. You need not remove them from the heat sink. Be careful removing the wires because you can damage these devices rather easily. Using a standard battery powered ohm meter, set it to the lowest scale on ohms. Place one lead on one connection point and the other on the connection point opposite it. If you do not get a reading, reverse the leads to the same points (you should only get a reading in direction depending on the polarity of the meter. If you do not get a reading in either direction, check the others in the same manner. If you get a reading on the others, the one that did not read is the bad one. (if these are three wire SCR's, they may require a trigger pulse of current to the third lead before you will get a reading of any kind. These are not widely used in smaller machines but anything is possible. If you identify that you have the three wire SCR's and can't get any readings from these, put everything back the way you found it and find a good repair shop to work on it.) Welding machines are somewhat simple to trace out even without a schematic. If the recitifiers check good, you need to follow the outgoing wires through the rest of the circuits. You should find a relay / contactor somewhere past the rectifiers which breaks the weld current circuit. This will be controlled indirectly by the trigger switch. There should be two small wires on this going to the coil. If there are no in-line connectors on these leads, follow them back to the next connection point. Remove one connection and check for continuity through the coil with the ohm meter on the lowest scale. If this is good, replace the wire you disconnected. If you don't get a reading, the coil is bad and you need a new contactor assembly. Check the conatcts for the weld current and the rest of this relay / contactor for physical damage, any signs of why it would not function. You should be able to cycle it easily by pushing on the moving piece that makes the contacts, be sure all parts move freely and easily with only the return (open) spring resisting the movement. If everything is good with this device, continue following the leads looking for an in-line weld current fuse, broken or loose connections, ect. If you don't find anything here, your beyond the typical problems that are easily field repairable and need to find a good repair shop to take it to. From the sounds of it, your more than likely going to find a bad connection, open rectifier or bad relay / contactor. These are the common instant fail things that go out as you described happened however, sometimes one part failure can cause several problems in circuit cards and such.
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