Marquette 12-205 Mig Welder (with bad transformer)

The following information is provided to assist any service personnel in the replacement of the main transformer. It is assumed that the personnel are familiar with all safety precautions and that they have prior experience with this or similar equipment. It is further assumed that the personnel understands basic electronics and they are familiar with the terms used in this procedure.

Symptoms of a bad transformer are usually obvious. The customer generally describes them as one or ALL of the following.

Smoke and/or sparks coming from inside unit
Wall circuit breaker tripping
Loud noise coming from inside unit and lights dimming
Smell of something burning

It's not a good idea to plug in a welder with these symptoms!! I generally start by removing all the covers and visually inspect the entire welder from AC input to the welding power output.

In the case of the following welder, I do not believe that the customer was being totally honest with me when he brought it in. The unit was still under factory warranty and I think he thought it best not to divulge much information less he be blamed for the failure. He should have been aware of one or more of the above symptoms, but he mentioned none. In fact, he said he was just welding along and it quit. He said that it had done this before and he would wait awhile and it would start up again. That's usually a symptom of exceeding the duty cycle of the welder and too much of that can be a bad thing (i.e. transformer and/or the rectifier overheating)

I started here by removing the covers, and it became apparent right away that the transformer was burned up. There were obvious burn marks on the transformer insulation wrap that surrounds the winding.

In order to get to all the transformer mounting bolts, I loosened the front panel and pulled it away as far as I could. I unplugged the circuit board and also detached the rate control switch from the front panel. Next I labeled all the wires and disconnect the transformer from the rectifier. I left the rate control attached to the transformer and disconnected only the wires necessary to allow the rate control switch to come out with the transformer (see photo below.)

The photo on the right shows the the burned windings. As you can see, its the secondary winding (outer winding connected to rectifier) that is damaged. Usually when a transformer goes bad, it is the primary winding (inner winding connected to rate control switch) that burns.

I have found that when the primary winding fails, I usually find a shorted rate control switch or that the customer connected the machine to the wrong input line voltage. There's not much protection that can save a transformer from those two situations. The secondary winding failure is a different story, however. Cooling fans and thermal overloads are generally sufficient to save the transformer from component failure (like a shorted rectifier diode) or from the user exceeding the duty cycle of the machine. Situations like this warrant further investigation (i.e. checking the fan motor, rectifier, overload, etc..)

This rectifier assy (shown left) is composed of six diodes. Four are in a bridge configuration and two are connected connected across the transformer secondary with their anodes in common. The numbers show the relationship between the photo and diagram 12-205 Wiring Diagram

Since the secondary winding was burned, I checked the rectifier assy for short circuits. With all the leads disconnected from the rectifier (except the negative output), I checked across each diode. With my VOM set on X1000 scale, I looked for a high resistance one way and then reversed the VOM leads and looked for a low resistance the opposite direction. I found no shorted diodes. The fan checked okay, also.

12-205 transformer with thermal overload

When I checked the thermal overload, I found (much to my surprise) that it was open. Since the overload prevents the transformer from operating, the only thing I could think of that would cause this situation would be if the overload had been cycling on and off (due to the transformer being hot) and it eventually broke down. Like I mentioned before, I still think the customer new more than what he was letting on.

12-205 new transformer with rate control connected

The new transformer came with a rate control switch already connected. This makes changing the transformer much easier, and it eliminates the possible damage to the transformer that would occur if it was connected improperly.

The new transformer, rate control switch, and thermal overload; put this welder back in operating condition.

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