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This is my first time posting but I have gained a lot of knowledge from this community over the past few months.

Here is the situation: I have an old Hobart OM-321 Tig welder which my tech has told me that the high frequency starter is not functioning and generating no arcs. The rest of the welder is fully functional. In attempt to debug things step by step I first checked all the capacitors, switches and fuses to the HF circuitry. No shorted or blown caps, switches were functional, and fuses were all good.

I began looking at the transformer, and here is where my knowledge is a bit shaky. I saw three wires, two attached to a smaller coil. A third wire was attached to the larger coil and a ground wire connected to the case. I'm used to seeing step down transformers which has one coil with multiple wires for the selection of voltage level. There is no spec sheet or data sheet and supplier is not responding. If there a method to debug this transformer to see if it's still functional?

effective resistance across the two leads to smaller coil is 7Ohmns effective resistance across single lead from large coil to the ground connection is 2.7KOhms

That is all the information I currently have. The goal is to verify if this transformer still works. Currently have the assembly removed from the welder so if there is a way to debug without power, that's ideal. Otherwise instructions with power would be sufficient as well.

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    \$\begingroup\$ Pictures would help. Manufacturer's model number, you know the normal stuff. \$\endgroup\$ – Andy aka Oct 8 '13 at 7:19
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You could try to couple a signal generator with sine signal (e.g 50 or 60 Hz, amplitude starting with just couple of volts -> increase some if needed) into the smaller coil. Then an oscillosscope to to high voltage coil's heads. You should see how many times voltage is multiplied in the magnetic circuit.

Before oscilloscope coupling better to use some multimeter or similar to get clue of the voltage level it outputs..actually you perhaps don't need oscilloscope at all if you get Voltage RMS value of high voltage coil by your multimeter.

If input voltage gets much bigger (like it should) so then it can be ok. If not so then transformer is basically malfunctioning. Fixing of it may not be practical?

WARNING: The voltage can be still pretty high in the output high voltage coil even if you apply relatively small input signal to the low voltage coil from the signal generator. Be sure to work safely with some dielectric gloves and active monitoring of the situation and its surroundings.

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Sounds like you have a transformer that is Center tapped on the Low voltage side.

Can you verify if you have a 115V PRI / 24V CT.

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