Hello everyone, I am trying to repair a thermadyne pakmaster 25 plasma cutter. When I first started this project the unit was not cutting thru thick metal when I turned up the current knob like it used to. When I took the unit apart I found the power mosfets and two fast recovery diodes had been completely dry of heatsink grease. I did not smell any leaky caps or notice anything burned. I took a gamble and replaced all 8 fets and 1 fast recovery diode, now when I fire the pilot arc on the torch to initiate the cut I just get a sputtering arc and no blasting thru the metal. I was thinking I could of damaged them with ESD. I also replaced one each of npn and pnp transistors near the current adjustment knob with still the same thing.
Before I desolder the FETS, I was thinking of doing some poking around with my scope, I dont have any high current or voltage probes. I figured if I poke around the low current and low voltage control side I might find something. I just powered up the unit at idle nothing pressed, I found four test points near a series of driver ic's near the mosfets. The top two red circled points just measure a simple sine wave, the bottom two measure a negative dc offset shifted sine wave.
When I fired the pilot arc while the probing at the top test point I got some kind of modulated waveform with a frequency in the khz range which is what plasma cutters operate at. Here is some pics and the manual that has a schematic. I would greatly appreciate some help
Before I desolder the FETS, I was thinking of doing some poking around with my scope, I dont have any high current or voltage probes. I figured if I poke around the low current and low voltage control side I might find something. I just powered up the unit at idle nothing pressed, I found four test points near a series of driver ic's near the mosfets. The top two red circled points just measure a simple sine wave, the bottom two measure a negative dc offset shifted sine wave.
When I fired the pilot arc while the probing at the top test point I got some kind of modulated waveform with a frequency in the khz range which is what plasma cutters operate at. Here is some pics and the manual that has a schematic. I would greatly appreciate some help
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