Plasma Cutter Repair

[zupta82]

 Hello everyone, I am trying to repair a Thermadyne Pakmaster 25 plasma cutter. When I try to crank up the current to cut thicker metals the plasma arc extinguishes, also at lower current settings the unit does not have the power it used to have. I am thinking the problem might be somewhere in the power Mosfet stages. I don't have a curve tracer or high voltage or high current probes to possibly poke around with my oscilloscope. I am kind of rusty in my electronics knowledge, I am an applied electrical engineer but don't work with much in electronics in my current job . I never could get the grasp of linear electronics theory. I have a TDS 420 oscilloscope and a MeterMan true RMS meter. I guess there is a high voltage capacitive discharge circuit portion that drives the cutting torch nozzle. Also I guess these pieces of equipment work off a really high peak inrush current to start a pilot arc that jumps to the piece of metal along with the use of compressed air. I would greatly appreciate some help. Here is an attached manual with a schematic

[wizard69]

Don’t kill yourself, there are high voltages and currents in these machines!   

I’ve never repaired one so I can’t offer a lot of help however I believe that start is accomplished via a high frequency impressed on the high current DC.   I could be wrong about that as I’m actually thinking TIG here.   

As for breaking down under load that can be almost anything from a transistor to a loose connection.  It is always best to start with visual inspection for  loose stuff, bad solder joints and etc. 

[Gregg]

Your problems possibly all are centered around the torch handle, take it apart, clean everything and replace any consumables that are burned, badly pitted etc.

[zupta82]

 Sorry, I forgot to mention that I am in fact using new consumables(new tips and electrodes). However, I  am using a old gas distributor which is made out of ceramic. I would have to get some pics of it, it doesn’t look pitted or worn. It’s contained in the torch handle

[zupta82]

I’m having problems uploading pics. When I attach the photos inline the screen just refresh’s itself

[zupta82]

Here is one pic of the capacitor bank

[zupta82]

I guess this might be some kind of a choke

[zupta82]

Here is a pic of the gas distributor

[zupta82]

Here is the massive FETs

[zupta82]

I was thinking I did recall a chirping like sound during cutting. Plasma cutters usually make a loud hissing sound. It’s kind of hard to describe, is it possible for a audio range frequency sound to be generated from a improperly switching transistor.

[zupta82]

I went ahead and replaced all eight mosfets and 1 fast recovery diode now I’m getting no severing of metal but am getting arcing with a blue flash. Maybe I damaged the FETs with ESD.

[zupta82]

Before I pull the FETs out there are a few dip-8 driver chips with test points on the board close to the FETs  maybe they would give some sort of low voltage and  low current triggering waveform that I could measure with my scope?

[zupta82]

I was wondering about these dip-8 chips on the board near a test point. Also these are near some mosfets I was thinking there should be some kind of triggering waveform during idle and cutting. Here is a pic

[zupta82]

Ok I went ahead and probed a total of 4 test points with 6 waveforms. 5 of the waveforms seem to be idling at 60hz line frequency. Two of waveforms are shifting by a negative dc offset. Lastly I figured I would fire the pilot arc and was able to generate a modulated waveform I think??

[zupta82]

Here is the negative dc shifted waveform

[zupta82]

I guess this would be a modulated waveform??

[zupta82]

I went back and checked the voltages at each “gate” terminal of each of the power Mosfets. It was difficult to test the other pins because I did not want to short them with my dmm leads. All the measurements were taken referenced to ground. Also the measurements were taken with the unit just running at idle. I did not want to fire the pilot arc from the torch trigger and maybe I would exceed the current capacity of my meter.

Here is what I measured:

Q1-Q4 = +13vdc and +65vac
Q5-Q8= -140Vdc and +120vac

[zupta82]

Sorry, I forgot to mention the series of 4 test points correspond to the red circles in this picture. The top two are the ones that created the modulated waveform when the pilot arc was fired and just a sine wave with the unit at idle. The bottom two created a negative dc offset shift at idle. Here is the pic

[zupta82]

Ok everyone I’m sorry I was off to a rough inconsiderate start on this forum. I was worried that my topic would get buried and not get viewed whenever I make updates to a “single thread”. I now finally realize that it actually puts it near the top of the list. Which is why I mentioned the project again in separate threads before. Again, I’m sorry I’m new to fourms and have somewhat poor social skills and learning disabilities. I’m probably well known as pain here already

[zupta82]

I just replaced the consumables (electrode, copper nozzle) still the same thing. No blasting thru the metal, just sputtering and blue flashing light

[zupta82]

I went back and checked the idle waveforms of one of the mosfets with the unit just running. I’m afraid to fire the pilot arc because I don’t what kind of current these mosfets would be producing I went in the order from g-d-s. Here are some pics, I hope I’m getting close??

[zupta82]

Sorry the one before was gate and source. This one is drain with a positive dc offset shift

[zupta82]

Sorry this is the one that is drain