Electronics > Repair
Plasma Cutter board repair questions: SOLVED
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jerryk:
I'm working on a Hobart AirForce 250ci plasma cutter that I thought I could save.  It has several obvious blown parts that have been replaced.   A couple parts replaced out of caution. I attached a schematic of the problem area along with a photo of the board.  On the schematic I have circled in red the parts that have been replaced.

The board powers up fine and all is well till I press the trigger on the plasma cutter lead.  It's issue is that it will not power the 12V compressor.  The error code lights indicate a lack of current to the compressor.  I have checked all of the components that power the compressor and the only oddity I find is with T1 transformer.  I'm not sure I picked the correct transformer symbol from the Kicad library either.  The secondary side tests fine but it's on the primary side that I'm lost.  At T1 the resistance between pin 5 and 6 is around 350K.  Resistance between 3 and 4 is nil.  Resistance between 3 or 4 to pin 6 is 21.5K.  I'm confident in my schematic being reliable up to T1 however I doubt that I have the transformer represented correctly.  Right now I feel like blaming the transformer but am not sure how to test it.  I usually test for resistance across terminals get an idea of how it's wound but I'm stumped on this one.

What I want to do is probe the gates on Q2 and Q3 but don't want to screw that up as it's on the high voltage side.  What is the ground reference for these two gates?

I did probe the input signal to U6 (PROBE POINT 1) and the output at the gate driver transformer pin 1 (PROBE POINT 2) and have attached photos of the results. 

Any ideas on why the compressor power scheme is not working?

Jerry
jerryk:
I did go with my suspicion and removed T1 transformer and it appears like it's toast.  First photo shows the primary side with two windings.  On the right side the two red wires have continuity but on the circuit board (see backlit photo) the wire on the right side in a no connect pin.  I assume this winding is some sort of feedback loop?

The two multi strand copper color wires  on the left are open lead on the multimeter.  So my assumption is this transformer is toast.  If I'm off base please let me know.

Before I scrap this project is there any possibility of fixing this transformer by unwinding, finding the break primary winding wire, and splicing it.  Or is this just a fools dream?

Jerry
indeterminate:
The Transformer is fixable
Heat it up so that the glue softens and you can separate the cores
unwind each winding counting the number of turns and direction.
get sum new wire and start from scratch.
Harry_22:
Hi!
Check the transformer again before crack it.
fant:
Another solution for the transformer:
Leave it covered by Acetone for a couple of days and it will dissolve all the glue.

Mandi
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