Plasma Cutter board repair questions: SOLVED - Page 1
Forum section: electronics / repair

author: jerryk
author_rank: Regular Contributor
timestamp: July 07, 2024, 11:53:56 pm
content: 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

author: jerryk
author_rank: Regular Contributor
timestamp: July 08, 2024, 06:36:24 pm
content: 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

author: indeterminate
author_rank: Regular Contributor
timestamp: July 09, 2024, 04:11:44 am
content: The Transformer is fixableHeat it up so that the glue softens and you can separate the coresunwind each winding counting the number of turns and direction.get sum new wire and start from scratch.

author: Harry_22
author_rank: Frequent Contributor
timestamp: July 09, 2024, 07:17:04 pm
content: Hi!Check the transformer again before crack it.

author: fant
author_rank: Contributor
timestamp: July 09, 2024, 09:12:56 pm
content: Another solution for the transformer:Leave it covered by Acetone for a couple of days and it will dissolve all the glue.Mandi

author: jerryk
author_rank: Regular Contributor
timestamp: July 10, 2024, 01:16:01 am
content: Well that was funWhat kind of readily available glue can I use to reinstall the core?I understand the primary and secondary windings on this transformer but don't understand what the 4 turn winding between the primary and secondary does. It's only connected on one side. I assume it has a feed back or filter function? Any hints on it's purpose would be great.Attached some pictures of the process and welcome any suggestions to improve the process.I still have to prove it's worth when I reinstall it.Thanks - Jerry

author: indeterminate
author_rank: Regular Contributor
timestamp: July 10, 2024, 07:47:02 am
content: Well duna bit of 5 minuet epoxy will doIf there is someone in your area that rewinds electric motors you way be able to dunk the transformer in sum varnish.Its a bit hard to bye the stuff in small quantities these days.

author: fant
author_rank: Contributor
timestamp: July 10, 2024, 08:27:59 pm
content: The 1 wire connection coil is an electrostatic shield, to decrease the capacitive coupling between primary and secondary windings.Mandi

author: jerryk
author_rank: Regular Contributor
timestamp: July 11, 2024, 12:19:17 am
content: I sure do appreciate information and advice. I have always assumed that if an OEM transformer blows it's game over but in this case the repair was a success. I did not varnish the primary windings and not sure how critical that is for longevity. I'll return it to my friend and look forward to seeing how the repair holds up over time.Again Thanks - Jerry

author: coppercone2
author_rank: Super Contributor
timestamp: July 11, 2024, 03:51:15 am
content: this is a inspiration to all of us. My first attempt to rewind a solenoid went sour but I have faith knowing this technique can save hundreds of dollars