Update!! Plasma Cutter Repair

[zupta82]

Hello everyone, I mentioned in a previous post about a plasma cutter I am trying to repair I already did the following

1) checked for dry or cracked solder joints, all are good
2) caps are not bulged or smell like fish
3) Checked all large diodes for forward voltage drop in circuit with a meter

I kind of scared where to go next, I would like to apply some juice to it and maybe check some idle voltage at  the big power fets. I do have a dc power supply and function generator. Here is a pic of the entire board

[james_s]

What's the symptom?

[Gyro]

It might have been sensible to attach this to your original thread, rather than starting a new one. It would give people some context!

https://www.eevblog.com/forum/beginners/plasma-cutter-repair/

[zupta82]

Ok sorry about that!!

[zupta82]

I guess I’ll recap what’s going on, when I try to turn up the current to cut thru thick steel it leaves a lot of dross( partially melted metal with a really jagged like appearance). Also, I do recall when I was close to the maximum current setting by turning up the adjustment pot the arc did die out, I was not able to start it back up. I have had the unit for about 8 years and never got around to really fixing it. I also do remember it tripping the circuit breakers I bought it for like 50 bucks thinking I could fix it. Again sorry to everyone I was just typing from my IPhone and was away from a Pc and was having trouble figuring out pasting a link. I was really drained from work and not thinking clear.

[zupta82]

I heard of something about freezing the board and watching for the devices that start to melt. I think this would be impossible since there is sooo much conformal coating on the component side of the board

[zupta82]

The first time I looked at the schematic, it’s so incomplete, I mean it doesn’t even include all the small control circuitry like the logic gates and low power stuff. I’m thinking it’s somewhere in the high voltage and high power end. I noticed there is something called a gate driver transformer which is in the vicinity of the large power fets according to the cheesy schematic. It’s got some hefty size windings, maybe it’s not coupling like it should. I could check the primary and secondary resistance of all the many big transformers this thing has??

[james_s]

I'm curious why you suspect the transformers? They are probably the last components I would suspect, transformers are very reliable, occasionally a winding can break or an overload caused by another defective part can burn one out but that will normally be obvious just lookingat/smelling the thing.

[zupta82]

I went back and removed and tested all 8 fets out of circuit. One of them would not charge up and turn on by using the diode feature on my meter. I repeated the test on this one supposedly bad fet several times it seems it’s turning on intermittently. I’m guessing it would really show it’s bad operation  when it’s pushed with some high current and tested with a  curve tracer instead. Also all the fets were mounted with a paper sheet between the them and the heatsink. There was also hardly any heatsink grease. I’m guessing while I’m at it, I’ll replace both Fred diodes one is a HFA25BP60 which is readily available at Digi-Key. However, there is  the other diode which is a BYP102 diode, this is nearly impossible to find, can anyone possibly recommend a equivalent

[zupta82]

Hello everyone sorry for my improper etiquette. I didn’t realize that I was annoying people and getting on people’s nerves by posting the same subject on a separate fourm. I really don’t want to start off the wrong foot on this forum, there is a lot of cool stuff here. I was only trying to learn by getting my topic to some of the seasoned guys here so they could share some of their knowledge

[james_s]

It's not a big deal, in the future just try to keep everything in one thread, otherwise it gets confusing as most of us see new threads no matter which section they pop up in.