Plasma cutter will not fire...

[Peter Apalais]

Hi all,

Well I have been given an old plasma cutter. I think it was the first one ever made, well from the era anyway, lol. Beggers can't be choosers, It was free.

It is an old "Weldwell" brand 20 plasma. Weldwell is a NZ company btw. I was told that it was repaired and when It left the shop it was apparently working. That was like 5 years ago, I picked it up out a trailer ready to go to the dump.

It will turn on, fan spins, there is a set/run switch. When on the set position, the air will flow out of the torch and you can adjust the pressure on the regulator on the back of the unit. If it goes below about 35psi (from memory) the orange "ready" light will turn off.

Well at least it turn on and the solenoid for the air works in set mode. Then you flick it up to run and try it and nothing happens.

I put in a new nozzle spring, nozzle and electrode today, but nothing.

How do you fault find these things, where do I start?

There are some LED's on the mobo inside. The "air" LED lights up and another one does too. But the high frequency "HF" and the "DC" do not. There might have been one or two more LED's.

Anyway It would be great to get it going........it's about 24 amps so that should cut 1/4" and that will be sweet.

 

[rtbcomp]

Welcome to the Forum!

My first instinct would be to check for blown fuses and obvious signs of burning, loose wires and so on.

I'm sure you are aware that there are lethal voltages in there, possibly even when it's disconnected from the mains, so if you're unsure about anything maybe not a DIY repair?

 

[Peter Apalais]

I have look for any melted wires, popped caps, lose bits etc. There only seems to be one internal fuse which is okay.

I did find a lose wire in the trigger for the torch. Old mate who last had it, had cold soldered the two wires. He had some heat shrink tube over it, but it was not even shrunk. So I soldered them properly and put some new heat shrink over it but still nothing happened.

Yeah you have to let the caps bleed down before you go touching bits inside.

 

[Wedg1e]

Looks like a quality circuit board; however there seems to be a number of logic ICs (I can't read any numbers though)... high-frequency, high voltage and logic chips have never been good bedfellows, might be worth risking a few quid (dollars? roubles? shekels?) replacing them all to see if that makes any difference. I'll stick my neck out and say they'll be 4000-series CMOS but they could also be 74-series TTL (hateful stuff)... there's a distinct air of 1980s circuit design anyway
Read the ICs and see if they have numbers like 4093BE or 74LS235, that'll narrow the type. Replace with identical type numbers, insert them the correct way round and avoid wearing nylon clothing while handling them - does anyone still wear nylon shirts these days?

Of course it could be something completely different but in the absence of anything blown, smoking, cracked or missing, it'd be my next step.

 

[Peter Apalais]

Thanks Wedge1e, Yeah well I guess I could always replace every part on the pcb with a new part. I will disconnect the board one night and slowly write down and find out what IC's I need and shop around to replace. IC's are not that expensive each are they?

 

[Wedg1e]

Thanks Wedge1e, Yeah well I guess I could always replace every part on the pcb with a new part. I will disconnect the board one night and slowly write down and find out what IC's I need and shop around to replace. IC's are not that expensive each are they?

Not usually, CMOS and TTL are old-hat now. You may struggle to find a supplier that doesn't want to sell you 10,000 of each type though!