Cut 50 - No Arc, any help appreciated!

[TryingToWeld]

Hi all,

My latest project is trying to fix a Cut 50 plasma cutter that I picked up for a few quid. This is my first attempt at fixing a plasma cutter!

It powers on and when I pull the trigger, air flows and I can measure an open circuit voltage of approx 240vdc. I've replaced all the torch consumables, but it won't initiate an arc when I drag it over a bit of clean scrap.

I've took it apart and checked the mosfets, and their gate resistors on the top pcb, plus the diodes on the small motherboard. I've also checked the diodes on the middle pcb, which all appear to be okay.

On one of the bottom boards there are two wirewound resistors, which have got very hot and discoloured, so I can't tell what value they should be. They read 340K so I'm assuming they are 330K ? The associated diodes and hv capacitors read ok, but I don't get an arc across the spark gap.

What else should I be checking? Any suggestions will be gratefully received.

Steve

 

[voipio]

The case shows signs of water damage. It's worth checking to see if any of the internals have been affected by water.

 

[TryingToWeld]

Good point, I think it was kept somewhere damp, a garden shed or outbuilding.

I've checked it over and there's no sign of sitting water, drips, puddles, residue, etc or arcing on the pcb's, so hopefully that's not the problem.

 

[eddie49]

This earlier Cut 50 thread may help:

D

Thread 'CUT50 board connections?'

May 14, 2020

I've been given a CUT50 in bits, as the last stage before the bin, which I'm wading through to see what doesn't work. I've managed to put the boards back together and have traced the majority of the block diagram. I haven't got a compressor to test it with, so I'm limited to basic electrical functions.

The mains has a thermistor across it, and a relay with a 24V coil which I suppose should kick in under load. I can't find a 24V switched supply anywhere, nor where this relay is supposed to be switched from.

If I turn the box on, I get a continuous arc from the HF contacts on the HF board...

• Dragnet
• Replies: 59
• forum: Plasma Cutters

 

[voipio]

A 240V DC open circuit voltage is dangerously high!!

 

[TryingToWeld]

My wife complains I do everything with one hand in my pocket!

But when I was an apprentice 40 years ago, we would get struck across the knuckles with a ruler if we were caught putting two hands inside any (live or not) electrical machine. One hand means the current shouldn't cross the heart, that rubber matting, an isolated bench supply and common sense has kept me alive this long!

Rectified mains voltage across a smoothing capacitor is 300vdc, which is common in switch mode power supplies!

 

[zzr1200]

Have you checked the points for burned surface and gap?

These could "possibly" be a very basic set of points...... ..... maybe other would comment??

 

[TryingToWeld]

Hi zzr1200,

Yes, I did give go over them with a fibreglass brush and contact cleaner. They're nice and shiny now!

Sleeping on it I've decided to replace the two wire wound resistors, in case they are breaking down at high voltage and experiment with the arc gap distance.

I'll report back later, how I got on.

 

[gaz1]

check your torch connections as well

the firing pins need to be checked as well



the firing pins are the left brass bits middle of the board left side

 

[TryingToWeld]

Hi Gaz,
I've checked the continuity of the torch and earth lead, all good.
I'd already cleaned the firing pins, but I have just reduced the gap from 1mm to 0.8, but it made no difference.
I think the next step is to take a look at the input to the voltage multiplier. Time to get the scope out!

 

[zzr1200]

I would think that 1mm down to 0.8mm would still be too wide, personally for a plasma hf gap.... ....

Maybe 0.5mm gap???

I maybe wrong on this, someone mentioned a figure for the gap a little while ago, in another post.⁸...

 

[zzr1200]

Post in thread 'Unidentified component.'

Aug 11, 2024

What Eddie says.

Which function isn't working (or is it all of them)? The spark gap is only used on the plasma side and, being literally just two bits of metal and some air, won't be a reason for failure.
Have you had the boards out and hunted around for any soot on the components or boards?

If one of the IGBTs ("transistors", kinda) have failed then it's unlikely you'll spot it without removing them from the board and testing each of them.

• skotl

• 

Post 4 (Pete), says 7 thou gap which is 0.18mm

 

[gaz1]

this is skotls thread post where cut50 / cut 50 was repaired

Thread 'The xmas cut-50 - think I know what's wrong...'

Jan 20, 2015

Following on from this thread; http://www.mig-welding.co.uk/forum/threads/cut-50-busticated-yes-the-xmas-one.51675/

I got bored waiting for the replacement to show up so I decided I may as well take it apart and see if I can find what's wrong!

(I took many pictures, plus a video as I dismantled it so that I have some hope of being able to rebuild it afterwards )


This is what looks like the high-voltage / high-frequency board; can you spot what's up yet...

• skotl
• Replies: 99
• forum: Plasma Cutters

• 

 

[TryingToWeld]

I think I must have short circuited it with a test probe, there was a bang and a MOV blew. I 'll have to put to one side until the replacement arrives next week.
I think you might be right about the gap, reading around a figure of 0.4 - 0.76mm is often quoted, I'll give it a try.
Thanks for the link Gaz, I have a read through while I waiting for the part to come.
Have a good weekend everyone.
/Steve