Cebora Prof 52 - Fault Finding

[puffernutter]

Some of you may link this thread to a For Sale (now Sold) non-working Cebora Plasma Cutter. Access to the inside was relatively straightforward, the transport company had dropped something heavy on the rear corner where the regulator is and pushed it in! I had to take all the covers off and remove the rear panel to straighten it!

This is how far I have got!

I know I have 3 phase at the incoming on/off switch.
There is 3 phase at the transformer.
The rectifier works (about 200v DC?)
There is continuity along the lead to the tip of the torch.
The air pressure is sufficient (I know that as when it was too low I had a red light on the panel and the contactor didn't work, upping the pressure, the red light goes off and the contactor pulls in.)
The push button on the torch handle operates the air (air is present) and the contactor pulls in. This also proves the shield is present as it forms part of the electrical circuit.
With all this - I do not get an arc.

My immediate thought is that there is no HF to start the arc, but I'd appreciate others' opinions. I'm hoping I can look at any HF using a 'scope?

Cheers

Peter

 

[puffernutter]

Just to add some more thoughts, there are only 2 electronic boards in the unit, one is a "Control Board" the other "HF". By the fact that the inlet pressure is being monitored and the air valve opens and runs for a period of time suggests to me that the Control Board is doing something. It's either not triggering the HF or the HF board is kaput?

These could of course be red herrings!

Peter

 

[Pete.]

I'd start by holding the button down for a few seconds (so that the HF timer expires) then checking the torch voltage with the button still pressed. That'll tell you if you have your cutting voltage.

Wiring diagram here:

https://welding.cebora.it/en/assistance/items-out-of-production/art-940-plasma-prof-52/sche940.pdf/view

Manual:

https://welding.cebora.it/en/assistance/items-out-of-production/art-940-plasma-prof-52/ist611g.pdf/view

 

[puffernutter]

Advice please, I was looking for spares for the Cebora spares and came across this image on the R-Tech website, the other 3 are my tool - am I missing something or has their design changed?

Below is the image from the R-Tech website - the lower part of the tool - air and electrical connections - looks similar to mine

The following are my tool

 

[KemppiFrog]

Tjhe last one reminds me of my Prof 35 Cebora 3-phase. Great bit of kit! Only prob, no 3-phase here, so I sold it. Bought it in, errr, '87?

 

[Jan-DIY]

You have to remove swirl ring and electrode. Looks similar.

 

[puffernutter]

You have to remove swirl ring and electrode. Looks similar.

Thanks, yes looking at it last night, I realised the R-Tech picture was more "naked" so to speak!

So, not a simple problem, back to electrical fault finding!

Cheers

Peter

 

[puffernutter]

To test the HF side - Googling suggests I'm looking for 5kV at 2MHz, does that seem about right?

Cheers

Peter

 

[puffernutter]

Sorry, still here! I've had time for a quick play this afternoon. With air applied at the correct pressure and the trigger depressed and the torch above a clean piece of metal with a good earth clamp you get the contactor pull in for about 5 seconds, the torch voltage goes to about 250v DC, then the contactor drops out - I assume as it's not detected the arc, it resets.

Could it be the HF side not striking the arc? Looking at the circuit board, nothing looks as if it's had the smoke let out - but there is this item that I don't recognise - it has a small screw at the top and bottom that I assume adjusts the contacts. All I could think of (and I don't want to set hare's running!) is this supposed to be an air gap to create the HF that's induced into the DC line via a toroid?

 

[Pete.]

That looks similar to the points setup on my old Cebroa AC/DC tig. See if you can slip a feeler gauge into the gap. I think the TIG was 7 thou. There was a warning not to use abrasives so I just used a feeler to scrape of the corrosion bridging the gap.

 

[puffernutter]

@Pete. Thank you!
I freed off the nut and adjusted the gap to 7 thou. Operated the torch, same as usual, air, contactor for 5 s then nothing. I tried it with a greater gap, still nothing. So, let's go empirical!

Using an insulated screwdriver (albeit low current it is around 5kV!) I tweaked the screw and I initially got an intermittent buzz and as I adjusted the buzz got louder and higher in pitch - so at least I thought, this is what it is supposed to do!

I tried the torch on my scrap plate with an earth and still nothing. Most disappointing.

I took the end of the torch to pieces, unscrewed the shroud, removed the copper to, then what I hadn't done before, removed the circular insulating cylinder and then unscrewed the copper electrode. All looked Ok, no sign of arcing or problems.

I reassembled the torch, tried again and lo and behold we have success, I was using very thin sheet, but even then when compared to the small single phase R-Tech I have already, I could freehand with this one and move very quickly!

I'll try more tomorrow when I have time, I'll buy some more tips, but it has shown that it actually works!

Thanks to the collective for your help and suggestions!

Cheers

Peter