CUT50 board connections?

  1. Dragnet Member

    Messages:
    50
    Location:
    Cornwall, UK
    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. This fades as the thermistor heats up. There's a good 330V from the rectifier board, though they've saved some dosh with only having three filter capacitors rather than the four labelled on the board!

    I'll post some pictures once I get the board out again, as I believe these are all pretty much the same so no doubt someone has some idea of the connections. I've gone through all the threads I can find on here but can't find just the right views!
     
  2. skotl

    skotl Forum Supporter

    Messages:
    6,898
    Location:
    Edinburgh, UK
    I took some pics when I disassembled my cut50 for repair - do these help?

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  3. skotl

    skotl Forum Supporter

    Messages:
    6,898
    Location:
    Edinburgh, UK
    upload_2020-5-14_10-29-26.png


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  4. EBME2 Member

    Messages:
    40
    Location:
    UK Preston
    Hi

    The Cut40 is very similar to the 50 not exactly the same but you can work out the operation from this post which has a PDF for the Cut40 https://www.mig-welding.co.uk/forum/threads/cut-40-repair-info.56439/page-3#post-699143. The 24V comes from the auxiliary power module bottom left, the relay I think you are talking about shorts out the resistors used on power up to prevent high surge currents through the capacitors, it is common for this relay to fail. They are fairly basic electronics, and I have been done quite a bit of figuring out how they work.
     
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  5. Dragnet Member

    Messages:
    50
    Location:
    Cornwall, UK
    Thanks! I'll just wash the shopping (what times, eh?) and ingest the myriad of pictures above. I was confused as the relay contacts appear both to go to the same bus bar so I couldn't see how it would switch anything useful. Applying 24V does show it works reliably (though it has come to me with the top chopped off, presumably in a frenzy of repair).

    I have that Cut40 PDF saved. So much easier than reverse-engineering tiny components on 3D boards! What is the expected outcome of turning one on with no air connected? It would be handy to know what normal operation looks like under those conditions before I start chasing any will-o'-the-wisp faults.
     
  6. gaz1

    gaz1 Forum Supporter

    Messages:
    9,858
    Location:
    westyorkshire
    get those wires wrong and you will get a brown trouser moment

    turn volume up :D

     
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  7. EBME2 Member

    Messages:
    40
    Location:
    UK Preston
    Not sure I would pull the trigger without air, certainly not for long, maybe an inflated car tyre may provide you with enough air for a few seconds to test. Pity you're so far away I have a small compressor you could have had and it's not worth the cost of posting.
     
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  8. EBME2 Member

    Messages:
    40
    Location:
    UK Preston
    Wonder if he had just drunk all that beer in the box?
     
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  9. skotl

    skotl Forum Supporter

    Messages:
    6,898
    Location:
    Edinburgh, UK
    I ran mine without the air hose connected by accident once, and it acted like a puny arc welder. Made sparks and sputtered and I looked at it with a confused look, before I realised the cause.
    Didn't do it any harm.
     
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  10. Dragnet Member

    Messages:
    50
    Location:
    Cornwall, UK
    skotl - it looks like yours has the rectifier and HF boards combined into one, while I have two.

    There's a lot more going on than in mine in your rectifier section, including two relays where I have one! Perhaps this should be the 'CUT50 Board Photos Thread' (as I believe you were trying to instigate elsewhere after your repair.) Here are some pictures of mine. I have two top boards (one evidently had some MOSFET problems, and it's interesting to note the SMD resistors on the currently disconnected board compared to the through-hole components on the other).

    Unfortunately, the labels on the connections don't match up at each end, which would perhaps be too much to ask in something of this quality.

    Do you know where your relay cables are actuated from? I can't see in the maze of cabling where they go to on yours.
     
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  11. gaz1

    gaz1 Forum Supporter

    Messages:
    9,858
    Location:
    westyorkshire
    extra photos

    20160807_174701.jpg 20160807_180927.jpg 20160807_181148.jpg 20160807_181122.jpg
     
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  12. Dragnet Member

    Messages:
    50
    Location:
    Cornwall, UK
    Thank you, gaz1! Have you got a wide view of the back left corner? It looks like you have the same version as me, and it's that empty plug in my picture 230350 that is exercising my brain at the moment. Last night I had the HF board out and discovered a blow 24V zener next to the flyback transformer. The power transistor there was always conducting, so I think that means there was no switching of the relay that turns on the flyback transformer. I've replaced it with a vaguely similar NPN, so perhaps that was what was causing the spark gap to be on continuously and not respond to the torch.
     
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  13. gaz1

    gaz1 Forum Supporter

    Messages:
    9,858
    Location:
    westyorkshire
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  14. Dragnet Member

    Messages:
    50
    Location:
    Cornwall, UK
    Thank you! On the new board installed, that socket is at the front not the side, but following the output from D2 I found it. It turns out the 24V supply comes off that top board, so as power appears it should turn on the relay and keep the mains on after the thermistor surge. Let's see...
     
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  15. Dragnet Member

    Messages:
    50
    Location:
    Cornwall, UK
    I've found a blown 24V zener on the HF board, which has now been replaced. Following the circuit from the HF relay, I found the contacts for the small plug that connects to the torch switch had been soldered together so the spark gap was always on whenever the unit was plugged in.

    Now I'm tracing the grounds. Is anyone able to see whether the signal ground is commensurate with the chassis/earth ground anywhere? A continuity check between the large heatsinks in the centre and the earth pin of the plug would be perfect.
     
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  16. gaz1

    gaz1 Forum Supporter

    Messages:
    9,858
    Location:
    westyorkshire
    my first post has some earth pics in yellow wire yours i believe is the black wire on that board of yours

    the last pic first post has both top and middle wire going to chassi

    i also know that the chassi is earthed as thats the first thing i checked on when first wiring a plug onto end of it
     
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  17. Dragnet Member

    Messages:
    50
    Location:
    Cornwall, UK
    What I'm finding is that there's no link between signal ground and chassis ground, except on one of my top boards where it is passed through a ceramic capacitor. The heatsink (where one of the tags in your picture goes) appears to be floating. I'm pulling the whole thing to bits again...

    I have what I thought was a triggering position (white plug, far bottom right of your first post) which is in the relay circuit for operating the HF. The contacts were soldered together so it was always on, as the red/black next to it closed the relay. However, on yours the torch switch appears to go to the front of the middle board by the coil. I haven't dug down to that yet to see how it's connected.
     
  18. Dragnet Member

    Messages:
    50
    Location:
    Cornwall, UK
    Update: I got it switching OFF when the torch button was ON which got me thinking it was a logic fault, and there was some switched connection somewhere. Sure enough, the connection from the control board to the HF board has four pins and three wires. The yellow signal ground was going to the wrong pin (2 instead of 4). In the photograph, the yellow was previously in between the red and black.

    The control board (the perpendicular large sub-board at the front of the machine) should be grounded at pin 2, so checking for continuity between that and the four pins of the on-board plug should show which is the signal ground. Changing that and the spark gap fires on a trigger press! I have no connection on the new board for the 24V relay, so I think I'll have to piggy-back it off the same connection to the HF board. This cable connects the torch signalling to the air valve switch, which also operates as expected when the button is pressed.

    Now for testing with some air...
     
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  19. skotl

    skotl Forum Supporter

    Messages:
    6,898
    Location:
    Edinburgh, UK
    Nearly there! This is me, waiting for the next instalment :D

    [​IMG]
     
  20. Dragnet Member

    Messages:
    50
    Location:
    Cornwall, UK
    I've made a board aping the CUT40 relay operation switch, so now the relay clicks in after startup and the machine runs without the thermistor switching off. However, I have no cutting power.

    The spark gap sparks as long as the torch button is pressed, and air comes out at the recommended pressure as the valve solenoid actuates. There is an arc from the torch to the workpiece but no great flames, and no cutting, just some slight pitting of the material. I have a selection of new consumables and it doesn't work with any combination of them.

    I've never used a plasma cutter before, so I'm not sure what I should be expecting. I wonder if there is an insufficient DC potential to create the plasma, but I have 50V between the torch and clamp. The videos certainly suggest an easy cutting action but I'm not sure what to try next.
     
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