My own Kempotig 250 AC/DC problems

  1. MBConfuse

    MBConfuse Junior Member

    Hello everyone,

    I've recently got hold of a Kempotig 250 AC/DC and it has been working fine for about 3 weeks of light use. Today when i was doing some low current work on DC (neg) it stopped functioning properly.

    To elaborate i was welding away all fine, then I stopped to change position and tried to re-start the arc and got nothing. No high freq, no gas, nothing.

    I was using a foot pedal and thought that could be the problem, but taking that out of the loop didn't fix it. Then i noticed on the display the amps just showed zero. I've tried flipping all the switches back and forth etc to see if its a bad connection. I can get the display to show a massive 4 amps by turning the dial right up, but it still wont start. There is no (audible) sound of solenoids above the fan and cooler noise. The cooler is pumping water, not very much of it, but there is a steady trickle i would say.

    Can anyone start to help me out on this one? Thanks in advance.
  2. Seano

    Seano Member

    Not wanting to be the bad news bear, but as excellent as Kempii machines are, the do have a fondness for eating main boards. :(
  3. MBConfuse

    MBConfuse Junior Member

  4. matt1978


    UK, Cannock
    How old is it?

    Rapid Welding may be able to help in diagnosing whats wrong with it and will be able to offer a price for the repair if its serious.
  5. MBConfuse

    MBConfuse Junior Member

    Its pretty old, I would hazard a guess at 15 yrs. Good idea, rapid has helped with info on the foot pedal already.
  6. MBConfuse

    MBConfuse Junior Member

    A little pit of progress; I managed to locate a couple of fuses on the control boards, one of which was blown. I need to get hold of some of the same rated fuses, then I'll be able to try it again. I have a feeling it will just blow again which I guess will indicate an underlying problem..... Wish me luck
  7. MBConfuse

    MBConfuse Junior Member

    So unfortunately the fuse didn't fix it. I started delving a bit deeper and have started to map out the circuit within which the fuse sits. And it looks like there is a variable voltage regulator which I'm thinking could be the culprit. It's got a heat sink on it which suggests it is vulnerable.

    I've made some scribbles (attached). Not sure if anyone can make sense of them. But can anyone advise if voltage regulators are liable to fail? There is also a comparator and an op amp in the circuit but my imagination says they are less likely to be the cause of my problem.....???
    • ImageUploadedByTapatalk1338491562.487582.jpg
    • ImageUploadedByTapatalk1338491573.536604.jpg
    • ImageUploadedByTapatalk1338491586.507260.jpg
  8. MBConfuse

    MBConfuse Junior Member

    Well as it turned out it was none of the above. It was a capacitor! Crafty little ****** had blown, but it was only obvious once it was removed from the board. If anyone else has this issue I'll happily explain what I found.
  9. grr Member

    SW Holland
    Please do elaborate MBConfuse, it sounds just like what happened to mine.

    Would indeed be nice if it turns out to be the same failure needing the same fix.
  10. MBConfuse

    MBConfuse Junior Member

    I thought you'd never ask! hehe.

    So long story short, it was the right hand large blue capacitor in the photos above. The process i went through to deduce that it had to be one of those capacitors was -
    • try to trace the circuit for the blowing fuse
    • test each connecting item
    • if it tests ok, then either remove it from the system or carry on if cant dosconnect
    • first suspect was the voltage regulator, but the fuse still blew when it was removed
    • then a diode, but the fuse still blew when it was removed
    • Then i got fed up and decided to power the board from a desktop supply (12V is ok as i it should be powered with 18V). Somehow the fuse was reading 25V. Never found out how the voltage could be higher than my input.
    • Went round checking grounds and found both sides of the cap were gounded
    • replaced both caps and all was fixed.

    I never learnt all the ins and outs of the operation of the board, but just enough to get it working. Had a little bit of trouble with the final soldering as the earth and cap +ve are very close. Ended up having to run a seperate wire to the cap +ve to prevent bridging.

    Later on i was informed that capacitors are almost always the first thing to least i know for next time.
  11. MBConfuse

    MBConfuse Junior Member

    The installed caps.
    • ImageUploadedByTapatalk1340453227.934970.jpg
  12. grr Member

    SW Holland
    Thanks for the detailed reply.

    Ill be picking up mine on tuesday I guess and will keep you posted on what I actually find. Story is it was shut off just after welding and refused to work afterwards so Im hoping same issue.

    Caps are the likely culprits as far as ageing issues go. Even when working replacing them after a decade wont be a bad thing. Wise move to replace the neighbouring one as well.

    Do you happen to recall the value of the caps? I think Ill then get them on monday so I canb hit the ground running once I get it home on tuesday.

    never been good at waiting :)
  13. MBConfuse

    MBConfuse Junior Member

    470uF I think, 63V.

    See the base of one blew out a load of black crap.
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  14. grr Member

    SW Holland
    Tnx for looking again, as Arnie said Ill be back once I know more.
  15. MBConfuse

    MBConfuse Junior Member

    Lol. I don't think that's quite what he said.... But it probably would have been funnier.
  16. grr Member

    SW Holland
    In that case maybe I should run for president!
  17. tito New Member

    Mr confuse

    please can you tell me the reference of IC- D9, of PCB that you take photo's

  18. ironic New Member

    After looking at the photo of the board, i see the fuse, and remember that quite a few times there has been problems with these little fuse holders. they protect the pcb at manufacture by brushing with lacquer and some gets into those fuse clips, or sometimes they just get old and oxidise, and end up with a poor connection between the fuse and holder. the cause of the capacitor failures may have been poor contact. i have scraped them clean and tightened a few times before.
  19. Mbart New Member

    hi, I am new to this forum.
    I have the same error of blowing the fuse, replaced the capacitor, one with a load of black crap.
    but still my fuse keep burning. olso when removed the voltage regulator.
    what could there be wrong ?
    Bart from Belgium
    Last edited by a moderator: Nov 11, 2017
  20. cumbriasteve

    cumbriasteve Moderator

    Cumbria UK
    Welcome to the forum. :waving:

    Please do not place your company links into your posts.

    Hopefully someone will be along to offer help with your question soon.