Murex tradesmig 351 feed issues

  1. tom-madbiker Member

    Messages:
    223
    Location:
    Chesham, UK
    OK my old murex has developed a fault the feed is erratic it slows down and speeds up whilst welding so one minute it's burning back and the next blobing on the job I thought the feed motor was bad so I have replaced that with another also the pot for the feed as it looked worn but it's still doing it any ideas it's fairly basic inside copper wound transformers ect and just the one board as far as I can see a few years ago I replaced the two transistors on the board as it started feeding continuously even without the trigger being pressed

    Thanks in advance Tom
     
  2. eddie49 Member

    Maybe the brake on the spool spindle is too tight?
    Does the motor speed up and slow down when you run it without load ? ( i.e. slacken off the pressure roller ).
    Could the torch liner be blocked or rusty? Try with the Eurotorch removed ( short the two pin sockets with a wire to jumper the torch trigger ), or use a different Eurotorch.
    Can you see the drive roller speed vary ? ( maybe put a felt pen mark on it ). If you set the speed pot mid-to-high and put pressure on the feed roller with a gloved hand, can you feel the speed vary?
    When you changed the motor, did that include the gearbox too? May need cleaning and fresh grease in the gearbox.
    Was the replacement motor new or used?

    Power for the motor comes from the small transformer, T2. Going in to the Control board at Pins a8 and a9 there should be 27v AC. After rectification and variable power control on the board, the feed to the motor comes out on pins a7 ( + ) and b9 ( - ). Depending on the setting of the speed pot, with the motor still connected, the voltage on those pins probably ranges from 8 to 24v DC. It might be possible to see that voltage fluctuating with a voltmeter - i.e. set a mid-range speed and see if the voltage varies. An old-fashioned analogue meter might show this better. However, it is probably a Pulse-Width-Modulated ( PWM ) circuit, so you'd really need an oscilloscope to see it.

    Alternatively, try disconnecting the motor and replacing it with 2 x 12v 21Watt car stop light bulbs in series. If the cause of the erratic wire feed speed is a fluctuating voltage, it may be possible to see these bulbs flicker.
    If that is the case, there's a fault in the motor speed control circuit on the PCB. Several times faults on these PCBs have been fixed by re-soldering the joints, e.g.:
    https://www.mig-welding.co.uk/forum/threads/help-me-if-you-can-please.90091/
     
  3. tom-madbiker Member

    Messages:
    223
    Location:
    Chesham, UK
    Thanks I will check the board joints the motor was a new one and a new gearbox I upgraded to a 4 roller feed it does it even when the wire is not feeding I'm not sure but I wondered if the power switch changes the wire speed too ie the higher its set the faster it goes but I'm not sure on this maybe I can check the speed with my lazer rev counter

    Thanks Tom
     
  4. mike 109444

    mike 109444 Member

    Messages:
    4,302
    uk Bristol
    Maybe.....
     
  5. tom-madbiker Member

    Messages:
    223
    Location:
    Chesham, UK
    I will check the torch too but I replaced it a couple of years ago can't really see that being the issue Im still thinking electronic maybe it's fixable with a cheap pwm control
     
  6. tom-madbiker Member

    Messages:
    223
    Location:
    Chesham, UK
  7. eddie49 Member

    That's a well-conceived and well-made universal MIG control board, with a lot of features and options, from TecArc. It has though-hole components, some resistors on ceramic stand-offs, and protective varnish. I am surprised they only need 2 small ICs, 3 small transistors, one thyristor, and 3 power semiconductors. Just 3 large diodes, not 4 ? At £115, it seems rather a high price, but I guess it is a low-volume product, made in the UK with decent components and full support.
     
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