Contactor

  1. Jpudds New Member

    Messages:
    13
    Location:
    Nuneaton England
    I have a boc Transmig 140, when I operate the trigger the contactor pulls in, it operates the wire feed and gas, but drops straight out.
    Can anyone please help
     
  2. eddie49 Member

    Hello and welcome to the Forum !
    I suspect that there is a fault on the PCB in the spot/stitch/burnback circuits. I am not aware of a circuit diagram being available.
    The early BOC/Murex machines were very well designed and used high-quality components, but suffered from poor assembly work and quality control, especially the soldering on the printed circuit boards. As in the other Transmig thread:
    https://www.mig-welding.co.uk/forum/threads/transmig-140-pcb-component-values.98793/
    the way to start is by looking for contact problems at all the push-on tags, the plugs & sockets, the screw terminal blocks, and finally re-flowing all the solder joints on the PCB.
    Obviously, do the work in sections, and make a note of where everything goes before starting.
     
  3. Jpudds New Member

    Messages:
    13
    Location:
    Nuneaton England
    Thanks for your information, it’s infuriating, it worked great.
    I didn’t use it for a while then when I needed it it wouldn’t work
     
  4. eddie49 Member

    I should have mentioned that since the small relay on the PCB controls the bigger Contactor, check if that relay opening precedes the Contactor dropping out. If it is not obvious, you could remove the plastic case and hold the relay shut. It is unlikely that the Contactor itself is faulty.
     
  5. Jpudds New Member

    Messages:
    13
    Location:
    Nuneaton England
     
  6. Jpudds New Member

    Messages:
    13
    Location:
    Nuneaton England
    Removed the plastic case and held in relay, wire feed roller continues to work but contactor drops out, but roller continues to work without trigger pulled in. Tried new relay still the same as original fault
     
  7. Jpudds New Member

    Messages:
    13
    Location:
    Nuneaton England
    Resoldered pcb joints, don’t know which one was at fault, but it’s now working.
    Thanks for your help much appreciated.
     
  8. eddie49 Member

    Thanks for the update, and I'm pleased that your Transmig is working again.
     
    monky harris likes this.
  9. Jpudds New Member

    Messages:
    13
    Location:
    Nuneaton England
    Sorry to be a nuisance, yet again.
    everything seems to be working now contractors wire feed and gas but I now find I have no spark, any suggestions please.
     
  10. Jpudds New Member

    Messages:
    13
    Location:
    Nuneaton England
    Checked the tip to earth nothing on it
     
  11. a111r Member

    Messages:
    898
    Location:
    London
    No spark /arc = too low welding voltage.
    Test the diodes on the rectifier with a DVM set to diode test mode.

    Each should read above 0.5 one way and 0 / 'OC' the other.
    You may well find one or more of the diodes have died.
     
  12. Jpudds New Member

    Messages:
    13
    Location:
    Nuneaton England
    sorry to seem ignorant, down to no experience, can you tell me which is the rectifier?
    Also have you any idea where I can purchase new brushes for the rheostat
     
  13. a111r Member

    Messages:
    898
    Location:
    London
  14. eddie49 Member

    There is an internal photo of a Transmig in post #12 of this thread:
    https://www.mig-welding.co.uk/forum/threads/who-made-the-boc-transmig-range.1921/

    At the centre of the base of the welder there is large transformer, the largest of three. The secondary is wound with flat copper strip. The two outer ends of the secondary connect via bolted terminal blocks to the flexible woven pigtails of the two diodes. These are stud-mounted diodes, bolted to a thick aluminium plate - which is the positive welding current output.
    The centre of the secondary is two copper strips, which go together to the back of the front-panel Dinse socket for the work return cable and clamp ( "Earth" clamp ). This is the negative of the welding current.

    Pressing the torch trigger should activate the small relay on the PCB, and it causes the main Contactor to pull in. There is a button in the centre of the Contactor, which can be used to manually force it closed without pressing the torch trigger ( use an insulated screwdriver to do this ).

    With the Contactor on, AC mains power will be applied to the main transformer. On the secondary, across the two outer ends where they bolt to the diode pigtails, you should see about 40v AC. From either of those two outer ends to the centre tap, i.e. the back of the front-panel Dinse socket, should be about half that - 20v AC.

    The thick aluminium plate that the two diodes are mounted on is the positive output of the rectifier. With your meter set to DC, that plate should show about 20v DC with respect to the negative Dinse socket. The voltage should vary as you turn the power control knob up and down. If all these checks are correct, you should be getting an arc at the MIG wire.

    The two large rotary controls on the front panel are not rheostats, they are variable transformers ( commonly known as Variacs ). The small one controls the wirefeed motor, and the large one provides fully-variable control of the welding power. They are both made in USA. I doubt if spare carbon brushes are available. You would have to make them by careful filing of a suitable brush, taken from a redundant electric motor or car alternator. Some imported power tools are shipped with spare brushes.
     
    Last edited: Nov 9, 2020
  15. eddie49 Member

    The brand name is "Powerstat", the manufacturer is The Superior Electric Co., Bristol, Conn., USA.

    Spare brushes are available - Google "superior electric powerstat brushes". But Farnell want £116 for one brush.

    Note: There are two circuit breakers on the rear panel of the Transmig 140; check that they are not open.
     
  16. Jpudds New Member

    Messages:
    13
    Location:
    Nuneaton England
    Thanks for your help.
    I removed and tested the two diodes on the aluminium plate and got no reading either way.
    When I hold in the contactor in nothing happens, I will have another look while testing using your advice.
     
  17. Jpudds New Member

    Messages:
    13
    Location:
    Nuneaton England
    The diodes are hopefully the Plessey ones I have inserted an image of
     
    • image.jpg
  18. eddie49 Member

    The Plessey components are small capacitors, designed to suppress spikes. The stud-mounted diodes on the aluminium plate look like this:
    Stud_Diode.jpg
    It should be enough to measure the AC secondary voltage from the transformer, and then check for DC after the rectifier, rather than removing the diodes.
    If there is no AC from the transformer, even with the Contactor switched or pressed on, then the fault is elsewhere, not the diodes.
     
  19. Jpudds New Member

    Messages:
    13
    Location:
    Nuneaton England
    Thanks to all involved for your advice, after checking the circuits as advised by Eddie 49 and making a new bush for the variac controlling the voltage all seems to be working.
    When I tested the diodes the right one was0.6 the left one was 0.47 I hope these readings are ok.
    Again thanks for your help and good advice
     
    eddie49 likes this.
  20. eddie49 Member

    That is a good result, I am glad you were able to track down and solve the true cause of the problem.
    The Transmig is a clever design for that era, using a variable transformer to feed the primary of the welding transformer, giving stepless fully-variable control of the output power.
    The values of 0.6 and 0.47 are the forward voltages across each diode when they are conducting. Ideally they should be closer, but in this rectifier circuit the two diodes are not sharing the current in parallel, so the difference doesn't matter.
     
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