Reviving a BOC DC 130 Transarc

  1. BR_T New Member

    Messages:
    5
    Location:
    UK
    I've picked up one of these and trying to get it to work. No output at the terminals, nothing when I try to arc. Leads seem fine, have checked the thyristors with a multimeter and seem to be working fine, transformer hums away, fan constantly on and I have a 13a fuse in the plug. Other than the board I'm not sure what could be the problem, I have checked the board components best I can with a multimeter but everything I test checks out - I'm not an electrician so might have to just take the whole thing to a shop. Having another look this weekend. Any thoughts before I do?

    In addition - when I got it there was a brass fuse in the plug and the capacitors on the thyristors had blown. I've replaced the capacitors with modern equivalents and a new 13a fuse.
     
  2. eddie49 Member

  3. minimutly Member

    Messages:
    1,510
    Location:
    Pembrokeshire Wales
    Transarc tradesman?
    Used to have one of them in work for stick and dc tig use - 30 years ago - it took some abuse.
     
  4. BR_T New Member

    Messages:
    5
    Location:
    UK
    Thanks Eddie, there is AC output from the transformer but checked the thyristors with a multimeter (took them out first) and they seemed to work. I checked the gates using an external power source, but I will check again tomorrow - still learning.
    Could it be that the diode/gate is working but there is no signal to the gate from the pcb?
    Thanks for the input.
     
  5. BR_T New Member

    Messages:
    5
    Location:
    UK
    Yeah that's it. Looks solid and fairly simple inside apart from the pcb. I picked it up to learn to weld but so far not been able to make it work.
     
  6. minimutly Member

    Messages:
    1,510
    Location:
    Pembrokeshire Wales
    Old elecrtronics - mostly obsolete. Do you have a circuit diagram?
    Bare in mind if you are spending money on it, an equivalent modern machine would cost sub £200?
     
  7. BR_T New Member

    Messages:
    5
    Location:
    UK
    I have a print out of the circuit diagram linked above and so far the capacitors have only cost me a couple of quid. I'm trying not to spend any more money on it so hoping it's a cheap electronic part and still available, otherwise like you said I'd be better off getting a new machine with warranty. I'll have another look tomorrow.
     
  8. unimog Member

    Messages:
    32
    Location:
    scotland
    hi try replace the thyristors with a 1n4007 diode if you get dc out put then with a scope check the dc op is bridge op wave form and check the op wave from the pc is ok
    i think the ic is a opamp ie quad 741

    good luck
    unimog
     
  9. eddie49 Member

    That's a great idea from unimog, to temporarily replace the thyristors with normal diodes.
    Apparently the DC output voltage of the welder is 42v, so the secondary of the transformer should be providing about 42 / 1.414 = 29.7v AC. With two diodes - any diodes really - replacing the thyristors, you should get the 42v DC on the bridge rectifier output. It will now not be a controlled bridge, it will be on at full power all the time. Of course, if you have used two small diodes, you won't be able to weld with it, but it will prove that the other two diodes are OK.
    If the DC is now OK, then either the thyristors ( SCRs ) are failing to turn on, or the trigger signals are missing.
    A multimeter on "Diode Test" will confirm that the thyristors are not shorted - there will be no conduction in either direction.
    To check that they can be triggered on, you could rig up a series circuit: 12v car battery, positive to the anode of a thyristor, cathode of the thyristor ( normally the stud ) to 21W car bulb, back to battery negative. A small positive voltage applied to the gate ( e.g. 1.5v or 3v battery ), with negative on the cathode, should trigger the SCR on. Once on, with a DC supply, it will stay on.
    If the thyristors do conduct, the next step would be to get hold of an oscilloscope to check why the trigger pulses are missing. They will be short pulses, synchronised to the 50Hz mains frequency. Changing the power control knob will alter the time when the pulses occur, in relation to the start of the AC waveform, allowing part or all of each cycle of the AC through the rectifier, and thus controlling the welding power. The same principle is used in the speed controller of a mains electric drill, or a light dimmer. This is called phase control. [​IMG]
    The welder uses two thyristors, one conducts the positive half-cycle, the other the negative.
    A digital meter might show something on the gates, but you really need a 'scope. Both trigger pulses are driven by transistor TR1 on the PCB.
     
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  10. minimutly Member

    Messages:
    1,510
    Location:
    Pembrokeshire Wales
    While trying to fix an old 2 to 3 phase inverter a while ago, and afraid to switch on before fully testing the driver circuits to the gto thyristors I used 12v 3w lamps wired instead of the gto inputs - varying the frequency gave a nice change in glow - easy to see any difference, able to leave on soak for a while, test with a scope etc.
    I know the welder only changes phase angle, but the lamp brightness should change with amps setting?
     
  11. eddie49 Member

    Not sure about using 3W bulbs to confirm that the thyristor trigger pulses are active - that may be too much current draw for the single transistor ( TR1 ) that supplies both triggers. Perhaps a small 20mA LED in series with a 220 Ohm resistor would be safer. However, for the next step in troubleshooting, you would still probably need to have access to an oscilloscope.
     
  12. Rig Pig

    Rig Pig Member

    Messages:
    3,755
    Location:
    Narrwich! U.K.
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