Sealey Supermig 195/9 - would a failed bridge rectifier take out the aux transformer?

  1. a111r Member

    Messages:
    888
    Location:
    London
    Hello chaps.
    My first 'technical post' here! Much searching has been done, so it is now time to ask the many great experts here, I think...

    Background:
    As an avid fixer of 'old stuff', I bought this 1997 example recently, to replace my various small 'hobby migs'.

    It came with the declared fault of 'gas always on' and the seller's mitigating action was to replace the Euro torch. Cleaning out the gas solenoid valve sorted that issue.

    I then replaced all the usual consumable / wear items, cleaned all the internal spade connections, the six position voltage switch and the contactor.

    Fired up first time and the arc 'was not' good... a red glow. And a 'BUZZ'. Indicative of AC to DC failure?

    I then metered the OCV from + to - and the 1.5 amp in line fuse on the aux. transformer blew as soon as power was supplied. And continues to do so. Bugger...

    Any advice? Have I missed something?
    Cheers.
     
  2. a111r Member

    Messages:
    888
    Location:
    London
    Just to add, I metered the aux transformer primary winding.
    22 Ohms. Too high, as the tinternet says it might be?
     
  3. gaz1

    gaz1 Member

    Messages:
    11,332
    Location:
    westyorkshire
    no the diodes are after the transformer and change ac to dc current

    this would create a power less weld when trying to weld material i suspect you would get only blobs of wire metal whilst trying to weld
     
  4. MBB Forum Supporter

    Messages:
    1,127
    Location:
    northumberland
    Seems like you've a short if its blowing a fuse, check live/neutral to earth with a meter.
     
    a111r likes this.
  5. eddie49 Member

    If the fuse that supplies AC mains power to the auxiliary transformer keeps blowing, that could imply that the resistance of the primary winding of that transformer is too low ( rather than too high ) - i.e. the transformer primary has shorted, or, as MBB suggested, a short to ground.

    In fact, the 22 Ohms primary resistance that you have measured is probably correct.
    My Eland 200A MIG has a substantial auxiliary transformer, which supplies 24v to the wirefeed motor ( I guess 24v 3A ? ), the Contactor coil, and the gas valve. The transformer stack is about 3" square and 2" thick, I suppose it is rated at 100 - 150 Watts. The primary resistance is 12.3 Ohms.
    I do not know if the auxiliary supply in your Supermig 195 runs the wirefeed motor etc., or if it just lets a small relay on the PCB click on? If so, that type of small plastic-encapsulated PCB-mount transformer, rated at 2 or 3 Watts, typically has a primary resistance of 200 to 550 Ohms.

    To diagnose the error on the auxiliary supply, I would suggest disconnecting one leg of the transformer secondary, and temporarily wiring the primary direct to AC mains via a short 3-pin lead with a 3 Amp fuse in the plug. If that does not blow, measure the secondary voltage - should be around 24v AC. The load on that transformer ( rectifier, capacitors, wirefeed motor speed control ) could be faulty, or there could be an AC wiring error.

    Regarding the main issue of low welding power: what was the OCV when you metered it? By OCV, it is normally considered to be the main welding voltage, from the torch tip to the work return cable ( "Earth" clamp ). That should start at about 14v DC on the lowest power setting, and rise steadily to about 26v as you step up through the power ranges.
    The low power and buzzing could indicate an open-circuit diode in the bridge rectifier.
    You mentioned cleaning the 6-position power range switch - was that done internally?
     
  6. mike 109444

    mike 109444 Member

    Messages:
    4,296
    uk Bristol
    FYI schematic and manual
     
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  7. MBB Forum Supporter

    Messages:
    1,127
    Location:
    northumberland
    Isolate the output from the transformer from the rest of the welder if it still blows the fise its the transformer. if not its something down the line . Gas solenoid on all the time as in first post may be the culprit.
     
  8. a111r Member

    Messages:
    888
    Location:
    London
    Thanks guys, very useful suggestions there that have spurred me on.
     
  9. a111r Member

    Messages:
    888
    Location:
    London
    That's good to know potential failures of these two are not linked.
     
  10. a111r Member

    Messages:
    888
    Location:
    London
    Thanks Mike, I've been using that diagram all along. As a mech eng, it's not too useful as a diagnostic tool.
     
  11. a111r Member

    Messages:
    888
    Location:
    London
    I've isolated the bridge rectifier from the main welding transformer, the 240v fan and the 24vac gas solenoid.
    And the aux transformer stiil blows its 1.5 amp fuse.
    I think this pointing somewhere...
     
  12. a111r Member

    Messages:
    888
    Location:
    London

    Many thanks. Mine also has a big 3' x 3' aux transformer, mounted on the casing which feeds the same as yours. There's no relay on the pcb. Not sure of the VA rating of this transformer?

    I never got round to noting the OCvs as that's when the fuses started to blow. Next step I think, as I'm sure the rectifier has diode problems.

    I only cleaned up the external contacts on the cam switch; it felt fine operationally and I've read the posts here about dismantling them!

    YAY! Before I posted yesterday, I was thinking the next step was to isolate the aux and power up independently, as you and another suggested.
    So tonight I did it with a meter on the secondary....
    3 amp ..... fuse blown.
    Slipped in a 5 amp....! secondary o/p = 5vac, then the magic smoke !!
     
  13. MBB Forum Supporter

    Messages:
    1,127
    Location:
    northumberland
    It looks like you're going to change the transformer.
     
    a111r likes this.
  14. MBB Forum Supporter

    Messages:
    1,127
    Location:
    northumberland
    This is a diagram from charliecambs earlier post he used a power supply from a dell charger it may be of some use. The charger is only 19vbut there are plenty 24v ones around. @charliecambs
     
    • welder layout.png
    Last edited: Jan 12, 2020
  15. a111r Member

    Messages:
    888
    Location:
    London

    Thanks. I recently used a Dell 19.5 volt SMPS to upgrade my old SIP to a separate feed motor power source, a common mod.

    The big aux step down transformer that's failed is prob 100VA, so I'll look for something of a similar size. Sealey want £120 for a replacement...
     
  16. eddie49 Member

    It's unusual that when you increased the size of the primary fuse, the transformer held out for a while with just 5v AC on the output, until it finally gave up. I can't imagine there was a short on the secondary, must have been a partial and then a complete primary short. Anyway, it is all over now.
    That is about ten times what it should cost! I'll have a look if I've got anything suitable....
     
  17. a111r Member

    Messages:
    888
    Location:
    London
    Thank you Eddie, that's be great.
     
  18. eddie49 Member

    My spare transformer won't work, but I have sent a PM about an alternative.
     
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  19. a111r Member

    Messages:
    888
    Location:
    London
    Things have moved forward, so thanks to all for your good advice.

    Eddie49 is a legend and a fine gentleman; via PM he sent me (... a 'newb') a verified, suitable transformer, some hand made brackets, nuts and bolts, for very low cost, all posted...before I'd even paid him! Thank you so much, Eddie.

    First task was to sort out a suitable fuse, 1.5 amps, as the original transformer had it clipped via the primary, so I used a car audio inline 'sprung' type.
    I then wired it all up with fly leads, 'out of the box', and no load. Secondary voltage was 22.4 vac.
    Looks good. No blown fuses, nor any of that magic smoke.

    Ammeter and pcb was then connected to the secondary. The resultant o/p was very low, 6 mA ... but this might be because I've never used this DVM to measure amps and hence messed up somewhat?

    So, with nothing untoward 'that evident', I metered the OCVs from settings 1 to 6 and got a well spaced range of c. 22v to 40v.
     
    mike 109444, anjum and gaz1 like this.
  20. a111r Member

    Messages:
    888
    Location:
    London
    The aux transformer has now been mounted using Eddie's nice brackets, wired up and cable tidied.
    New earth clamp and gas regulator.
    Date expired 2 Kg extinguisher plumbed in. No need for a gas mix as I'm welding car body panels so the welds will need taking back with a flap disc.
    Case cleaned up with thinners and T-Cut to remove over spray.

    So we will see if it will do welds shortly...
     
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