Vintage Migmate ... reunited!

  1. a111r Member

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    304
    Location:
    London
    Going to be poking about this old machine which I sold 6 years ago. Didn't realised then that it was of superior build quality. DSCN3519.JPG DSCN3520.JPG
     
  2. a111r Member

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    304
    Location:
    London
    New liner and de-rusted the externals. Now it needs a good clean out internally.
    There's probably a high resistance joint to check for, too or maybe an issue with the 'fun-sized' PCB, as after a minute's welding the wire feed slows and the arc is erratic.
    Also it's got a bit of a slow feed range, 2 m/min to 9 m/min?

    I recall I had no qualms moving it on, thinking it was the usual SIP issues but now I think should be a nice machine. DSCN3562.JPG

    I think that TIP3055 would be worth de-mounting and testing?
     
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  3. eddie49 Member

    That machine is so primitive and basic, it's like a stone-age MIG ! The tiny PCB, the open relay, the uninsulated spade tags, the windscreen wiper motor for wirefeed, even the crude welded-on transformer brackets fastened with a single self-tapper. It is functional, but so very minimalist.
    Maybe the wirefeed motor gearbox needs to be cleaned out and refilled.
     
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  4. tom2207 Member

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    474
    Location:
    uk northern ireland
    but it can be sorted ,,, a handy thing to have under the bench.
     
  5. a111r Member

    Messages:
    304
    Location:
    London
    I think SIP realised (early 80's) that they could ramp down the build quality and give Practical Classics a bung to rate the new, inferior models?
    It's a nice old thing. I'd like to get it working properly, good for another 40 years service.

    I cleaned up all the bolted connections earlier. Trans to choke was a bit loose but not burnt, like the Clarke last week. I think even I could refurb the PCB...

    The wire tensioner mech. is similarly Stone Age. Rigid with a wide, 'non-roller bearing' roller. The case has a nice patina, too.

    The gas valve solenoid is a nice touch on a 'low-end'. That means there's no mech. gas valve in the torch, nor a pesky swan neck liner, so the liner runs straight to the contact tip

    I wonder if it will actually do nice welds, nostalgia aside...?
     
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  6. a111r Member

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    304
    Location:
    London
    Photos of the said items. The terminal nerds may spot a retro-fitted Clarke handle replacing the broken one ... or maybe not!:
    DSCN3563.JPG DSCN3564.JPG
     
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  7. charliecambs Member

    Messages:
    16
    Location:
    cambridgeshire
    I have a similar one i bought new unused from car boot many years ago. Transformer or maybe rectifiers had date stamp 1987 I think. internals look the same. used it a lot then decided to swap to separate power and PWM controller from ebay for the wire feed and its really good now.

    It came with wire liner in torch. The wire feed as we know operates from weld voltage so varies with arc length so ends up chasing itself up and down. Since yours may have an issue anyway I'd suggest the change to separate power. I used to get frustrated each time i got it out for a quick job due to stuttering arc start. Now its set power, set speed and off you go.

    I've added a cheap digital voltmeter so you can set speed before welding ( as it no longer automatically changes with the power settings).

    Just noticed the wire feed tensioner setting on yours, mines good pretty well no spring pressure but a bit sensitive to having gun feed straight.
     
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  8. a111r Member

    Messages:
    304
    Location:
    London
    Interesting, thanks.
    I've done most of the SIP mods (power supply / bracing) before on another 1990s example. I concluded no real step change to = 'nice machine', unfortunately. I'm not going to be doing that again, although it was educational.
    This one had a black plastic liner. Now it has white 4 mm curtain wire.
     
  9. a111r Member

    Messages:
    304
    Location:
    London
    I took a look inside the motor's gearbox as Eddie suggested. The grease is like new.

    I also found that the feed roller wasn't aligned as accurately as it should be. This was because the drive shaft is Woodruff slotted (from when it was a wiper motor?) hence tightening the grub screw moved the roller towards to bottom of the curved slot.

    DSCN3565.JPG
     
  10. eddie49 Member

    Yes, nothing wrong with that gearbox.
    So perhaps the fault is electrical, rather than mechanical. Rectifier diodes breaking down when warm? Maybe a voltmeter - an analog one, even - might show some fluctuation whilst welding, or after gentle warming of the rectifier with a hairdryer.
    If you did a quick "separate PSU for the wirefeed motor" mod, that would help to diagnose where the problem lies.
     
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  11. a111r Member

    Messages:
    304
    Location:
    London
    The hair dryer is a great idea, thanks I'll try that first.

    I have some SMPS around and I'm sure one's 20 volt, plus a suitable relay.
     
  12. a111r Member

    Messages:
    304
    Location:
    London
    Just finished some rectifier testing, although most of the time was spent figuring out why my Beckman meter was acting up, measuring voltage but not resistance or diode function. One of the leads has a break it seems, so had to make do with a wire.

    Testing cold, I think one of the four diodes is failing 'open', as it tested O/L and around 0.2*, compared to the others at O/L and 0.5

    Testing warmed:
    Two were O/L and 0.4
    One was O/L and 0.02 (* can't remember if it was this one)
    One was 0.005 both ways (shorted?)

    Spot on Eddie, as usual! I have a spare from a similar amperage machine that will physically fit the space. It has four diodes but two heat sink plates, would that substitute? I don't really 'get' translating a rectifier schematic into how they're constructed...

    DSCN3566.JPG
     
  13. eddie49 Member

    Yes, I find them baffling and frustrating too, especially if there's only two effective diodes for use with a centre-tapped secondary, or if half the diodes, even ones on the same plate, are actually reverse-polarity style. I have to make composite sketches with both physical outlines and electrical symbols to figure them out. Give me 4 gurt big fat stud-mounted diodes each on their own heatsink any day!

    With only one plate and only 4 diodes and all of them pressed into the one plate, the original rectifier has to be one of those two-effective-diodes versions, to be used with a centre-tapped transformer. Each of the two effective diodes is a pair of actual diodes, and all of them have normal polarity ( stud or case is cathode ). I can't quite work out where the spike suppressor ( MOV ? ) goes, I think it's across the two AC inputs - which are the two thick metal tabs or ears, with holes for M6 bolts.
    The splash of red paint on the aluminium plate shows it is the positive, and will be bolted to the torch cable.
    The bolted junction between the transformer centre-tap and the inductor input ( negative ) is another give-away. That is where the infamous Clarke splice can be found, with two wires on one side because it is the centre-tapping-point of the two "half" windings.

    The potential replacement rectifier with two plates had me puzzled for a while, until I realised that it is in fact identical to the original, except that it has been built backwards! The common point where the cathodes of the two effective diodes meet is the centre metal tab. This is the positive output of the rectifier and will go to the torch. So in this rectifier, the diodes are reverse polarity, the cases pressed into the plates are anodes.
    The two AC inputs, from the two outer ends of the centre-tapped transformer secondary, are in this one the two outer square plates. Again, the MOV is across those AC inputs.
     
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  14. a111r Member

    Messages:
    304
    Location:
    London
    Thanks for the very detailed explanation, Eddie. I'll fit the Clarke rectifier later. The only modification needed is trimming down the mounting stud an inch to better clear the choke.

    I think it would be worth a minor wiring mod to utilise the Clarke rectifier's 100 C thermal cut out (removed in the above photo).
    The SIP hasn't got one, which could explain the diode failure from heavy, prolonged use.
    I'll wire it in series with the trigger wire's DC +ve, the same as with some early Clarkes. Later but similar Clarke models have it alternatively wired in via the 'indicator' mains on/off switch.

    So this old Franken-Clark has been very useful. The PCB, torch, wire feeder, handle and now the rectifier have all been re-used on other machines. Maybe I'll use the non-rusty dividing panel for fabricating repair sections. (There's similarly stripped Mightymig behind it).

    DSCN3570.JPG
     
    Last edited: May 14, 2020
  15. a111r Member

    Messages:
    304
    Location:
    London
    Finished. I had to relocate the choke 15 mm sideways to get enough room for the wider rectifier.
    No gas to test it but I ran beads totalling 1 m and it was at least, consistent.

    DSCN3571.JPG
     
  16. eddie49 Member

    Andy, now that I have seen a side-on view of the new rectifier I am not so sure that my advice on it was correct.
    It has two square plates, and I now see two thick metal "ears", so four connections in total. Two AC in, two DC out, it is a full wave bridge rectifier? But there's only 4 diodes in total, they would not be big enough to take the current alone, and the Clarke 90EN that it's been labelled with is a 2-diode centre-tapped jobbie, and maybe one of those two ears is in fact butted up against the back plate anyway.
    Regards, Confused of Reading
    P.S. Should a two-diode-plus-centre-tapped-transformer rectifier be called a full wave half bridge rectifier ?
     
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  17. a111r Member

    Messages:
    304
    Location:
    London
    Thanks Eddie, no I think you were initially correct. And nothing went bang!
    The unused 'ear' with the confusing blob of red paint is an AC connection, an extension of the rear square plate. Here is how it's wired on a small Clarke, you can see the motor, torch and welding voltage all to the centre ear.

    The Sealey Mightymig that I dismantled I think had a full wave rectifier, so non-centre tapped transformer? There were 8 diodes over 2 plates with 2 DC outputs.

    DSCN3569.JPG DSCN3567.JPG
     
    Last edited: May 15, 2020
  18. Reman

    Reman Member

    Messages:
    89
    Location:
    Bristol, UK.
    So the wire feed is in essence a wiper motor? Is it a truck unit running on the same 24v transformer tap that the newer Migmate's use, or is it a car unit and yours has a factory 12v tap/supply instead?........ Or is it all a modification that was done some time in the past?
     
  19. a111r Member

    Messages:
    304
    Location:
    London
    Yes, that's it. The motor's Italian, 24v. It's powered from the welding dc voltage off the rectifier, as is normal.
    It's not been previously converted, it's how they started off building them.
    Then they realised it would be much cheaper to use 'toy grade motors', driving plastic reduction gearing, within a less rigid enclosure.
     
    Last edited: May 15, 2020
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  20. a111r Member

    Messages:
    304
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    London
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