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Old Model 250 ac/dc HF Problems

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  • Jmcghee
    started a topic Old Model 250 ac/dc HF Problems

    Old Model 250 ac/dc HF Problems

    Hey guys-

    I recently picked up an old transformer machine, Model 250 ac/dc HF tig & stick. I wired it in (100amp breaker) gave it a quick bath, snugged everything up, hooked up a bottle, & started welding. I put it through it's paces, and everything went great. Spent probably an hour or so trying everything... tig, stick, ac, dc, hf start & continuous, mild steel, aluminum, etc and it all worked perfectly. The only adjustment I made was on the HF spark gap, the manual called for .008" & it was way over that. I also replaced all the tig torch consumables with new. After getting acclimated I had lunch, came back a couple hours later planning on getting started building a small welding table. I'm not sure what happened in that time, but the machine is no longer happy in DC. AC works perfectly, but in DC I'm getting a pretty strong vibration that directly relates to what the amperage is set at (150 amps vibrates hard enough to shake a screwdriver off the top of the machine). It also won't really operate in DC. It'll try to strike an arc, but just barely.

    I'm way out of my depth as far as diagnosing this & was hoping somebody might be able to steer me in the right direction? The machine sat for about 10 years before I got it, but everything seems to be visually fine. There's no obvious burned/frayed/loose connections, leaking caps, etc. Anyone have any idea(s) what might be going on?

  • Aeronca41
    replied
    Fantastic! Success is sweet.

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  • ryanjones2150
    replied
    That would be colonel John "Hannibal" Smith.

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  • BukitCase
    replied
    Cool; to quote the guy on the A team, "I LOVE it when a plan comes together" :=)) ...Steve

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  • Jmcghee
    replied
    The diode came in yesterday. I swapped it out, fired it up, and I'm back up & running! Thanks again everyone for walking me through it

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  • Aeronca41
    replied
    Good advice.

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  • BukitCase
    replied
    "I'd just change the one--others opinions may differ. Can you find a part number on it?"

    I agree - IMO, the three that survived are proven TOUGHER :=) -

    on part #, watch out for an "R" in it - a lot of bridge rectifiers on old stuff only used TWO heat sinks (save money) - so HALF of the diodes had the CATHODE on the end that BOLTS on, the OTHER HALF had the ANODE on the bolt.


    Other diodes may just have different part # for straight or reverse polarity, so...

    Lacking an "R" in the part #, if your machine has 2 heat sinks and 4 diodes, make SURE you check the pn# of the one that's BAD, not the one that's easier to read... Steve

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  • Meltedmetal
    replied
    Internet craziness today, ignore me.
    ---Meltedmetal
    Last edited by Meltedmetal; 08-09-2018, 06:51 AM.

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  • Aeronca41
    replied
    Yep--what he said.

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  • ryanjones2150
    replied
    I'm in the camp of if it ain't broke, or obviously about to break, don't fix it.

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  • Aeronca41
    replied
    I have to say--another old guy thing, I guess--I never use the diode test/beeper unless I'm simply testing for wire continuity and I need to hear the beep from many feet away. Don't think I've ever used it even once to check a diode.

    I'd just change the one--others opinions may differ. Can you find a part number on it?

    Leave a comment:


  • Jmcghee
    replied
    You guys were right... the beeper selection has a ceiling below the resistance I was getting (300 I believe). I selected a higher range, and presto. 3 of them check out & one is trashed. Should I just replace the one, or all 4 & the resistors while I’m at it?

    Leave a comment:


  • Aeronca41
    replied
    I agree with Steve--I'm not sure you have four failed diodes. While I have more digital meters than any reasonable person needs (I'm like that with welders, too.....), when I'm testing diodes I like to grab one of my trusty old Simpson 260s. Love to see that needle swing. Not that you can't do it with a digital meter--I can and do, but being an old guy I still like some of the old ways, and this is one of them.

    How did you test the ones that gave no indications? Here's what I would do: Don't use the diode test/beeper range on the meter--use ohms. Connect the leads solidly with alligator clips, and get a resistance reading in one direction. If your meter has autoranging, turn it off as Steve suggests. You may have to switch scales until you get a reading. Then do the same thing with the leads reversed. I'm guessing you only have one bad diode. I think you are going to find the other three have low resistance one way and high the other.

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  • BukitCase
    replied
    Digital meters can be tricky, I'm wondering if your other three diodes are actually OK - I would check those 3 again, this time setting your meter to NON autorange, and check each one on several different MANUAL ranges - it's possible you might get a reading of some kind, also if your meter has a "diode" setting THAT's the one I'd use... Steve

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  • ryanjones2150
    replied
    Since it works perfectly in AC, like you said, I'd think the transformer is probably unscathed, by and large at least.

    Leave a comment:

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