Model 250 ac/dc HF Problems
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  1. #1

    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?

  2. #2
    Join Date
    Jun 2012
    Location
    CT
    Posts
    2,893

    Re: Model 250 ac/dc HF Problems

    Sounds like you fried an SCR. Not sure why or how. If it's not an scr, it is one of the gating diodes
    Millermatic 252
    Syncrowave 250
    Purox Metalmaster

  3. #3

    Re: Model 250 ac/dc HF Problems

    Is there a way to test the SCR?

  4. #4
    Join Date
    Mar 2010
    Location
    Ceres, California
    Posts
    3,261

    Re: Model 250 ac/dc HF Problems

    Full model name and serial number please. Is it a Dial arc or a syncrowave or some thing else?
    If it is the very old miller 250. you will have a crank on the front to move iron in and out of the transformer to control the current.
    If it is this machine you may have a shorted weld diode. In the SR1 assembly. Metal plates with diodes bolted to them.
    Get a meter and test for a shorted diode.
    Yes SCRs can be tested. but lets figure out if you even have any. An SCR machine is controlled by a circuit pcb and a small
    Potentiometer. The dial arc uses a large one, the miller 250 doesn't use one.
    Last edited by ccawgc; 08-10-2018 at 03:30 PM.

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