Shopmaster 300 output will not go below 100amps
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  1. #1

    Shopmaster 300 output will not go below 100amps

    Shopmaster 300 AC/DC (1993?)
    Serial KD473805
    HF 251 D1

    So my output when running GTAW is not controllable. When using my thumbwheel and all settings are confirmed correct, If I start a weld with the amp control set anywhere below max amps, I get 100amps on the amp meter. Even when set to minimum and remote/panel switch set to panel. If I set the amp control to max and turn off remote amp control I get max amps. So basically no matter whether I scratch start or use my thumbwheel, I get 100 amps or 300amps and nothing else.

    I pulled up the owners manual and the trouble shooting basically says have Authorized Service center check PC1 and HD1. My problem is that all my service centers in a reasonable distance wont even look at this thing because its so old. Thankfully I have a fairly strong electrical background, so Im comfortable tearing into this thing and testing components. I just need info on how to test HD1 and what to look for on PC1.

    Was just getting started on building a stainless exhaust system for my brothers Harley, so I need to get this fixed asap.

    Would be nice to find a service manual with test values for each component

  2. #2
    Join Date
    Mar 2010
    Ceres, California

    Re: Shopmaster 300 output will not go below 100amps

    If I remember correctly. The Panel knob will set the maximum output and when set in remote the remote should control between 0 and panel set. Have to make sure both the HF box and the welder switches are in the correct position, for it to work right. If the pcb has failed you will have to repair it or send it away for repair. Miller does not have any more, last time I checked.
    Send me by PM your email and serial number.

  3. #3

    Re: Shopmaster 300 output will not go below 100amps

    HF and main panel settings are definitely correct. I also tried with or without the HF box on (ie scratch start)

    Pulled the pcb and don't see anything that looks scorched. Two trimpots at locations R132 and R144 look like they got hot, and the glue that was used to lock the knobs in place looks a bit cooked... and the coating is split on a capacitor on c12 and on the 33ohm resistor(?) at position r17. But the capacitor doesn't appear to have leaked..

    Ill test everything on this board I know how to test.. But Im not to confident in the issue being the board

    The trimpots

  4. #4

    Re: Shopmaster 300 output will not go below 100amps

    Sine I was very tired when posting yesterday I did do a run test again before disassembling the case to check the pcb.

    ccawg is correct in that the when in remote, the panel knob sets the maximum current.

    If I have the knob set at say 50amps, I get 100amps no matter the position of my thumbwheel when the rig is set to use the remote.
    If I turn off the HF box and set everything to panel and scratch start, I get 100amps

    This happens with the amp control knob set at anything under max... I get 100amps even with the knob at minimum.

    IF I set the amp knob to max, I get what I assume is max. I don't have a tungsten rig capable of handling the max of this machine, or a piece of metal thick enough to burn on at that setting for long.

    But whether set remote or scratch start without the HF box, knob at max, when an arc is started I immediately see the amp gauge go shooting towards the top end of the scale and well beyond 100amps.

  5. #5

    Re: Shopmaster 300 output will not go below 100amps

    Ccawg... havent seen an email?

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