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Shopmaster 300 amp control

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  • Shopmaster 300 amp control

    Shopmaster 300 AC/DC #
    KD473805 with HF 251-d1

    Was welding great one day, next day working with same materials and untouched settings I went to strike and arc and it immediately blew through the material.

    I was set at like 50amps on the panel and using a thumbwheel on my torch, but was getting 100amps on the machine gauge no matter what I did with the thumbwheel. Immediately suspected the thumbwheel so I set the machine to panel control instead of remote and got the same thing 100 amps even with panel knob set to minimum. Disabled the HF box and put the remote cable direct to the shopmaster, same result with scratch start.
    Then I tried setting the panel higher than 100amps, and it stayed putting out 100amps until I went to max panel at 300amps. It then hit 300amps right away.

    I downloaded the owners manual and the trouble shooting guide says go to service center for checking the HD1 and PC1. Problem is, no local service centers are even willing to look at the machine because of its age (1993 I think).

    I work with circuit boards and replacing components in them as part of my day to day job, but that's all digital circuitry and I don't really ever do any component checking. (Usually just replace a few caps and chips to make an ecu programmable). I have basic tools for doing this stuff, so I can probably test this stuff, but I cant find any specs to go by. Nor can I find a service manual. Any guidance on testing would be greatly appreciated.

  • #2
    On your model there are 2 fuses. I know you probably checked them.
    With machine power off, HD1 pins 1 and 3 should read 1600 ohms.
    With machine on, wire 78 to 42 or any ground you should have 24 VDC.
    With someone welding as you don't have a load bank wire 79 to 42 you should have 1.6 VDC for every 100 AMPs.

    I'm hoping it's your fuse.


    • #3
      I found one fuse with a spare... is their another functioning fuse? That fuse checked out with an ohm meter. Hd1 pins 1 and 3 show 1636 ohms.

      I'll check the other values in the morning while my wife is around... she gonna freak when I tell her I need her to weld haha
      Last edited by Fire67; 12-05-2018, 10:01 PM.


      • #4
        Some models had 2 fuses. If it isn't right next to F1 than no, you only have the one.


        • #5
          Yeah just f1 and a 2nd spot with a fuse but no wires going to it.. I actually swapped the wires from one to the other just to try it.. just about to do the other checks now


          • #6
            Ok with help this got interesting....

            78 to 42 im showing 24.3vdc

            79 to 42 Im getting 2.4-2.8vdc if the panel is set anywhere from 50 to about the 200 mark on the panel IF I use the thumb wheel. The amp gauge never goes below 150amps

            If I set it to panel amperage control and just use the finger wheel for the contactor, I get similar amps and volts. BUT if I manually turn up the amperage while she was holding an arc the 79-42 voltage starts to climb when I get passed the 150-200amp mark on the panel knob. Between the 150ish and 300amp marks on the panel knob, my reference voltage climbs linearly from the 2.8 mark to about 3.2-3.4vdc

            So I need to check the finger wheel output for sure, and going to go figure out how to do that now.

            In panel control I have 79 wire reference volts that roughly match the output according to the gauge even though the setting is lower than were seeing. So I think I also need to check the potentiometer that is the amp control knob. Weird that it would fail without moving it (when the problem started I was working on a project with thin steels and never adjusted that knob between day one and day two)

            Does the low reference voltage at 300 amps indicate anything with the HD1? Or is this just natural variance? Im leaning towards controls more than anything so Im checking the panel and finger wheel pots now. Looking forward to insight from 1997cst or anyone else


            • #7
              Welp… no 10vdc anything on the 14 pin receptacle... and only seeing 12vac on the contactor output pin when I should be seeing 24vac, however the contactor does work. But as we all know without the 10vdc reference, we have nothing to control amps.
              Last edited by Fire67; 12-06-2018, 01:22 PM.


              • #8
                Looks like your board needs repair just like mine.


                • #9
                  Sounds about right unfortunately.


                  • #10
                    Fire67, I’ve got a electronics guy going through my board now. He’s found problems with the op amps on it. They send a signal to SCRs to control them. Just some insight. I hope it helps


                    • #11
                      Miller sent me a diagram for the PCB.. Mine is part number 155826, if anyone wants the diagram pm me with an email address. I started from the 10v reference output pin and started going backwards through the circuit. Found 6 resistors out of their individual spec (diagram doesn't give exact specs on each component just miller part number. So this is going on the resistor bands color codes)… I don't have the ability to test the op amps or capacitors in the circuit.. The varistors in the board don't have an exact spec either, and I have a feeling they are "tuned" to get a desired output from each circuit.. I have experience with this stuff and have zero patience for it at this point in my life..

                      So, I priced things out.. Found a brand new old stock pcb for $1075, yeah ****ing right... The repair places want a minimum of $400 to go through the board and replace components they find bad; but ZERO guarantee on the work... so spend 4-600 to maybe fix it??? I don't even use half the capability of this machine... No thanks.

                      Im leaning towards selling the HF251 box (they are currently selling for 700+ on ebay) and literally scrapping all the copper and steel in the shopmaster. I can probably get 3-400 if I spend the time to strip down the unit and segregate the copper from everything else and send it to scrap. **** I bet I could find a scrapper to do all that work for me and still pay me $150...

                      So minimum $650 and I can go buy a non miller inverter machine for a couple hundred extra... I don't use this thing for production, so if I get a cheap machine that breaks down, I send it in for warranty. If it breaks after warranty, I can still buy 2 more with 5yr warranties before I spend as much as another miller machine with equal capabilities now. Hate to put it that way, but Im a born and bread capitalist.. Bang for the buck my friends, and I don't weld day in and day out, so I do not need the reliability.

                      My Shopmaster rig is 25-26yr old.. time for me to move on...

                      Anyone want a parts machine, HF-251, or a water cooler?