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Millermatic 250 Problem, No Arc

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  • #16
    Be careful please....


    • #17
      The 2 post on the left pointing up and down, those are feed power from the transformer?? right??
      I didn't have any power on them.


      • #18
        You are not getting 56 vac with power on?


        • #19
          You're correct--That is SR1. No voltage there is sounding like a bad transformer, but let's disconnect it and be sure something isn't pulling the voltage down. Seems unlikely the breaker wouldn't have tripped if that were the case, but always good to check.

          The cables from the transformer should have number tags on them 6 and 7 as shown in the schematic (first pic). If not, you can just follow the wires.

          I also included marked-up scans of the wiring diagrams. Red is the two ends of the secondary, and green is the center tap. On the second picture, ignore the green note "to C7"-that's wrong. C7 is down below.

          I missed PopScott's post. Right on.

          Click image for larger version  Name:	scan2181221.jpg Views:	0 Size:	756.1 KB ID:	607779Click image for larger version  Name:	scan2182256.jpg Views:	0 Size:	1.09 MB ID:	607780Click image for larger version  Name:	scan21825675.jpg Views:	0 Size:	650.8 KB ID:	6077817
          Last edited by Aeronca41; 04-18-2020, 08:34 PM.


          • #20
            Go back with power off and check resistance at same locations... should be less than 1 ohm on both outside plates to each other and same to outside plates to work clamp and neg terminal.


            • #21
              I'll check tomorrow and let you know what I find.

              I really appreciate all the help you guys are giving me!


              • #22
                Ok. I disconnected the transformer leads from the rectifier and checked the voltage on the transformer, I had 60 VAC, 30 VAC on each leg. So then I checked the diodes in the rectifier and one was open, no flow either direction, and the other side flowed both ways.

                The way I checked the diodes was with a 12v test light and 12v battery, I hooked it up one direction the reversed the direction.
                Is that an acceptable way to test the diodes?

                I did a google and ebay search and didn't have any luck coming up with the exact part. Miller number 154 242

                So those diodes/rectifier must be the weak link in a lot welders, battery chargers and generators. So far I've had to fix both my big battery chargers and my backup generator, all they needed was the rectifier. That's the only thing that went wrong with them in 30+ years, lol.

                Thanks again for all the help,I like to diagnose the problem and nail it the first time. When I'm diagnosing driveability and no start problems in automotive it really bugs me if I don't hit it the first try. I always try and cross check or double check what I come up with before buying any parts or doing any work.


                • #23
                  Yep, that sounds like a great way to test the diodes. The shorted one was pulling down the voltage on the secondary. Did you notice the buzzing was not as loud with the shorted diode disconnected?

                  I'm a little surprised the shorted diode didn't pop the breaker, but I'm a systems engineering guy, not a welder repairman.

                  If you call Miller tech support, they can probably tell you a new part number for the diodes,but they will likely be pretty pricey. If the price is out of sight, you could try buying generic hockey puck SCRs from Mouser, Digikey, Allied, Hamilton, etc. The parts list says they are 325 amp, 300 volt rated. The parts list calls them thyristors, but that term isn't used nearly as much as SCR now. Get your calipers out and measure them for the various diameters and thicknesses and see if you can find something the exact same size. The contact surface area is essential for cooling, so don't try anything that won't mount up just like the originals. When you take them off, see if there is any clear or white paste (thermal mounting grease) on them--often used to improve heat transfer on high-power semiconductors. You can buy it at electronics supply places. And look on the diodes themselves--there might be an OEM part number somewhere.

                  Another thought--I don't know what part of the country you live in, but if you have thunderstorms, make it a habit to unplug your welders when you aren't using them. If they're hard-wired, even turning off the power feed switch is not as effective as completely disconnecting them from the power source. A lightning strike even far from your place can travel down the wires and on occasion toast semiconductor components, even if powered off. Diodes and SCRs do go bad on their own from old age, but they can also be taken out by power surges. Or they may be damaged, and not quit working until months or even longer.

                  Glad you got it figured out!!


                  • #24
                    Diodes? They are SCR's... 154242 is not a valid number... 143818 THYRISTOR, SCR 325A 300V hockey puck is the number..$201.. if you have a shorted SCR then you may now have an another issue at your board RC2 gate signals caused by that short......unplug and measure RC2 1 and 4 or 2 and 3 red/white wires and they should be 20 ohms or there about...if either is shorted it may have possibly damaged the board.
                    Last edited by Popscott; 04-21-2020, 04:52 AM.


                    • #25
                      I checked with the welding supply that I bought the machine from and they had to call Miller.
                      Miller said that part has been obsolete since 2006. So that was a strike out.
                      I'll check the items you guys mentioned see if everything else good.

                      Is that 143818 Thyristor something that will work in place of the Miller rectifier bridge?


                      • #26
                        143818 is the hockey puck in post #16.. there are 2... $201 from Miller ea.... Disassemble it... note the polarity when taking apart... They are torque sensitive so note the position of the clamp gage (see pic) and tighten it up to the same value when re-assembling. Miller shows they have these.. I am seeing them on Ebay for $150
                        Please look at post #24 and be sure the gate reading is not shorted... A short puts the power circuit voltage directly back into the board.
                        Last edited by Popscott; 04-22-2020, 08:56 PM.


                        • #27
                          The way to test is here

                          but using your crude setup with battery and bulb...leave the transformer leads unhooked... unplug rc2.......make sure there is no residual voltage on the caps ... put negative on plate puck end with red wire and positive on the other plate.. get a resistor.. looks like 270 or so ohms and touch one end to the positive plate and the other end touch off on the white wire in the RC2 plug... the bulb should come on and stay on until you unhook the positive from the plate.
                          Last edited by Popscott; 04-22-2020, 09:21 PM.