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Miller intelliweld blowing fuses

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  • Miller intelliweld blowing fuses

    Just picked up a miller intelliweld 650 serial is jk574295 however it won’t stay powered up. With these units you basically manually apply the relay then it energizes the magnet to hold it in... so I’m assuming that means it isn’t getting its 24v supply to do so. I followed the troubleshooting in the owners manual and the f7 and f8 fuses were blown so I replaced as it instructed and they immediately blew upon startup. The Manuel says these two fuses power the main control board and the control rectifier. But since My relay doesn’t stay energized I was guessing rectifier issues????So what I’m looking for is someone that’s a tad smarter than I to tell me how to narrow this down a little farther. Any component test procedures or more info would be greatly appreciated.


    I know enough a decent bit about components so I should be able to follow the more complex stuff but pictures and diagrams always help!

  • #2
    You seem to be on the most logical track. I would start by checking bridge rectifier SR4; I think you are going to find shorted diodes in there. Do you know how to check the diodes in a bridge circuit? If not, please locate SR4 and post a picture of it and I'll see if I can help. Since a bridge rectifier can be physically made several different ways, and I don't know what this one looks like, will need a picture to guide you.

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    • #3
      Is it this ?? The little black box ? Sr3 in the diagram ?

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      • #4
        Yep-the black box at the extreme upper right of the first picture, with all the wires on it, is a diode bridge. Assuming you're done your homework in locating it, it will be SR3. There are other bridges in the machine. Typo in my earlier post where I called it SR4.

        To test a diode bridge with an ohmmeter, all of its wires must be disconnected. Obviously, be sure all the wires are labeled, and the terminals on the bridge are labeled. Imperative not to mix them up when you reconnect it after testing, and you need to know which terminal is which to do the tests. Mixing them up can cause some very very bad things to happen, from which you may not be able to recover.

        You have to test each DC output terminal (the plus and the minus, terminals for wires 46 and 47, respectively) against its associated AC input terminal (wires 44 and 45). There will be a total of 8 readings you need to check to test all four diodes.

        The bridge terminals may already be marked with two labeled AC, and one each -- and +.

        You probably already know this, but I will include it in case you don't, or someone else looks at this in the future. A diode works by passing electrons in one direction (electrons flow against the direction of the arrow; I know that feels backwards, but that's how it is) and blocking them in the other. Your ohmmeter works by measuring how much current from its internal battery flows through whatever you are testing. To test a bridge, you just have to be sure each diode conducts in one direction, and not the other. If it is shorted, it will conduct pretty much equally well in both directions (bad). If open, it will not conduct in either direction. (also bad).

        So, looking at the schematic diagram, to test the top right diode, put your meter on Ohms, connect the red (positive) lead to the "top" terminal where AC feed wire 44 was connected, and the black (negative) lead to the bridge's positive DC output terminal, where wire 46 was connected. You should read a very low resistance, near zero, since the diode should conduct with that polarity of voltage from the ohmmeter applied. Now reverse the meter leads, and you should read a very high resistance; your meter should show OL for "overload" since the diode should not be conducting.

        Next, do the same test for the top left diode, using terminals for wires 44 and 47. This time you should get conduction (low ohms) with the black lead on the top and the red lead on the left, and high ohms (OL) when you reverse them.

        Now, do the same test on the bottom two diodes, one meter lead on the AC terminal that was connected to wire 45, and check to wires 46 and 47 the same way you did for the top diodes. All in all, you should have taken 8 readings, one in each direction for each of the four diodes. Each diode must show conduction (low ohms) in one direction and no conduction (high ohms, OL) in the other. I think you are going to find at least one with low ohms both ways.

        If this isn't clear, ask some more questions!
        Last edited by Aeronca41; 08-25-2019, 02:44 PM.

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        • #5
          Well I used the diode test on the fluke and they all check out ... all take about half a volt to open and are closed tight when the leads are reversed.... so where to next ...

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          • #6
            So I checked what I believe to be the coil between the f7 and f8 fuses and it reads .3 ohms.... seems way low for a 1a fuse

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            • #7
              I just went from fuse base to fuse base with the ohmmeter to clarify

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              • #8
                As far as I can tell where my leads are hooked up on the fuse bases corresponds to the wire coming in from the top on the diagram( the ones that go to coil) when your holding the relay/contactor on it produces 42vac which is right ?? The diagram shows two 19v coils in series and my line voltage here is a decent bit higher than normal so that would account for the discrepancy?

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                • #9
                  Not sure what you mean by "the coil". If you mean the transformer secondary winding (wires 41 and 42), that's OK, because that is just the transformer winding that is providing voltage to the bridge. Next thing I would do is unplug plug PLG51 that connects to the PC1 board and see if the fuses still blow. If they don't, the problem is on the PC board or some input to it. If they do, you probably have either a shorted capacitor C14 or C15, Resistor R4 has gone way down in value (very unlikely, because it would have probably blown up by now), or maybe a shorted protection diode around the W contactor relay. You can check caps for shorts with your ohmmeter, and check those two diodes in the usual way. There could also be a wire shorting to ground somewhere around that circuit.

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                  • #10
                    Well I’ve been sitting here banging my head against the side panel with the diagram on it... I only have 2 fuses left and I’m not sure when I was gonna be able to make it back to kirby to get some more so I was gonna try and wait for the destructive testing as long as possible lol. And yes I meant the transformer... I apologize for my lack of accuracy in terms... I normally stick to the automotive electrical and most guys can barely even recognize symbols on a diagram in my field so that’s where I get it from. however I did check the resistance of the coil on the contactor and it was at 1.1 ohms does this sound low to you ? Or normal?

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                    • #11
                      Well I think I got her nailed down to a bad contactor magnet... I believe it was internally shorted causing the low resistance. Both fuses lived with it unplugged and plg51 still connected. But thanks a million for the help! And as long as you think that makes sense I’ll have one ordered Monday and hopefully a working intelliweld by next weekend !

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                      • #12
                        Fantastic! I don't think the DC resistance being that low is necessarily indicative of the problem, but it could be.If unplugging it solved the blowing fuses problem, that's likely the issue. You could also try checking the resistance between either one of the coil's end wires and the metal structure of the magnet itself. Could be the coil is shorted to the metal parts of the relay, which would certainly cause the fuse to blow.

                        But, there are a couple of other things I'd verify before ordering a new relay. First, make sure the two diodes near that relay coil are good. If either of them is shorted, it could also be causing the fuse to blow. Not knowing how you disconnected the relay, I don't know if the diodes are still in the circuit or not. If one or both of them was disconnected along with the relay coil, they could still be the problem. Also, there could be a wire shorted to ground somewhere in that circuit that is popping the fuses, and disconnecting the relay coil would also disconnect those wires from the fuses. Just to verify that, remove both fuses, then connect one lead of your ohmmeter to the chassis (a good connection on a bolt, or a location where you have scraped off the paint), and the other to wire 101 where it connects to the STOP button. Hold the STOP button down, and check the ohms reading. Unless I've missed something in the schematic just now, you should read an open (OL). If you get an ohms reading, let me know and I'll look harder, but I think you should be isolated from ground under those conditions.

                        However, my gut says you found the problem already. Good work!

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                        • #13
                          The way it’s wired when you unplug the magnet both diodes are still in the circuit and I have checked every thing for a short to ground and couldn’t find anything however I have not checked the stop circuit while being isolated like you described. However if I’m thinking correctly there’s 42 ish volts that should be going to that magnet and at 1 ohm of resistance that would mean 42 amps it’s capable of drawing (way high for the 1a fuse) ? The only thing is if there is a voltage drop before the magnet windings ? I didn’t see any resistors in there but I’m also not sure what the little circle marks were on the diagram. I guess I should’ve actually measured the voltage at the connecters for the magnet to

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                          • #14
                            I'd still verify both diodes with your ohmmeter, just to be sure.

                            Diode D1 is going to half-wave rectify the AC, so you are immediately at an average voltage of 42 x 2.828 divided by 2 times .637. Explanation: 42 volts is RMS; what you read on an AC volt meter. 42 volts RMS is multiplied times 2.828 (twice the square root of 2) to get peak to peak voltage, then cut in half by the half wave rectifier diode, then multiplied by .637 to get the average DC voltage at the coil. RMS values are irrelevant on pulsating
                            dc like you have here. That comes out to about 38 volts if I did the math correctly, not far from 42, but a little less. Since it is not filtered DC, the "hump-backed" waveform will generate some degree of reactance (which will be essentially added to the DC resistance) in the coil, limiting the current. However, the 1 ohm does sound a bit low for DC resistance. Take a look at the relay and see if there is a manufacturer name or number on it--someone made it for Miller. If you can find it, you should be able to look up the specs, including coil resistance, on the web.
                            Last edited by Aeronca41; 08-25-2019, 10:21 PM.

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                            • #15
                              I'm not sure what that was that went over my head, but it went over it. Yet I find it interesting just the same.

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