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MM251 No digital display and fan runs constantly

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  • MM251 No digital display and fan runs constantly

    Hey all,

    I picked up a MM251 (serial number LB170736) with no history on it and it's doing a couple of things unusual. As soon as you power it up, the fan comes on and stays on. The digital displays are completely blank all the time (powered on, and while striking an arc). It seems to weld fine and both the voltage and wire speed knobs adjust those two settings as you would expect....I intentionally tried things like having a high voltage with low wire feed, got sputtering and was able to adjust the wire speed up until I got a nice sizzle, etc. I checked the OCV with it powered on and the voltage knob maxed out and got a touch over 40VDC between the positive and negative terminals by the wire feed.

    I have two other 251s that both work perfectly, but figured I'd post here before I start pulling parts to test things out. Any input is appreciated!

    Bart

  • #2
    Probably a bad display. The fan is wired to come on as soon as the unit is powered up according to the schematic I'm looking at.

    https://www.red-d-arc.com/pdf/Miller...s%20Manual.pdf

    Comment


    • #3
      CR1 controls the fan on demand. Could be a fused relay. Someone may have wired the fan to stay on. I check wiring, especially at CR1. Could be a thermostat.
      As far as display, if it welds fine I wouldn't worry. But if it bugs you replace it.

      Comment


      • #4
        A bit of an update. I found another thread where someone had a display that wasn't working. I followed that advice and found there was 35V going to the display board, and at that point I pulled the display from my other 251, hooked it up and it worked just fine and responded to dial changes. So, the display board needs to be repaired/replaced.

        I also found a few threads about people with machines with dead fans, and used some of that troubleshooting along with the wiring diagram and found at least one problem. I had it powered up, fan running, and saw a bit of smoke come off the varistor attached to CR-1. I shut down the machine and got a good look at the varistor...just touching it showed it's charcoal! I turned the machine back on, and no fan for a few seconds, then it kicked on and then a puff of smoke from the varistor before I shut it down.

        Replacing the varistor should be no problem, but it makes me think that something else was wrong. The thermistor should dictate when the fan comes on, not the varistor, unless I'm missing something obvious. I can't find any evidence the wiring was changed to make the fan run continuously but I'm going to spend a little more time with the other 251 next to it so I can look back and forth between them.

        I'd appreciate any thoughts...

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        • #5
          Originally posted by 1997CST View Post
          CR1 controls the fan on demand. Could be a fused relay. Someone may have wired the fan to stay on. I check wiring, especially at CR1. Could be a thermostat.
          As far as display, if it welds fine I wouldn't worry. But if it bugs you replace it.
          Thanks...that's what I'm trying to work through now. This is a project for a buddy who loves my 251 and asked me to find one for him, so I'm not keeping it, but want to make sure it's right.

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          • #6
            Another update...I disconnected the thermistor on both welders, checked resistance and got similar figures for both then pointed a heat gun at each thermistor and the resistance dropped similar amounts.

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            • #7
              It could be Q7 on your main board. If you can find it, measure the resistance across it's outside terminals (there should be three in line or close to it). It should be open. Also check the resistance from the middle leg to board ground. It should not be open. I could probably walk you through troubleshooting the display board if you want to try to fix it yourself.

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              • #8
                Originally posted by jjohn76 View Post
                It could be Q7 on your main board. If you can find it, measure the resistance across it's outside terminals (there should be three in line or close to it). It should be open. Also check the resistance from the middle leg to board ground. It should not be open. I could probably walk you through troubleshooting the display board if you want to try to fix it yourself.
                Thanks! I spent some time with a multimeter and the display board yesterday. I found one resistor that was way low (reading 6K instead of 39.2K) but since it was still attached to the board I'm not sure it's bad. While I was using a magnifying glass to read the markings on resistors I did a general look around with the magnifying glass and saw that the top of the step down switching regulator had a crack, and then saw that the first pin was melted...crack led right to that pin. That pin traces back to the 35V input pin so there's no way the board will work like that.

                I have a new step down regulator on the way along with a couple of resistors (just in case).

                I also have a replacement varistor for CR-1 on the way. Now I'm hoping I just have to figure out why the fan runs constantly since it appears the thermistor is working properly.

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                • #9
                  Q7 is the transistor on the main PCB that takes a signal directly from the microcontroller (through resistor R96) to turn on your fan. It is right by connector RC8. It takes an active signal (+5V) from your microcontroller to turn it on, and since everything else works, I wouldn't think it is your microcontroller. If you do the check above with the power off, you should know whether or not that is it. If you don't get anything conclusive, you can measure the voltage on R96. Just find which side is connected to the middle pin on Q7 and check the voltage on the opposite side (this should be connected to pin 23 on the microcontroller). If you have 5V on the microcontroller side of R96, then it's probably your thermal sensing circuit. If it's 0V, then it's probably Q7, which costs a couple bucks to replace.

                  If it's the thermal circuit, it's probably worth checking the resistance from pin 10 on A2 to board ground (the middle pin on VR1). It should be equal to your thermistor's resistance. Also, it's worth checking resistance from A2 pin 10 to +5V (the right pin on VR1).

                  Hope this helps.
                  Last edited by jjohn76; 01-02-2020, 09:34 AM.

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                  • #10
                    What resistance do you measure on the output of VR1 (pins 2 and 5)? Also, what resistor is showing a low resistance? Either one of the transistors or diodes sounds like it is toast if you're getting a low reading on a resistor in circuit because I don't see any resistors in parallel on the schematic.

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                    • #11
                      You guys are making my head spin. All this smart stuff....I had a conversation with a guy at the track this morning how we’re not teaching the kids how to do regular ole arithmetic or to write in cursive anymore. Here you guys are fixing, by hand, something that a machine in China made. I think you guys kick the proverbial ass. Good on ya.

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                      • #12
                        Originally posted by jjohn76 View Post
                        What resistance do you measure on the output of VR1 (pins 2 and 5)? Also, what resistor is showing a low resistance? Either one of the transistors or diodes sounds like it is toast if you're getting a low reading on a resistor in circuit because I don't see any resistors in parallel on the schematic.
                        Thanks...all of this helps. I'll follow your advice above to test Q7 etc tonight when I get home.

                        I'll have to check the resistance between 2 and 5 on VR1, but pin 1 on VR1 is melted, and there is a slight crack across the top of VR1 from the pin 1 corner, so it's toast.

                        The resistor giving me a low reading is R2. I got 6.35K Ohms which should have been 39.2K. R3 which is marked the same gave me 39.2K. I checked all the other resistors and they were within specs.

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                        • #13
                          Checking between pin 2 and 5 just confirms whether or not a short caused the voltage regulator to go. It might be worth using an old USB outlet charger, if you have a spare.one you can sacrifice, to power up that board and check voltages before soldering in a new vr1. I have more USB wall chargers than projects for them, and they give out a good current limited 5V.

                          I don't know what to say about the difference between R2 and R3, those are part of the communication circuit with your main board, and R2 is on the input side. Please check to see what you get across R4. If it's the same measurement, there could be an issue with your microcontroller. Pin 2 on your microcontroller should not be connected to ground, which would be the case if R2 and R4 read the same.

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                          • #14
                            Originally posted by jjohn76 View Post
                            Checking between pin 2 and 5 just confirms whether or not a short caused the voltage regulator to go. It might be worth using an old USB outlet charger, if you have a spare.one you can sacrifice, to power up that board and check voltages before soldering in a new vr1. I have more USB wall chargers than projects for them, and they give out a good current limited 5V.

                            I don't know what to say about the difference between R2 and R3, those are part of the communication circuit with your main board, and R2 is on the input side. Please check to see what you get across R4. If it's the same measurement, there could be an issue with your microcontroller. Pin 2 on your microcontroller should not be connected to ground, which would be the case if R2 and R4 read the same.

                            I got 2.6 Ohms between pins 2 and 5 on VR1.

                            R4 was 504.6 Ohms and it's marked as a 500 Ohm resistor. I checked R6 as well, and got 509.6 Ohms...it's marked the same as R4.

                            I went through all the resistors on the board and got numbers that matched up with their markings except for R2.

                            I have a spare 5V USB charger. Where would I put the leads to power the board?
                            Last edited by G-ManBart; 01-02-2020, 10:03 PM.

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                            • #15
                              Originally posted by jjohn76 View Post
                              Q7 is the transistor on the main PCB that takes a signal directly from the microcontroller (through resistor R96) to turn on your fan. It is right by connector RC8. It takes an active signal (+5V) from your microcontroller to turn it on, and since everything else works, I wouldn't think it is your microcontroller. If you do the check above with the power off, you should know whether or not that is it. If you don't get anything conclusive, you can measure the voltage on R96. Just find which side is connected to the middle pin on Q7 and check the voltage on the opposite side (this should be connected to pin 23 on the microcontroller). If you have 5V on the microcontroller side of R96, then it's probably your thermal sensing circuit. If it's 0V, then it's probably Q7, which costs a couple bucks to replace.

                              If it's the thermal circuit, it's probably worth checking the resistance from pin 10 on A2 to board ground (the middle pin on VR1). It should be equal to your thermistor's resistance. Also, it's worth checking resistance from A2 pin 10 to +5V (the right pin on VR1).

                              Hope this helps.
                              The main board has me pretty confused. I was having a hard time getting readings off Q7 because of the sealer they use so I flipped the board over and put the test leads on the outside pins where they protrude out the bottom. I got 420K Ohms with the leads one way and 282K Ohms with the leads the other reversed (accidentally noticed this). From the middle leg to board ground I got 1.9K Ohms.

                              I reinstalled the board, powered up the welder and got 4.99V between R96 and board ground. On a related note, there was about a five or ten second delay where the fan didn't start after I turned on the power. I wasn't fast enough to check voltage on R96 before the fan started, but as soon as the fan started the varistor on VR1 started to smoke.

                              Prior to checking voltage I checked resistance from pin 10 on A2 to board ground and got 17.53K Ohms.

                              I also checked resistance from A2 pin 10 and got 15.81K Ohms.

                              Needless to say, it seems like something doesn't add up!

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