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MM251 No digital display and fan runs constantly

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  • #46
    Do those display modules pop out? I have worked on an Extreme 12VS where they were soldered through the black standoffs to the board, and remember a guy with a Syncrowave 250 who could just pop them out. That way you can test them separately, and check underneath for broken leads before replacing Q3. If the LEDs are inoperational most any common anode 7 segment display module would work, but it may be worth replacing all six. It's hard to match the appearance - some displays have slanted numbers, some have rounded corners, and you usually can't just buy one anyways.

    D2 is a zener diode, probably a 1N4733A because it needs to clamp the voltage on that input line to a safe 5V for the micro input. The body of the diode has the numbers on it. I can see the "1N" and "47" on D2, so the last three digits should be just visible if you look at the side of D2. If not, D3 is the same diode. What do you get when you measure resistance or R2 in the opposite direction of how you measured it before?
    C5 is just a little monolithic ceramic filter capacitor, either 0.1uf (if you see a 104 on any of those capacitors) or 0.01uf (if you see a 103 on it). Either a 35V or 50V one should work for that line.

    Q3 is probably a 2N3906, but you should be able to make out it's component number on the flat face. It sometimes takes scratching the conformal coating off with your fingernail to read it.

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    • #47
      Originally posted by jjohn76 View Post
      Do those display modules pop out? I have worked on an Extreme 12VS where they were soldered through the black standoffs to the board, and remember a guy with a Syncrowave 250 who could just pop them out. That way you can test them separately, and check underneath for broken leads before replacing Q3. If the LEDs are inoperational most any common anode 7 segment display module would work, but it may be worth replacing all six. It's hard to match the appearance - some displays have slanted numbers, some have rounded corners, and you usually can't just buy one anyways.

      D2 is a zener diode, probably a 1N4733A because it needs to clamp the voltage on that input line to a safe 5V for the micro input. The body of the diode has the numbers on it. I can see the "1N" and "47" on D2, so the last three digits should be just visible if you look at the side of D2. If not, D3 is the same diode. What do you get when you measure resistance or R2 in the opposite direction of how you measured it before?
      C5 is just a little monolithic ceramic filter capacitor, either 0.1uf (if you see a 104 on any of those capacitors) or 0.01uf (if you see a 103 on it). Either a 35V or 50V one should work for that line.

      Q3 is probably a 2N3906, but you should be able to make out it's component number on the flat face. It sometimes takes scratching the conformal coating off with your fingernail to read it.
      As best I can tell, the modules don't pop out. I pulled on them and found no sign of movement.

      When I measure R2 one way I get 6.76K Ohms and 6.70 the other way. When I measure R3 the same way I get 35.54K and 39.17K Ohms.

      D2 and D3 are marked 1N4732A. That appears easy to find.

      C5 is marked BC 019 and nothing else. After spending some time on Mouser I have it narrowed down, but can't tell whether it's 0.1uf or 0.01uf. All the others on the board have different numbers on them.

      Q3 has multiple lines of text on it. It's got a big F (Fairchild, I'm guessing) then D117 on the top line, then TN on the middle line and 2907A on the bottom line. My searches show TN2907A as obsolete, and I haven't found a cross-reference yet. Doing that sort of search makes my head hurt...lol.

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      • #48
        Were you able to get every light segment to light up if you power directly with your diode tester? You should have a capacitance tester on your multimeter. It looks like that transistor is PNP in a TO-226 package, needs at least 60V and 800mA continuous current.

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        • #49
          Originally posted by jjohn76 View Post
          Were you able to get every light segment to light up if you power directly with your diode tester? You should have a capacitance tester on your multimeter. It looks like that transistor is PNP in a TO-226 package, needs at least 60V and 800mA continuous current.
          No, only some segments would light up. Some were bright and normal, some were dim, and some were dead.

          Good point about checking the capacitor with the multimeter...I'm pretty sure I'll have to unsolder one to check, but that's not a big deal.

          I'll do some more digging on the transistor. Thanks,

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          • #50
            The dim ones would lead me to believe the current is going somewhere else. What resistance do you get across those segments in both directions? It's probably worth checking before pulling those modules. If they're like the ones on the Extreme 12VS, the leads to those led modules went through the black standoffs and soldered directly to the board. I could not find replacements with that length of leads, so I replaced them with the black female header that I soldered into the board and plugged the modules into the socket. I glued the modules into the Extreme 12VS because of the shocks that suitcase feeder experiences, but would think you'd be ok with them just plugged in.

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            • #51
              Originally posted by jjohn76 View Post
              The dim ones would lead me to believe the current is going somewhere else. What resistance do you get across those segments in both directions? It's probably worth checking before pulling those modules. If they're like the ones on the Extreme 12VS, the leads to those led modules went through the black standoffs and soldered directly to the board. I could not find replacements with that length of leads, so I replaced them with the black female header that I soldered into the board and plugged the modules into the socket. I glued the modules into the Extreme 12VS because of the shocks that suitcase feeder experiences, but would think you'd be ok with them just plugged in.
              The sections that don't light up give me around 400K Ohms. The sections that are dim give me around 1M Ohm. The sections that light up properly are all 1.4M Ohms. When I reverse the leads I get Open on all of them. I checked the good display board and got 1.4M Ohms on all sections and Open when reversed.

              The displays go through the black standoff and then are soldered to the board as best I can tell.

              I've done more searches and posted a thread on another site and one person said the conditions described are common following a lightning strike, or when the machine was connected to higher voltage than it was set for. With the way the display board voltage regulator cracked and failed, along with the varistor being charcoal, I think that's pretty likely.

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              • #52
                Yeah, some type of surge makes sense. The display board definitely experienced some type of overvoltage. If the microcontroller is fried, it's tough to replace without a programmer and the original software.

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                • #53
                  Originally posted by jjohn76 View Post
                  Yeah, some type of surge makes sense. The display board definitely experienced some type of overvoltage. If the microcontroller is fried, it's tough to replace without a programmer and the original software.
                  I had to buy a couple of other parts online so I decided it made sense to just buy a new display board which put me into the free shipping range. The new board showed up yesterday and it works perfectly. I haven't completely given up trying to fix the bad board, but wanted to get the machine back in one piece and useable.

                  Thanks for all the help! If I manage to get the bad board fixed I'll post an update.

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                  • #54
                    Sounds good! If you mess around with Arduino micrcontroller kits, you could see what's communicated from the main board to the display board and set up your own tester for it, but that gets you even further away from welding with your time...

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