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Boss HM-2 Issue

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  • Boss HM-2 Issue

    I have a Boss HM-2 that powers up but doesn't pass any signal in bypass or on. I've probed the IC's and they seem to functioning properly. I traced back to the transistor at Q3 to the last point of signal. I've replaced Q3 though, and it still doesn't work. I'm getting nearly 9V to the center pin of the transistor, so it doesn't seem to be an issue there. I do have another working HM-2 that I've used as a basis, and the only difference I can find at Q3 is the working pedal has voltage along with signal on the collector and emitter sides of the transistor (about 3+V) while the broken one does not. Am I way off base here? What could be causing no sound?

  • #2
    Maybe Q3 is bust?
    What forward and reverse voltages do you get when testing the b-e and b-c junctions, on Q3 of each pedal?
    My band:-


    • #3
      Schematic? I have no idea where all the Qs might be.
      Education is what you're left with after you have forgotten what you have learned.


      • #4
        Click image for larger version

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        • #5
          Signal comes out of the volume board through C32, then through the FET Q10, then through C7 to the base of Q3. Do you have signal anywhere on that path? If so, where does it get lost?
          "Yeah, well, you know, that's just, like, your opinion, man."


          • #6
            Did you replace Q3 with a 2SC732? Or did you sub it. Subbing is OK, but different types have different pinouts. "Center pin"? The pinout of a typical 2SC small transistor is ECB across the front, looking at the flat face. The schematic shows it as a emitter follower, so the collector is normally at V+, 9v here. TO me that is the center pin.

            The base should have about +4.5vDC on it, plus any signal. That should be the right leg. In any case, that 4.5v reference is used by many parts in the unit. Look left center right by the power jack. That is a simple voltage divider that produces that 4.5v. The emitter of Q3 should sit at a few volts, and have the signal. C3 blocks that DC but passes the signal to the output.

            If you have about 3v on two legs of Q3, I suspect you are calling the wrong names for each leg.

            Does your signal look OK on both sides of Q10?
            Education is what you're left with after you have forgotten what you have learned.


            • #7
              Ah that's my fault when I was referring to the E and C pins. Q10 was problematic with the signal, but also the voltage was incorrect. I went back to the area of the pedal with the 4.5v production, and I was getting the 9v on the one side but not the 4.5v on the other. I replaced R5 and R6 which were reading out of spec with no success. I then pulled C25 and it was blown. Upon putting in a new capacitor, the 4.5v was restored to the pedal and the pedal worked again. Thanks for the input on the transistor needing the 4.5v on E-B. All good now!


              • #8
                Originally posted by EmmaQ
                I also replaced R5 and R6 and it helped me... But your way of solving the problem is also good)
                What was the fault with your unit?
                "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey


                • #9
                  I smell a bot, check other posts by same.
                  Education is what you're left with after you have forgotten what you have learned.