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Electro-Harmonix LPB-2 True Bypass Mod

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  • Electro-Harmonix LPB-2 True Bypass Mod

    Hi everyone,

    I've been attempting to convert a EHX LPB-2 power booster from the 70s to true bypass, and add a DC jack. I was hoping this would be relatively simple, since the circuit is very basic (schematic attached).

    This is the layout before I changed anything. The red and black battery cables were actually switched from what they are supposed to be, as far as I can tell.


    Click image for larger version  Name:	IMG_8347.png Views:	0 Size:	1.11 MB ID:	921596

    Before the mod, there was a clear volume boost, and the amp started to distort even at low volume.

    After attempting the mod, the boost seems to have gone, with the potentiometer all the way up it just about reaches bypass volume. Could someone point me in the right direction, from the photos?



    Click image for larger version  Name:	IMG_8357.png Views:	0 Size:	3.16 MB ID:	921589 Click image for larger version  Name:	IMG_8358.png Views:	0 Size:	2.21 MB ID:	921590 Click image for larger version  Name:	IMG_8359.png Views:	0 Size:	2.00 MB ID:	921592
    Attached Files
    Last edited by clarisso11; 01-02-2021, 07:42 PM.

  • #2
    Since my last post, I did a bunch more trouble-shooting:
    • Re-wired footswitch in two different ways
    • Added LED, checked with and without LED wired in
    • Switched some ground connections
    • Changed 3PDT switch with a different, brand new one
    • Tested on battery alone, and DC alone

    The result is always the same: The boost is gone, all I get when engaged is the bypass volume, and only if the potentiometer is fully cranked.

    I guess my questions now are:
    1. Could the 3PDT switch be somehow reducing the power supply to the transistor?
    2. Are there any parts that need to be added, when switching to 3PDT?

    Here is the latest wiring:

    Click image for larger version  Name:	IMG_1867.JPG Views:	0 Size:	2.27 MB ID:	921637 Click image for larger version  Name:	IMG_1868.JPG Views:	0 Size:	2.29 MB ID:	921638

    Comment


    • #3
      Originally posted by clarisso11 View Post

      The red and black battery cables were actually switched from what they are supposed to be, as far as I can tell.
      Seems the positive battery wire was connected to ground.

      If it worked like that your unit might have a PNP transistor. Can you identify the type? Please measure transistor voltages.

      What's the purpose of the slide switch?

      - Own Opinions Only -

      Comment


      • #4
        Originally posted by Helmholtz View Post

        Seems the positive battery wire was connected to ground.

        If it worked like that your unit might have a PNP transistor. Can you identify the type? Please measure transistor voltages.

        What's the purpose of the slide switch?
        Good point! The switch turns off power supply to the circuit, I'm guessing to preserve battery.
        The transistor is a BC239, here are the voltages:

        Click image for larger version  Name:	transistor_voltages.jpg Views:	0 Size:	637.0 KB ID:	921645

        I think you're on to something here. Would the circuit even have working with the battery contacts the other way around? I just assumed they were mistakingly switched, since I saw photos of the same pedal with the opposite orientation.

        Comment


        • #5
          Ok so put it back the way it was ...Does it still work?
          BC239 is a NPN.
          nosaj

          Just for arguments sake Why do you want true bypass and do you understand the reasons for it?

          What's the difference between true bypass and buffered pedals? Which type is better for me? We pit true bypass vs buffered pedals and find out.
          Binkie McFartnuggets‏:If we really wanted to know the meaning of life we would have fed Stephen Hawking shrooms a long time ago.

          Comment


          • #6
            Binkie McFartnuggets‏:If we really wanted to know the meaning of life we would have fed Stephen Hawking shrooms a long time ago.

            Comment


            • #7
              https://alltransistors.com/adv/pdfda...c238_bc239.pdf

              I interprete your measurements as positive voltages wrt battery minus. So I see C: +3.98V, B: +4.58V, E: 8.8V, correct?
              Emitter voltage being the highest value makes sense if ground is at +9V, but the difference between base and emitter voltages should be only 0.6V. Could mean a damaged transistor.

              Voltages don't look right to me and the schematic's values seem to be different, so I suggest you draw your own schematic.
              - Own Opinions Only -

              Comment


              • #8
                I wired it up positive ground, and it's working again! Looks like these circuits went out in both configurations, I presume depending on whatever transistor they were using at the time.

                Sure hope I didn't damage the transistor by wiring it up the other way!

                Originally posted by nosaj View Post
                Just for arguments sake Why do you want true bypass and do you understand the reasons for it?
                The original switch seemed a bit dodgy and I had a good quality 3PDT lying around, so I figured why not. I'm don't think it makes much of a differenc.

                Comment

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