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  • 760 troubleshoot

    A few years ago, my dad picked up a Leslie 760 he found for cheap but never worked correctly. I decided I wanted to start gigging with it so I grabbed it from my parents house. There are a few problems:
    The first I think is with the 9-pin Dr. Fishsticks controller, the footswitch will turn the rotors from off to fast, but won’t turn it to slow or back off. Neither the fishsticks or the footswitch look like they have any bad connections with the soldering though...

    The second is that the treble horn isn’t producing any noise. When I opened up the amplifier I noticed that the speaker wire wasn’t even plugged in to the circuit board, but when I plugged it in nothing changed. When I switched them and put the treble speaker cord in to the bass circuit board, I could hear the treble but not the bass rotor. Any ideas what my issue could be? I looked at the schematics and some pictures of other 760 amplifiers, and strangely enough the circuit boards look slightly different. I’ve added a few photos of the amplifier... I can try to get clearer and brighter ones tomorrow if needed....
    Attached Files

  • #2
    Originally posted by Pomcq View Post
    Any ideas what my issue could be?
    Consider this analogy. You talk to an auto mechanic and say, "My car won't start. Any ideas what's wrong with it?" I could throw out a lot of guesses. One of them might be correct, but the way to get a diagnosis is by narrowing down the problem -- except for the occasions where something is clearly visible to the naked eye.

    On earlier 760s, the treble boards and bass boards are different and have different interstage transformers coupling the driver to the output transistors, so that's normal.

    The operation of the Fishsticks controllers can be confusing. Try hitting repeated combinations of the buttons to see if, maybe, you get the slow motors to come on. If not, there may be a problem in the motor control circuit.

    My personal experience as a professional technician is that 760s tend to need a lot of work at this point in history to be put back into reliable working order. That includes motor stack service and circuit board parts replacement. I know that's probably not what you want to hear, so I apologize for being the bearer of that news.
    I'm David. 'Dave' is someone else's name.

    Comment


    • #3
      Originally posted by David Anderson View Post
      Consider this analogy. You talk to an auto mechanic and say, "My car won't start. Any ideas what's wrong with it?" I could throw out a lot of guesses. One of them might be correct, but the way to get a diagnosis is by narrowing down the problem -- except for the occasions where something is clearly visible to the naked eye.

      On earlier 760s, the treble boards and bass boards are different and have different interstage transformers coupling the driver to the output transistors, so that's normal.

      The operation of the Fishsticks controllers can be confusing. Try hitting repeated combinations of the buttons to see if, maybe, you get the slow motors to come on. If not, there may be a problem in the motor control circuit.

      My personal experience as a professional technician is that 760s tend to need a lot of work at this point in history to be put back into reliable working order. That includes motor stack service and circuit board parts replacement. I know that's probably not what you want to hear, so I apologize for being the bearer of that news.
      Thanks for the response. I was worried that my question was maybe too general, but thought it couldn’t hurt to ask if anyone has had similar issues with the treble circuit board. Anyway I’m clearly in over my head so I should probably just take it to the tech...

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      • #4
        It wouldn't hurt to check the horn driver by hooking it to a different amp. Try to keep any test signals above 800Hz and don't drive it too hard. I usually sweep them with a test signal at varying frequencies to check for buzzes. The diaphragms sometimes need to be re-centered.

        The motor controls are handled via transistors that control relays that control Triacs or SCRs, depending on the vintage. They're not too hard to fix if you understand the circuit as shown on the schematic.

        But it's a bit beyond the scope of what I have time to do on here to take you through a full diagnostic procedure.
        I'm David. 'Dave' is someone else's name.

        Comment


        • #5
          Originally posted by David Anderson View Post

          My personal experience as a professional technician is that 760s tend to need a lot of work at this point in history to be put back into reliable working order. That includes motor stack service and circuit board parts replacement. I know that's probably not what you want to hear, so I apologize for being the bearer of that news.
          Amen to that brother!

          Ive redone about four 760s and an 825 and every single one has had problems in the PSU in the switching circuit - either no fast or no slow or both trying to run at once. It took me a long time to learn how to fix them and would suggest you take it to a professional for a service if you want a reliable gigger.

          The earlier yellow boards are prone to shedding traces as well. I have had to do some point to point wiring on one of these to get full function back.

          Best
          Dave
          Hammond C3, M102, XB3, XB5, X5
          Lowrey Heritage DSO-1, H25-3, Yamaha E70
          Farfisa Compact Duo Mk2, Vox Continental 300, Korg BX3 Mk1, Leslie 122, 145, 910, 415
          www.drawbardave.co.uk

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