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Fender stage 100 oscillates with new output transistors

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  • Fender stage 100 oscillates with new output transistors

    It had a blown fuse, 2 shorted output transistors. TIP 142 and 147 darlingtons. I replaced all 6 transistors, and now it oscillates at about 750 kHz. Fun. I guess the new parts are higher gain or higher Ft.

    Anybody solved this before I go sprinkling capacitors all around the neighbourhood?
    making 63 and 66 T-bird pickups at ThunderBucker Ranch

  • #2
    Originally posted by marku52 View Post
    It had a blown fuse, 2 shorted output transistors. TIP 142 and 147 darlingtons. I replaced all 6 transistors, and now it oscillates at about 750 kHz. Fun. I guess the new parts are higher gain or higher Ft.

    Anybody solved this before I go sprinkling capacitors all around the neighbourhood?

    R104, R105, R106 or C48 open circuit?

    Comment


    • #3
      Fender Stage 100 Schematic...

      Update: Let me try a new file without the hyphen.
      Attached Files
      Last edited by TomCarlos; 02-16-2020, 07:19 PM.

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      • #4
        Originally posted by TomCarlos View Post
        Fender Stage 100 Schematic...
        Sorry, doesn't open here.
        - Own Opinions Only -

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        • #5
          Let me try a zip file...
          Attached Files

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          • #6
            Originally posted by Kevina View Post
            R104, R105, R106 or C48 open circuit?
            Thanks
            Interesting. Have you seen failures there? Looks like a zero in the FB loop.
            making 63 and 66 T-bird pickups at ThunderBucker Ranch

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            • #7
              Originally posted by marku52 View Post
              Thanks
              Interesting. Have you seen failures there? Looks like a zero in the FB loop.
              No, never seen one, but I spent my life remote diagnosing faults in the music industry.

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              • #8
                Originally posted by Kevina View Post
                No, never seen one, but I spent my life remote diagnosing faults in the music industry.
                Here's some more data. This amp has a weird output structure, the usual output node is grounded, and the speaker output actually is tied to the center tap of the power supply. I notice with the oscillation going, the power supply voltages are unbalanced (NPN side lower), but there is no unusual power consumption out of the AC mains.

                My theory of the nonce is that the NPN darlingtons are self-oscillating. Something I distantly remember from E school was that an emitter follower with inductance on the emitter gets converted into a negative R on the base. Leading to an oscillation in the NPNs alone (PNPs are usually slower). This output stage is complementary emitter follower. With inductive emitter resistors.

                I'm going to try adding some R to the bases of the NPNs, maybe 100 ohms or so.

                I'll report what happens.
                making 63 and 66 T-bird pickups at ThunderBucker Ranch

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                • #9
                  That is a fairly common arrangement. Look up "flying rail" amplifiers.
                  Education is what you're left with after you have forgotten what you have learned.

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                  • #10
                    Is there any point to it other than "We could do it so we did!"? I've seen it used where you could expand the output range of an op amp by flying output transistors on either side of the op amp rails.

                    But this......Why bother?
                    making 63 and 66 T-bird pickups at ThunderBucker Ranch

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                    • #11
                      I don't see it as a why bother. Imagine a battery circuit. Say a wireless transmitter or even a transistor radio. it works fine, but has no ground connection. it has a "ground" inside, but that is really a circuit common, not a ground.

                      Now imagine a solid state amp. It has power supplies with a common. You have output stages connected to that power supply, and it gathers into a common, a common we call the output bus. The speaker load connects between those two commons, power supply and output. If we built that on a wooden plank, it would work even without a real ground.

                      Well we DO have a real ground, so we can connect one end of the speaker to it, or the other. We are used to connecting the power supply to it, but we can just as easily do it the other way, the way this amp is wired. otherwise the circuits are darn similar.
                      Education is what you're left with after you have forgotten what you have learned.

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                      • #12
                        I believe one of the advantages is being able to run the front end of the power amp off the low voltage supplies. I think there are others but of course there are trade offs as well.

                        edit: with a bit of different implementation, this design allows all collectors to be directly bolted to heatsink ("grounded").
                        Last edited by g1; 02-19-2020, 12:31 AM.
                        Originally posted by Enzo
                        I have a sign in my shop that says, "Never think up reasons not to check something."


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