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Oscilloscope Settings to trace oscillation

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  • #31
    The link given for the schematic doesn't work. It just takes you to your personal adobe area . How about a model type /number. "Laney 2010 50w" doesn't give much away.

    Do you actually need an input signal to get it to oscillate?

    Being from 2010 it's probably PCB based so not easy to more wires around and, as has been mentioned, may be inherent to the model. In other words if the factory couldn't fix it then your in for a rough time. It happens as a signal from one place ( usually the output tubes) is getting back to an earlier place. It can there by capacitive coupling i.e. wires, traces or components hear each other, inductive where you have circuit loops near each other or by a common impedance that causes current flowing in one circuit to affect the operation off another unintentionally. Often it's a combination.

    A pictures of the works will help.

    You can't really trace it with a scope, and every PC scope I've ever used has been garbage in any case. But you can use it to monitor the output so you know if it's happening or not. Start by identifying which parts of the circuit are involved. You can often do that by adjusting controls. Once the area of search is reduced you can try putting small capacitors (say 100pF) to ground in various key places to see how it affects the oscillation. If there is an effect (on the oscillation) then that part of the circuit is involved. Once you done that you have to figure out how the output manages to couple to it and see if you can do anything about it.

    If you can't fix the coupling then you have to work to lower the gain ( actually loop gain) to prevent the oscillation. A common method is a capacitor between the phase splitter's plates. Maybe a cap to ground will do it. The problem with that is it might adversely affect the high frequency response and change the tone in a way you don't like. Make sure the wires from the output tube's plates are twisted together and as far away from any other part of the circuit as possible.

    Or, if it only happens on extreme settings, then just make peace with the fact that that is just the way it is and don't that.

    Good luck!
    Last edited by nickb; 03-12-2022, 06:24 PM.
    Experience is something you get, just after you really needed it.

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    • #32
      Originally posted by nickb View Post
      Once the area of search is reduced you can try putting small capacitors (say 100pF) to ground in various key places to see how it affects the oscillation.
      Nick, the amp is a Laney TT50H. The presence and damping circuits are not working and I believe they could be related or causing the oscillation so suggest fixing those faults (and any others) first.
      If you have any other comments (regarding the repair), could you please go to the laney thread here: https://music-electronics-forum.com/...-outout/page25
      Last edited by g1; 03-13-2022, 03:45 AM.
      "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

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      • #33
        I am closing this thread because it's redundant. g1 has a link to the original thread about this matter just above. But here it is again so that it's at the end of this thread.

        https://music-electronics-forum.com/...-outout/page25
        "Take two placebos, works twice as well." Enzo

        "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

        "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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