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Oscilloscope Settings to trace oscillation

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  • Chuck H
    replied
    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

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  • g1
    replied
    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.

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  • nickb
    replied
    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.

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  • ca7922303
    replied
    Would anyone be interested in walking me thru how to trace down the oscillation. Schematic link is in these lists. Not able to not have a fully functioning, half power, red plating amp. Appreciate the help.

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  • J M Fahey
    replied
    Short/trivial answer: do not use those "wrong" settings together.

    Amp apparently "works" on most settings, and sometimes 1 or 2 combinations are "problematic", probably because they lead to very high gain, high top boost, etc. , all together.

    Donīt think your amp is "broken" as in "it worked fine, a part broke, I replace it and we are back to normal" but "it is as it is".

    Solving that problem MOST probably will require redesigning an I am not only talking "circuit" redesigning but also include layout, such as "pass this wire here instead of there" or "ground here instead of there", maybe 1 inch away.

    Such things are well beyond the "repair" stage, are typically solved at the Factory, one reason year after year we have *different* amp versions.

    Again: I think the sane solution is to avoid that couple extreme settings, period.

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  • ca7922303
    replied
    Oscillation only occurs thru a speaker is when xpand or boost are engaged separately. When engaged together, no oscillation. When engaged together with modern and or bright switch, no oscillation occurs.

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  • ca7922303
    replied
    Modern/bright, xpand/boost engaged

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  • ca7922303
    replied
    Xpand engaged

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  • ca7922303
    replied
    Channel 3 with g,v,mv on 10, b,m,t on 0, boost switch engaged.

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  • ca7922303
    replied
    Channel 3 g,v,mv,b,Mt on 10, reverb on(0).

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  • ca7922303
    replied
    Channel 3 with g,v,mv on 10, b,m,t on 3 reverb(0).

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  • ca7922303
    replied
    Channel 2 with g,v,mv,b,m,t on 10.

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  • ca7922303
    replied
    Channel 2, same but with modern switch engaged.

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  • J M Fahey
    replied
    Originally posted by ca7922303 View Post

    Having driver issues with PC based oscilloscope, can we use signal generator and DMM on ac volts to trace?
    FIRST you must *see* oscillation or there is nothing to trace.

    We still donīt know what you have there.

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  • ca7922303
    replied
    Channel 2 b,t,m, drive/volume/master volume on 10. Reverb(0).

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