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    ST-120 Audio Circuit = floating ground?

    dalemurray
    dalemurray

    Posts : 42
    Join date : 2018-09-25
    Location : Wheaton Illinois

    ST-120 Audio Circuit = floating ground? Empty ST-120 Audio Circuit = floating ground?

    Post by dalemurray on Mon Feb 10, 2020 11:55 pm

    I would like to verify if the st-120 is properly assembled, the output stage(speaker terminal), if not the entire audio circuit, is not earth ground reference?

    If correct, I should be able to take measurements using an earth ground referenced oscilloscope at the speaker terminals without fear of blowing up the amp or scope?

    Input source = function generator
    Output = 100 watt 8ohm dummy load

    I'm at 99% on the statements above but that last 1% gives me the heebee-jeebees.
    I have not probed the circuit to determine if this is true, yet. (Unplugged, of course)
    I do not have a proper isolation transformer, thus my question.

    Bob Latino
    Bob Latino
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    Location : Massachusetts

    ST-120 Audio Circuit = floating ground? Empty Re: ST-120 Audio Circuit = floating ground?

    Post by Bob Latino on Tue Feb 11, 2020 8:32 am

    dalemurray wrote:I would like to verify if the st-120 is properly assembled, the output stage(speaker terminal), if not the entire audio circuit, is not earth ground reference?

    If correct, I should be able to take measurements using an earth ground referenced oscilloscope at the speaker terminals without fear of blowing up the amp or scope?

    Input source = function generator
    Output = 100 watt 8ohm dummy load

    I'm at 99% on the statements above but that last 1% gives me the heebee-jeebees.
    I have not probed the circuit to determine if this is true, yet. (Unplugged, of course)
    I do not have a proper isolation transformer, thus my question.


    Each of the two output transformer secondaries is "star grounded" directly to the chassis. There is a wire that goes directly from the black binding post on each channel to one of the two ground lugs that is next to the quad cap.

    Bob
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    StevieRay

    Posts : 54
    Join date : 2009-01-09
    Age : 57
    Location : Central VA

    ST-120 Audio Circuit = floating ground? Empty Re: ST-120 Audio Circuit = floating ground?

    Post by StevieRay on Tue Feb 11, 2020 9:49 am

    I haven't had to deal with this for years, since I now use a Fluke Scopemeter that is not tied to earth in any way -- battery powered scopes are always better.

    In your case, since the amplifier uses a transformer for all voltages -- in other words, nothing is coming straight from the power plug; everything is isolated -- you are OK for temporary actions.....

    Just to be sure, and in this one case, get a cheater plug that will disconnect the ground on the scope plug.  This will keep anything from being earthed, which is probably more important to prevent ground loops rather than keep anything from blowing up.

    Use the cheater ONLY when needed.
    dalemurray
    dalemurray

    Posts : 42
    Join date : 2018-09-25
    Location : Wheaton Illinois

    ST-120 Audio Circuit = floating ground? Empty Re: ST-120 Audio Circuit = floating ground?

    Post by dalemurray on Tue Feb 11, 2020 9:53 am

    Bob Latino wrote:Each of the two output transformer secondaries is "star grounded" directly to the chassis. There is a wire that goes directly from the black binding post on each channel to one of the two ground lugs that is next to the quad cap.

    Bob

    Correct me if my statement below is not true.
    The transformer is effectively an isolation transformer for the entire audio path - it does not provide a path to earth ground/neutral wire for the chassis or components; the only component tied to earth ground/neutral wire is the internal components of the transformer itself.

    Like I said, I was at 99% certain and now 99.5% certain this is the case.
    dalemurray
    dalemurray

    Posts : 42
    Join date : 2018-09-25
    Location : Wheaton Illinois

    ST-120 Audio Circuit = floating ground? Empty Re: ST-120 Audio Circuit = floating ground?

    Post by dalemurray on Tue Feb 11, 2020 9:59 am

    StevieRay wrote:I haven't had to deal with this for years, since I now use a Fluke Scopemeter that is not tied to earth in any way -- battery powered scopes are always better.

    In your case, since the amplifier uses a transformer for all voltages -- in other words, nothing is coming straight from the power plug; everything is isolated -- you are OK for temporary actions.....

    Just to be sure, and in this one case, get a cheater plug that will disconnect the ground on the scope plug.  This will keep anything from being earthed, which is probably more important to prevent ground loops rather than keep anything from blowing up.

    Use the cheater ONLY when needed.

    I appreciate the feedback.

    Code where I live requires conduit for all electrical thus all outlets have earth ground and neutral ties to earth at the box. In this case, I do not believe a cheater would buy me any safety.

    I suppose I could connect my scope to my UPS and unplug the UPS from the wall, thus scope would be free of earth ground. Issue is the UPS provides a modified sine output, not sure if that would dink the readings or mess with the scopes power supply. I imagine it would not but not sure; eevblog may be able to provide that answer.

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