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    Silencing the Lamb (ST70)

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    Drugolf

    Posts : 25
    Join date : 2014-12-29

    Silencing the Lamb (ST70)

    Post by Drugolf on Tue Jan 27, 2015 9:23 am

    Sorry, had to use a creative different thread title as they all seem the same after a while.

    I have been updating/upgrading my ST70 and a couple issues remain. One of which is a fairly loud hum/buzz from the left side. Now, when I connect my cables to my preamp, the noise goes away so I am assuming I have a missing or bad ground somewhere? Also, when I was checking values on the underside (for the 2nd issue) when I came near the left side socket with my red test lead (black grounded to chassis) the noise reduced significantly. Don't know if that helps but thought it might help indicate an area where things are emminating from.

    As far as the board goes, I have replaced the caps and am now using the socket adapters and 6g8HA tubes from Dynakit. Swapping the adapters and tubes does not change the side the noise is coming from.

    The 2nd issue is wandering bias, but unless these need to be considered together, I'll save that for another thread if needed.

    corndog71

    Posts : 451
    Join date : 2013-03-19
    Location : It can get windy here

    Re: Silencing the Lamb (ST70)

    Post by corndog71 on Tue Jan 27, 2015 11:13 am

    Drugolf wrote:Sorry, had to use a creative different thread title as they all seem the same after a while.

    I have been updating/upgrading my ST70 and a couple issues remain.  One of which is a fairly loud hum/buzz from the left side.  Now, when I connect my cables to my preamp, the noise goes away so I am assuming I have a missing or bad ground somewhere?  Also, when I was checking values on the underside (for the 2nd issue) when I came near the left side socket with my red test lead (black grounded to chassis) the noise reduced significantly.  Don't know if that helps but thought it might help indicate an area where things are emminating from.

    As far as the board goes, I have replaced the caps and am now using the socket adapters and 6g8HA tubes from Dynakit.  Swapping the adapters and tubes does not change the side the noise is coming from.

    The 2nd issue is wandering bias, but unless these need to be considered together, I'll save that for another thread if needed.

    I would recheck all of your ground connections first. Loose RCAs, unsoldered connections, etc. Have you shorted the inputs and heard noise?

    tubes4hifi
    Admin

    Posts : 1291
    Join date : 2008-11-30

    Re: Silencing the Lamb (ST70)

    Post by tubes4hifi on Wed Jan 28, 2015 5:55 pm

    most likely a bad ground on the noisy channel, could be either the input or the output.
    Could also be a bad capacitor on that channel.
    As for bias, that could be a bad capacitor also.
    These amps are 50 years old. All caps and resistors should be replaced, or better yet, buy a new modern VTA70 board that sounds 10X better !

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