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    volume control

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    rfbro

    Posts : 26
    Join date : 2014-12-28
    Age : 61
    Location : Vancouver WA

    volume control

    Post by rfbro on Thu Apr 09, 2015 11:59 am

    The volume control on my st120 is making noise when I Change the level and sometimes the right channel is not as loud as the left would contact cleaner help if not how difficult is it to bypass the volume control all together. ...I do have a pre amp. Bob B

    GP49

    Posts : 732
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: volume control

    Post by GP49 on Thu Apr 09, 2015 5:58 pm

    I don't have a schematic so I cannot say for sure if this applies to your amplifier, but a level control potentiometer should not be removed if it is the only resistance between the hot input terminal and ground, unless you are absolutely, positively certain that with a completely open circuit, the input stage will be unconditionally stable.  

    Every tube amplifier I have ever seen has a shunt resistor at the input, approximately ranging from 100KΩ up to 1MΩ. Remove it and you risk oscillation if the input cable is accidentally unplugged.

    You could reconnect the hot pin of the input pin to the top of the level control potentiometer, if you wish.  Electrically
    that will be the same as having it turned all the way up and will bypass the wiper and any associated contact issues.

    sKiZo

    Posts : 1309
    Join date : 2013-04-01
    Location : Michigan USA

    Re: volume control

    Post by sKiZo on Thu Apr 09, 2015 8:03 pm

    If it's the Alps control that Bob & Co supplies, it's a simple insertion into the signal path.

    Problem being, it gets really tight in there. First thing I'd do is check all the connections for cold joints, solder blobs or whiskers, or potential shorts to the chassis  - that could make for intermittent noise and other such issues. Good luck getting any cleaner into the switch. Those are sealed up pretty tight. One thing you CAN try is to just exercise the control. Turn it slowly back and forth through the entire range maybe 20 times and see if that helps. Could be some sort of mechanical crud or droppings that are getting in the way.

    Another thing to try - turn the volume on the amp up all the way and leave it there. That should give you the best possible signal through the control. Not all that necessary with a pre anyway, although I do find it handy to balance the line level between the components on my system.

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: volume control

    Post by Bob Latino on Fri Apr 10, 2015 8:28 am

    rfbro wrote:The volume  control on my  st120 is making  noise  when I Change the level and sometimes the right channel is not as loud as the  left would contact cleaner help if not how difficult is it to bypass the volume  control all together. ...I do have a pre amp.  Bob B

    Hi Bob,

    I would try to first clean the stepped attenuator by spraying the control with an electrical contact cleaner and then (with the amp OFF) turning the attenuator from one end to the other about 10 times. Even though the attenuator is "sealed" enough cleaner should work its way inside to do its job.

    RE: "Bypass the attenuator". If you turn the attenuator all the way to the right it is basically out of the circuit and the input jacks send the audio signal directly to the driver board input eyelet. You would, however, now need some other means to control volume. (a preamp)

    It could also be a bad solder connection on the attenuator itself. You could try resoldering the 6 connections on the attenuator and/or the four connections on the input board.

    Bob

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