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    ST-120 bad left channel

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    dellwood

    Posts : 4
    Join date : 2013-10-18

    ST-120 bad left channel

    Post by dellwood on Fri Dec 06, 2013 8:52 am

    After about 30 hours on a newly built ST-120, I heard a loud pop, and a percieved loss of volume. Before I could get to the system, I heard another loud pop and the left channel was gone.

    I swapped the left and right driver tubes and the left side remained dead. All tubes glow.

    What is the recommended troubleshooting procedure?

    Thanks

    deellwood

    Bob Latino
    Admin

    Posts : 2414
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-120 bad left channel

    Post by Bob Latino on Fri Dec 06, 2013 9:00 am

    The CENTER diver tube also affects the left channel. Try a NEW driver tube in the center position.

    It could also be one of the POWER tubes on the left channel. Try swapping the right channels power tubes into the left channel and then try the left channel alone (no output tubes in the right channel)

    Nearly always when you hear a "pop" like that, the issue you are having is tube related.

    Bob

    arledgsc

    Posts : 340
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: ST-120 bad left channel

    Post by arledgsc on Fri Dec 06, 2013 10:05 am

    Could be a lot of things but if no help changing the center tube what about the two left power tubes?  Do the bias voltage readings look close to normal?  My amp "popped" several months ago and one of the power tubes opened (no current) yet I could hear audio from the channel on the remaining good tube. Only had about fifty hours on the tube set.

    dellwood

    Posts : 4
    Join date : 2013-10-18

    Re: ST-120 bad left channel

    Post by dellwood on Fri Dec 06, 2013 11:36 am

    Are spare power tubes and driver tubes available from Tubes4HiFi?

    dellwood

    sKiZo

    Posts : 1312
    Join date : 2013-04-01
    Location : Michigan USA

    Re: ST-120 bad left channel

    Post by sKiZo on Fri Dec 06, 2013 8:47 pm

    First things first ... I wouldn't plug in different tubes until you have a chance to go thru all the connections with a magnifying glass for weak solder joints or bridges. A few hot/cold cycles will bring out the worst in those. Be VERY critical - if one looks at all odd or not up to standard, heat it up again. Shouldn't be any issues with incorrect wiring as you had it working for a good length of time before problems developed.

    Also pull out the VOM and check for continuity and correct voltages at each connection. You're ahead of the game in that you have one good channel to compare with.

    dellwood

    Posts : 4
    Join date : 2013-10-18

    Re: ST-120 bad left channel

    Post by dellwood on Tue Dec 10, 2013 8:36 am

    I found the 10 ohm 2 watt resistor from pin 8 to the chassis ground to be bad on each of the two left channel power tube sockets. They were bulged, had spalled of some of the ceramic coating and showed no continutity.

    Any ideas what may have caused this?

    Thanks,

    dellwood

    Bob Latino
    Admin

    Posts : 2414
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-120 bad left channel

    Post by Bob Latino on Tue Dec 10, 2013 9:17 am

    dellwood wrote:I found the 10 ohm 2 watt resistor from pin 8 to the chassis ground to be bad on each of the two left channel power tube sockets. They were bulged, had spalled of some of the ceramic coating and showed no continutity.

    Any ideas what may have caused this?

    Thanks,

    dellwood

    Hi David,

     Sometimes it is hard to say exactly why something happened. In most cases with a blown bias resistor, it is caused by excessive current being drawn by the tube that was on that socket. There is a problem on the tube sockets on that channel. It could be a bad tube(s) OR it could be a bad connection on that (those) tube sockets. The fact that the amp played fine for 30 hours before you ran into an issue leads me to believe it could be a solder connection. Sometimes after a few ON/OFF cyles where the amp heats up and then cools down a few times (expansion and contraction of the solder joint) a solder connection can become a partial connection. If it happens on pin#5 you will lose all bias control of that tube. Again it COULD be a bad tube - BUT - to happen on TWO tubes at the same time leads me away from thinking it is bad tube.

    I will send you out some new bias resistors later today  ..

    Bob

    dellwood

    Posts : 4
    Join date : 2013-10-18

    Re: ST-120 bad left channel

    Post by dellwood on Tue Dec 10, 2013 9:21 am

    Great Bob!, I'll break out the soldering iron and magnifying specs and replace the resistors and inspect the other solder joints.

    Thanks a bunch.

    dellwood

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