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    st-120 unbalanced L-R output ?

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    tajanes

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    st-120 unbalanced L-R output ?

    Post by tajanes on Mon Aug 06, 2018 2:34 pm

    I've been having to use my pre-amp balance control more and more recently to 'up' the right channel output (rel to the left) and thnking a tube is going bad, I would appreciate suggestions on which tube(s) to swap out first. Thinking pre-amp tubes first choice but thought I'd get some real advice before I start down the road.

    Thanks,
    Andy
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    Bob Latino
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    Re: st-120 unbalanced L-R output ?

    Post by Bob Latino on Mon Aug 06, 2018 3:15 pm

    tajanes wrote:I've been having to use my pre-amp balance control more and more recently to 'up' the right channel output (rel to the left) and thnking a tube is going bad, I would appreciate suggestions on which tube(s) to swap out first.  Thinking pre-amp tubes first choice but thought I'd get some real advice before I start down the road.

    Thanks,
    Andy

    This sounds like a driver tube problem to me .. Either the center or the right driver tube .. More likely the CENTER driver tube. Buy one driver tube and replace the center tube with the new one. If that doesn't do it, try the new tube in the right position.

    Bob
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    tajanes

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    Re: st-120 unbalanced L-R output ?

    Post by tajanes on Mon Aug 06, 2018 4:09 pm

    I tried the center, no diff
    chged rt, no dif, chged lt no diff

    exchanged the power tubes, now having trouble getting the bias set within reason, they are all over the place, and making adj to one seems to rest others?
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    Bob Latino
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    Re: st-120 unbalanced L-R output ?

    Post by Bob Latino on Mon Aug 06, 2018 4:44 pm

    I believe that you have the VTA stepped attenuator. Try resoldering all the connections on the attenuator, input jacks and the IN connections on the driver board.

    Bob
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    tajanes

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    Re: st-120 unbalanced L-R output ?

    Post by tajanes on Mon Aug 06, 2018 5:00 pm

    I didn't go with the stepped attenuator in the final build, but may be toast biz I ordered boards preassembled and as such don't know which connections to check.

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    Bob Latino
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    Re: st-120 unbalanced L-R output ?

    Post by Bob Latino on Mon Aug 06, 2018 5:34 pm

    tajanes wrote:I didn't go with the stepped attenuator in the final build, but may be toast biz I ordered boards preassembled and as such don't know which connections to check.


    Andy > I have send you an Email about this at your regular Email address ..

    Bob
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    Peter W.

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    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: st-120 unbalanced L-R output ?

    Post by Peter W. on Tue Aug 07, 2018 9:42 am

    It seems to me that it might be useful to develop a network of local individuals who have some basic diagnostic tools such as:

    a) Tube Tester - swapping tubes to look for problems can be frustrating at the very least. And small-signal tubes seldom fail (after the first 100 hours or so). So access to a tube-tester might eliminate an entire class of problems.

    b) Good VOM - will point to a host of possible issues. Even just from channel-to-channel comparisons.

    c) ESR Meter - test those caps.

    d) Second Set of Eyes - best diagnostic tool on the planet.

    We are in southeastern PA, in Zip 19027. We have a summer house in 17859. With notice, I could have all four items available - gratis. If others so-equipped were to chime in, maybe we could cover a some parts of the world. I know that there is a person in Ranger, TX (76470) who would kick in as well - although he is not specifically a Dynaco person, just one helluva diagnostician.
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    tajanes

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    Location : Chicago

    Re: st-120 unbalanced L-R output ?

    Post by tajanes on Tue Aug 07, 2018 10:34 am

    Peter,
    Thanks, if only I was near PA (I'm in shoot-em up Chi)....

    Bob has been a big help (again and again) and he had me take bias readings off of output tube socket pins- so thinking I have issue(s) with solder to the remote bias pickups. Not really sure whats going on but its good to know the bias is set properly and may just be failing tubes- although I've replaced and moved some around w/o finding the bad one(s).
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    corndog71

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    Location : It can get windy here

    Re: st-120 unbalanced L-R output ?

    Post by corndog71 on Tue Aug 07, 2018 2:50 pm

    I’m in Chicago too!  While I don’t have a tube tester I have a little experience.  Let me know if I can help.
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    10-E-C

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    Location : upper east tn

    Re: st-120 unbalanced L-R output ?

    Post by 10-E-C on Tue Aug 07, 2018 4:24 pm

    I'm in Upper East Tn, about 20 miles from both the NC and Va border if anyone is close to me.

    TM
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    tajanes

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    Join date : 2015-02-07
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    Re: st-120 unbalanced L-R output ?

    Post by tajanes on Thu Aug 09, 2018 9:19 pm

    As I mentioned to Bob, I went in and changed my preamp tubes, and then measured bias directly at the power tube sockets- and all is back and sounding great, and balanced left-right! Thinking I have a bad solder in my bias pickup combined with a bad / failing tube.

    Thanks again Bob, and all in the community for pitching in !

    Andy

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