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    my ST-120 moving woes

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    baddog1946

    Posts : 304
    Join date : 2010-02-03
    Location : Costa Rica

    my ST-120 moving woes

    Post by baddog1946 on Fri Feb 17, 2012 9:48 pm

    After moving to a new house I set up my ST-120 and I had the following problem: The amp seemed to fire normally as far as the tubes lighting up at first but one channel had lower volume then one of the output tubes flared up. I shut it down to right away and then began to check what was up.
    First I checked the bias and it was close and easy to re-set to normal. I then switched the input cables from my DAC and the other channel was low volume. I tried replacing each side of the driver tubes and the center as well and same channel remained low volume.
    After a short time during the testing period there was another output tube flare up and I again immediately shut down the amp hopefully not frying an output tube.
    BTW I also changed all the output tubes as well, re-biased and still had the same low volume problem on one channel and then another flare up. There was also a little noise when adjusting the bias kind of a crackling noise.
    I took the bottom off and everything looks OK no crispy parts or obvious issues.
    Since I am having these flare ups I don’t want to keep firing the amp and do something serious because I am in Costa Rica a long way from parts suppliers.
    I do have a spare Weber rectifier but no quad cap. Any ideas I could use some suggestions.
    Baddog

    j beede

    Posts : 316
    Join date : 2011-02-07
    Location : California

    Re: my ST-120 moving woes

    Post by j beede on Sat Feb 18, 2012 12:39 am

    baddog1946 wrote:After moving to a new house I set up my ST-120 and I had the following problem: The amp seemed to fire normally as far as the tubes lighting up at first but one channel had lower volume then one of the output tubes flared up. I shut it down to right away and then began to check what was up.
    First I checked the bias and it was close and easy to re-set to normal. I then switched the input cables from my DAC and the other channel was low volume. I tried replacing each side of the driver tubes and the center as well and same channel remained low volume.
    After a short time during the testing period there was another output tube flare up and I again immediately shut down the amp hopefully not frying an output tube.
    BTW I also changed all the output tubes as well, re-biased and still had the same low volume problem on one channel and then another flare up. There was also a little noise when adjusting the bias kind of a crackling noise.
    I took the bottom off and everything looks OK no crispy parts or obvious issues.
    Since I am having these flare ups I don’t want to keep firing the amp and do something serious because I am in Costa Rica a long way from parts suppliers.
    I do have a spare Weber rectifier but no quad cap. Any ideas I could use some suggestions.
    Baddog
    It sounds like the output from your DAC is imbalanced if I correctly understand your sentence above. Is there a Radio Shack near you? You could buy an RCA "Y" cable so can apply the LEFT or RIGHT output from your DAC to both inputs of your ST-120. The output level should match. Have you tried a different source? If your ST-120 has the attenuator option you could run your DVD player or CD ploayer output straight in to the ST-120.
    ...j

    j beede

    Posts : 316
    Join date : 2011-02-07
    Location : California

    Re: my ST-120 moving woes

    Post by j beede on Sat Feb 18, 2012 12:42 am

    Are you near Escazu? There should be a Radio Shack there.
    ...j

    baddog1946

    Posts : 304
    Join date : 2010-02-03
    Location : Costa Rica

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    Post by baddog1946 on Sat Feb 18, 2012 9:33 am


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    Last edited by baddog1946 on Sat Feb 18, 2012 9:39 am; edited 1 time in total

    baddog1946

    Posts : 304
    Join date : 2010-02-03
    Location : Costa Rica

    Re: my ST-120 moving woes

    Post by baddog1946 on Sat Feb 18, 2012 9:37 am

    Thanks for the advice but I still have the crackling bias and flare up problems going on so I'm thinking its not the DAC. I am going to prove the DAC today with another amp.
    The fact it occurred when I moved the amp is somehow related to it I think but one never knows. We have a lot of power issues and I am also checking the house power for correct phase. That is making sure the ground is on the right lug of the plug I am using for the amp.
    Failing that I am still hesitant to fire the amp until I can determine that something is not going to fry and expensive output tube.

    Bob,
    any ideas how I can check out the quad cap without a cap checker? Any ideas at all would be helpful.
    It is probably not a tube as I swapped all the tubes out with the same result.

    Dheick

    Posts : 13
    Join date : 2012-01-10

    Re: my ST-120 moving woes

    Post by Dheick on Sat Feb 18, 2012 9:53 am

    I had a simular problem. It turned out to be a bad solder conection on one of the output tube scockets. sometimes even if it looks like a good joint it might not be. Maybe it worked itself loose during the move. just a thouhgt.

    baddog1946

    Posts : 304
    Join date : 2010-02-03
    Location : Costa Rica

    Re: my ST-120 moving woes

    Post by baddog1946 on Sat Feb 18, 2012 10:12 am

    point well taken I will check that.
    Thanks

    j beede

    Posts : 316
    Join date : 2011-02-07
    Location : California

    Re: my ST-120 moving woes

    Post by j beede on Sat Feb 18, 2012 10:52 am

    On old amps I "clean" the bias pots by rotating them across their full range several times before setting the bias level. I do this with the amp off and after noting the approximate position before the "cleaning". I then return the bias to the pre-cleaning setting before turning the amp on setting the bias the usual way. I also apply a bit of downward force while rotating the bias control.

    GP49

    Posts : 717
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: my ST-120 moving woes

    Post by GP49 on Sat Feb 18, 2012 2:53 pm

    Resolder everything in the chain from the bias circuit, up to and including the tube pin for the control grid. If one of those connections opens up, you get instant tube runaway.

    Check the coupling capacitor between the driver stage and the control grid of the output tube. If it shorts or arcs over, you get instant tube runaway.

    tubes4hifi
    Admin

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    Join date : 2008-11-30

    Re: my ST-120 moving woes

    Post by tubes4hifi on Sun Feb 19, 2012 2:08 am

    it is likely a flaky solder connection, but my very first thought before reading other responses was that it very well could be the AC power in your new place. Take your meter, one lead in ground, and check the neutral and hot. Some places have flaky AC, like 50v on neutral compared to ground. Wouldn't matter if you use 2 wire power instead of 3 wire, but weird stuff like that can do weird things to amps.
    Meanwhile, yeah, hopefully a solder connection somewhere. Flaky socket pin on a tube . . .

    baddog1946

    Posts : 304
    Join date : 2010-02-03
    Location : Costa Rica

    Re: my ST-120 moving woes

    Post by baddog1946 on Sun Feb 19, 2012 11:20 am

    I'm on it thanks for the feedback to all. I will post what it was when I know.
    Baddog

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