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    Quiet Christmas: fuse, rectifier, 12AT7 fails and now can't bias (ST-120)

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    antipodean

    Posts : 6
    Join date : 2010-10-04
    Location : Sydney, Australia

    Quiet Christmas: fuse, rectifier, 12AT7 fails and now can't bias (ST-120)

    Post by antipodean on Tue Dec 27, 2016 6:06 am

    Hi All,

    After 7 years of continuous, flawless service my ST-120 has hit a rough spot.
    Right in the middle of serving the Christmas pud ... the music stopped :-(

    A few comments about being saved from more Christmas carols with lousy bit rates and awful dynamics, we all focused on the pud (hats off to the good wife for culinary excellence) before I wandered over to the amp to find it "lights out" and a blown fuse ... and no spares.

    Today, now that the shops are open, I grabbed a handful (4) new fuses from the shop today, pulled the power tubes and the rectifier: all good.
    Plugged in the rectifier and left channel power tubes: pfft. Fuse #2 (and Fuse #3 for a dim-witted confirmation)

    Anyway, it turned out the long-serving Weber CopperCap WZ34(The one that you're not supposed to use any more) had shorted one of the diodes inside (confirmed by meter).
    I replaced this with the GZ34 that Bob sold me with the kit and powered up again....

    This time, the left channel's 12AT7 didn't warm up ... I had a spare to roll in and tried again ...

    As a precaution I thought I'd check the bias: The right side came up OK (.55 ... the old setting ... sorry, only just read the .50v post)
    But the left side had .55 for the front tube, 0.00 for the rear. So I rolled the rear tube (Sovtek Kt-88, as are the others) and it stayed at 0.00v..

    I'm a little stumped about now: The right side all looks happy, but the left is definitely unwell.
    So I pulled all the tubes, opened the chassis and went looking for any potential issues. Nothing seen and the PIO coupling caps all read OK and showed charge/discharge but I'm still getting 0.00v for bias on the rear left tube.

    I'm starting to think that something else might have been the cause, and I'm just witnessing the symptoms.

    Any suggestions out there?

    Anthony
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    Peter W.

    Posts : 434
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: Quiet Christmas: fuse, rectifier, 12AT7 fails and now can't bias (ST-120)

    Post by Peter W. on Tue Dec 27, 2016 8:46 am

    Did you replace the shorted diode? (I more suspect open rather than shorted, but you did write "shorted").

    I would suggest you start looking where the voltage *stops*. And in most cases, such an event is that something goes open, or shorts to ground. So, an open diode or resistor, a shorted cap would be my first suspects. Followed by a cold-solder that went fully open under the stress. Either way, all those conditions could be very nearly invisible. Use your meter, comparing the left to the right, carefully. And start with the unit turned off and unplugged.

    Best of luck with it!
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    Bob Latino
    Admin

    Posts : 2542
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Quiet Christmas: fuse, rectifier, 12AT7 fails and now can't bias (ST-120)

    Post by Bob Latino on Tue Dec 27, 2016 9:22 am

    If the left rear will not bias and gives a reading of 0.0 then possibly ...

    1.  The tube in that socket is bad but you said that you tested the tube and it is OK
    2.  OR the 10 ohm resistor that goes from pins 1 and 8 to chassis ground does not measure 10 ohms as it should. Check to see if the 10 ohm resistor on that tube socket does measure 10 ohms
    3.  OR the 1000 ohm 1 watt resistor across pins 5 and 6 of the left rear socket does not measure 1000 ohms. Check that resistor
    4. The triode/pentode wiring or switch for that tube socket has a problem.

    Try this > Take out all the tubes. Center the 4 bias pots. Take the bottom cover off. Turn the amp ON. Measure the DC voltage at pins 5 and 6 to chassis ground on all four output tubes. You should get -50 to -60 VDC on all 8 measuring points. If you get the correct readings then the bias system is OK. If the left rear tube socket won't give you the correct reading on pins 5 and 6 then there is problem on the board with the bias system for just that tube.

    Bob

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    peterh

    Posts : 821
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Quiet Christmas: fuse, rectifier, 12AT7 fails and now can't bias (ST-120)

    Post by peterh on Tue Dec 27, 2016 10:06 am

    In addition 4 new matched powertubes. It's time , 7 years is some hours.
    It's entirely possible that it started with a flash-over in a powertube, that killed the weber and
    a 10ohm resistor.

    antipodean

    Posts : 6
    Join date : 2010-10-04
    Location : Sydney, Australia

    Back on track

    Post by antipodean on Sun Jan 01, 2017 7:45 am

    Thanks All,

    The suggestions narrowed things down.. thank you.

    After getting the tubes power tubes rolled back to some KT88s, I noticed that there was noise when I put my finger on the R1/R3/R5/R7 group.
    Somewhat dimmly, I removed these and with eyesight fading, taught my 9-year-old daughter how to solder the replacements in (I thought that was really cool!)
    Anyway, the problem persisted and a somewhat "annodised" looking solder joint on the back of the left channel RCA socket copped a heating and some fresh solder ...
    Left channel now solid.

    I still can't explain why this would affect the bias on the back power tube for the left channel, but everything is nice and stable and has been working for a few days now ...

    Still thankful for the fun time I had with my youngest ... and a female - so much for the STEM grinches!

    Happily back listening again with a new rectifier on order along with some 12AT7's and a matched set of KT120s (headroom is always important when you have kids)


    AB

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    Re: Quiet Christmas: fuse, rectifier, 12AT7 fails and now can't bias (ST-120)

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