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    ST-120 .. missing left channel

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    antipodean

    Posts : 8
    Join date : 2010-10-04
    Location : Sydney, Australia

    ST-120 .. missing left channel

    Post by antipodean on Fri Mar 23, 2018 8:50 pm

    Hi All,
    As a happy 9+year owner of one of Bob's terrific ST-120 amps ... I've got one that I can't solve: A missing left channel.

    It started as an intermittent fault and I thought that I'd seen the three left-side tubes lose their warm glow which I'd mentally noted as a potential dry joint to be resolved. But more recently when it stopped 100% the tubes have all been glowing nicely...
    This morining, I've don'e some pre-cautionary re-soldering of the heater line for all three tubes and they all measure ~6Volts as expected.

    Next, I did some rolling. Starting from the centre 12AT7 outwards including the power KT-120s - no change.
    As part of this I re-biased and whilst the "front" pot was OK levelling out quickly after warming up, regardless of the tube changed, the back was still quite floaty between 0 and 0.5v - which I can't explain.

    I also checked the rails and noted that outputz of the outer-left 12AT7 was a bit lower at about 200V than the right-hand(working) side at 230V (when the doc says 300V) - I will note that I'm in Australia and I'm using a stepdown tranny to drop from 240@50Hz to 110(still at 50Hz) - so there could be some differences there.

    I did have an oscilliscope but it's in storage right now and haven't been able to have a look at signal path but, when testing the outputs of the outer 12AT7s, when I test the working right-side, I get a crackle in the speakers with my analogue multimeter, but no such crackle on the left.

    Just out of interest, I took off the cover and checked that there's some continuity across the speaker tranny for the primary windings ... all good there.

    But now I've run out of ideas and looking for some help...

    Anthony

    antipodean

    Posts : 8
    Join date : 2010-10-04
    Location : Sydney, Australia

    Re: ST-120 .. missing left channel

    Post by antipodean on Sat Mar 24, 2018 1:16 am

    Ahhh.... stand down fellow tubers, the good wife found it. The speaker had disconnected on the black terminal. Might have had something to do with some new speakers #2 son bought recently and wanted to test on the Valve amp .... At least it was an inexpensive find as I hadn't actually thrown anything out!
    Gotta love a fairer half who knows her way around audio too :-)
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    CletusB

    Posts : 154
    Join date : 2018-02-11
    Age : 63
    Location : Trinidad & Tobago

    Re: ST-120 .. missing left channel

    Post by CletusB on Sat Mar 24, 2018 8:02 am

    antipodean wrote:Ahhh.... stand down fellow tubers, the good wife found it. The speaker had disconnected on the black terminal. Might have had something to do with some new speakers #2 son bought recently and wanted to test on the Valve amp ....  At least it was an inexpensive find as I hadn't actually thrown anything out!
    Gotta love a fairer half who knows her way around audio too :-)
    What would we do without them? Very Happy

    eickmewg

    Posts : 87
    Join date : 2014-08-29

    Re: ST-120 .. missing left channel

    Post by eickmewg on Sat Mar 24, 2018 4:23 pm

    First rule of troubleshooting: check the simple things first. The car quits running? Check the gas gauge.

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