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    Dynaco st70 output fault help is asked



    Dynaco st70 output fault help is asked

    Post by johannes on Mon Feb 21, 2011 3:12 am

    Hi all.

    I am the owner of a st70 in stock condition. Lovely unit.

    It has some hum which is, however, not noticeable when the amp has warmed up. The hum has been there since i bought the amp last november.

    Now, I suddenly the other day, noticed a lot of hum in the left speaker while the other speaker was quite.

    As i turned on the music, the left channel (the one with the hum) passed only a weak signal through, while the right speaker output was normal.

    I got a little scared and turned off the unit. The tubes should all be fine (they all glow and bias is stabile), Mullard GZ34 is new.

    Does anybody now of this fault and has some experience what to do next?

    The bias voltage is stabile in both channels at 1,5 V. The selenium rectifier, black cats and the quad caps have not been changed. Nor the big can at top, as i know off.

    Thanks everybody in advance.

    Bet regards, Johannes, Denmark

    Bob Latino

    Posts : 2382
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Dynaco st70 output fault help is asked

    Post by Bob Latino on Mon Feb 21, 2011 8:51 am

    Hi Johannes,

    If you had hum in both channels then it is nearly always a power supply problem and the most likely culprit is the quad cap. In any original ST-70 that has the original (40 to 50 year old) quad cap - that cap should be replaced. it was a real weak point on original ST-70's.

    If your hum is on one channel now, as you say, first try swapping the 7199 tubes from one channel to the other. If the hum/weak volume moves to the other channel it is the 7199 tube. If swapping the 7199 does not help the problem, then more than likely there is an issue on the left side of the driver board. You could try replacing those six black capacitors on the top of the driver board (4 X .10 uF and 2 X .05 uF). That might do it. IMHO you should probably replace the whole driver board. The parts on that board are 40 to 50 years old.



    Re: Dynaco st70 output fault help is asked

    Post by johannes on Mon Feb 21, 2011 9:33 am

    Hi Bob.

    Thanks for a very helpful answer.

    The hum from the unit itself has been there for a while i guess but the hum in left speaker suddenly occured last thursday as i wanted to hear music.

    Perhaps there has been hum in both speakers at all times but since my speakers at not that sensible (snell c/v) i have not paid attention to it.

    And now the signal is much weaker (at the side where the loud hum occurs) comparede to the other speaker where the signal is clear.

    I will try to swift those 7199 tubes and go forward as you recommended.

    Best regards, Johannes

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