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    unstable bias voltage

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    leo1254

    Posts: 2
    Join date: 2009-07-03
    Age: 59
    Location: near pittsburgh

    unstable bias voltage

    Post by leo1254 on Fri Jul 03, 2009 12:58 pm

    hello,
    I have a dyanco st-70- I am not a tech, and only have a very basic understanding of electronics.

    I replaced the caps on the driver board, the quad cap capacitor, the 50uf caps, and the choke. During this work, I managed to break two resistor leads. (1) the 6.8k ohm, 2w on the quad cap lug, and (2) the 10k ohm, 2w on the terminal strip. I wasnt able to find replacements, so I soldered a small wire to replace the broken lead on both resistors, and re-attached with solder. However, my meter reads the 6.8k to be 5.8k.

    With tubes in, and without a signal to amplify, I am able to set the 1.56v bias setting. When I play music through the amp, and monitor the bias voltage, it varies and slowly climbs reaching over 2v and I turn the bias adjustment pots back to compensate, but it continues to climb - I then turn it off before another tube gets damaged.

    Are there any other checks I can do which would pinpoint my problem?
    How do I get a stable bias voltage?

    Sal

    Posts: 222
    Join date: 2009-02-05
    Location: Central New Jersey Dynaco-ST70.com

    Re: unstable bias voltage

    Post by Sal on Fri Jul 03, 2009 3:42 pm

    Is your bias stable without any music playing at all? Is your bias changing on both the left and right channels? Just a FYI, varying the bias on one channel will affect the other channel. If one channel bias goes up while the other channel goes down its a good chance you have a bad tube on one side.

    The original printed circuit board is not very good by today's standards, make sure you don't have any broken traces on it.

    Let us know what you find.

    Sal

    leo1254

    Posts: 2
    Join date: 2009-07-03
    Age: 59
    Location: near pittsburgh

    Re: unstable bias voltage

    Post by leo1254 on Fri Jul 03, 2009 5:49 pm

    Thank you for the reply.
    Yes Sal, that was the problem. I returned to the bench and re-soldered a couple spots, and the instability resolved. My eyes sometimes are my worst enemy. I found 2w mf resistors at MCM Electronics, so the carbon comps will be pulled and replaced with new ones.

    I reallly am glad to have found this forum, and will re-post if any new problems arise after first checking the posts for an answer.

    Its playing! Smile
    Thanks
    leo1254

    Sal

    Posts: 222
    Join date: 2009-02-05
    Location: Central New Jersey Dynaco-ST70.com

    Re: unstable bias voltage

    Post by Sal on Fri Jul 03, 2009 6:11 pm

    I am glad you found the problem. Just so you know for future, if you need any parts for your Dynaco amp, go to Dynakitparts at http://www.dynakitparts.com

    Sal

      Current date/time is Tue Nov 25, 2014 6:46 pm