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    Bias run amok

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    deepee99

    Posts : 1337
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Bias run amok

    Post by deepee99 on Sat Jun 04, 2016 12:31 pm

    Just did the monthly bias check and one of my M-125 Big Boppers has a bias issue. Each of the four tubes can't be dialed down below 1.0 v. It checked out fine last time around but this is a bit scary.
    Since this is a problem common to all four tubes/holes in that amp, any suggestions?

    audiobill

    Posts : 270
    Join date : 2014-03-13
    Location : Philadelphia

    Re: Bias run amok

    Post by audiobill on Sat Jun 04, 2016 1:05 pm

    Have you checked your B+ voltage?

    deepee99

    Posts : 1337
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: Bias run amok

    Post by deepee99 on Sat Jun 04, 2016 1:16 pm

    Hi, Bill,
    No I haven't. Can't remember how/where to do that.

    audiobill

    Posts : 270
    Join date : 2014-03-13
    Location : Philadelphia

    Re: Bias run amok

    Post by audiobill on Sat Jun 04, 2016 1:19 pm

    On top of the board, you can see B+.....not the same as at the quad cap, but you can compare your amps easily.....

    Dave_in_Va

    Posts : 144
    Join date : 2013-04-02

    Re: Bias run amok

    Post by Dave_in_Va on Sat Jun 04, 2016 2:56 pm

    I'm not a tech.
    One time my VTA ST70 started sounding bad and upon checking the bias, I found my readings to be around 75-80mA.
    It was the (NOS Mullard) rectifier gone south. Swapped in a Sylvania all has been well since (40mA).

    sKiZo

    Posts : 1312
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Bias run amok

    Post by sKiZo on Sat Jun 04, 2016 3:16 pm

    Yup ... good idea to check some voltages, but a potentially simple fix would be to swap the rectifier for a known good spare.

    Before that, I'd just try cleaning the pins and socket. Also retension the socket so you're sure you've got good contact all around.

    No luck there, next best bet would be a cold solder joint. Easy enough to reflow if you know what you're doing, and a close eyeball inspection as well as all of the above should be the first steps any tech should try if you do take it in for service.

    arledgsc

    Posts : 340
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: Bias run amok

    Post by arledgsc on Sat Jun 04, 2016 5:41 pm

    Check the bias negative voltage power supply as well. You will run out of adjustment range if it starts going positive of nominal.

    Verify the power supply electrolytic caps look healthy (no outwardly domed end-caps or dried electrolyte clues). A leaky cap can load the supply and throw voltages out of whack.

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