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    runaway bias on mark III with vta pcb

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    wedg714

    Posts : 74
    Join date : 2011-11-23

    runaway bias on mark III with vta pcb

    Post by wedg714 on Sat Mar 24, 2012 6:02 pm

    HELP!! this is the second time that i've tried to install a vta pcb in a mark III. both times i get runaway bias. the tubes, choke, sockets, quad cap, resistors, and all wiring are new. the tubes test in another amp so i know they're good. the power tranny checks out. 430 on both pins. i'm including pics so if anyone sees a mistake or has some suggestions, please reply. thanks

    [url=http://www.servimg.com/image_preview.php?i=13&u=17384813][/url]

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: runaway bias on mark III with vta pcb

    Post by Bob Latino on Sat Mar 24, 2012 7:41 pm

    Hi,

    I don't see the two 100 uF 100 volt bias caps which should be sticking up from the bottom of the driver board on either side of the main bias resistor. It does not appear that you installed these two caps. Look at the photos below. The RED circles are where the two caps should be installed. NOTE - The bias system's 100 uF 100 volt caps are installed with the polarity reversed. The longer POSITIVE lead should face the INSIDE of the board in this installation. On the very bottom photo are the prototype M-125 boards. You can see on the left side of this photo (which shows the bottom of the board), the two caps sticking up from the bottom of the board. On the M-125 board bottom you can see a red jumper wire in place of the brown colored resistor you have. Install those two caps and you should be OK. Without them you really have no bias system at all - hence the runaway bias that you are getting.

    Bob





    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: runaway bias on mark III with vta pcb

    Post by Bob Latino on Sat Mar 24, 2012 9:39 pm

    Send Roy an Email and I am sure that he will send you out the four caps (two for each board) that you need ..

    Bob

    tubes4hifi
    Admin

    Posts : 1286
    Join date : 2008-11-30

    Re: runaway bias on mark III with vta pcb

    Post by tubes4hifi on Sun Mar 25, 2012 10:36 pm

    it's all taken care of, not sure how those caps got left off!! Maybe not enough sleep the night before!

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