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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


3 posters

    runaway bias on mark III with vta pcb

    wedg714
    wedg714
    wedg714


    Posts : 74
    Join date : 2011-11-23
    • Post n°1

    runaway bias on mark III with vta pcb Empty runaway bias on mark III with vta pcb

    Post by wedg714 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

    [urunaway bias on mark III with vta pcb Mark3_23rlrunaway bias on mark III with vta pcb Mark3_24=httrunaway bias on mark III with vta pcb Mark3_25p://wrunaway bias on mark III with vta pcb Mark3_26ww.servimg.com/image_preview.php?i=13&u=17384813]runaway bias on mark III with vta pcb Mark3_22[/url]
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    Bob Latino
    Bob Latino
    Bob Latino
    Admin


    Posts : 3276
    Join date : 2008-11-26
    Location : Massachusetts
    • Post n°2

    runaway bias on mark III with vta pcb Empty Re: runaway bias on mark III with vta pcb

    Post by Bob Latino 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

    runaway bias on mark III with vta pcb Mark3_25

    runaway bias on mark III with vta pcb TwoMarkIIIdriverboards
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    Bob Latino
    Bob Latino
    Bob Latino
    Admin


    Posts : 3276
    Join date : 2008-11-26
    Location : Massachusetts
    • Post n°3

    runaway bias on mark III with vta pcb Empty Re: runaway bias on mark III with vta pcb

    Post by Bob Latino 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
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    Roy Mottram
    Roy Mottram
    Roy Mottram
    Admin


    Posts : 1838
    Join date : 2008-11-30
    • Post n°4

    runaway bias on mark III with vta pcb Empty Re: runaway bias on mark III with vta pcb

    Post by Roy Mottram 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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    • Post n°5

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