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    New VTA70 upgrade and a bias problem

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    McGar

    Posts : 8
    Join date : 2015-05-29

    New VTA70 upgrade and a bias problem

    Post by McGar on Wed Jun 17, 2015 8:35 pm

    I have finished my VTA 70 install on my old 1960 or 61 ST70.  I have spent a couple of hours researching the board.  I think I have problem solved, but i thought I would ask for some help.  

    Bias is way too high.  With pots turned all the way down, bias is leveling off at .70.   I thoroughly cleaned and tightened all the sockets.  And still have the problem.

    Here are some pertinent values:

    red/blk wire bias connection  55vac

    Other end of diode   -45vdc Evil or Very Mad

    each location of pin 6 at board -22.3 to -23

    Across pin 2 and 7 of each output tube  7vac

    rec tube across pin 2 and 8  5.5 vac

    rec tube pin 4 and 6  379vac


    So have I got a bad diode (or did I toast it soldering it in place)?  Or should I look somewhere else?  Can I replace that diode with a 1N4007  or do I need to go the electronics store?

    Thanks in advance for any and all help.

    Bob Latino
    Admin

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

    Re: New VTA70 upgrade and a bias problem

    Post by Bob Latino on Wed Jun 17, 2015 9:17 pm

    Hi,

    You should get about -70 to -75 VDC on the other side of the diode. Try another diode in there.

    Yes - You can use a 1N4007 diode in there ..

    Bob

    McGar

    Posts : 8
    Join date : 2015-05-29

    Re: New VTA70 upgrade and a bias problem

    Post by McGar on Wed Jun 17, 2015 9:58 pm

    AWWWRRRRGGGGGGHHHHH!

    Replaced the diode and still getting about -45vdc. Any suggestions? Diode stripe is through the square hole next to the Bias with the red/blk wire. Any other values I should check?

    McGar

    Posts : 8
    Join date : 2015-05-29

    Re: New VTA70 upgrade and a bias problem

    Post by McGar on Thu Jun 18, 2015 7:55 am

    I think I may have it! Sloppy solder connection at C17. Now I am getting 75vdc at the end of the diode.

    Bob Latino
    Admin

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

    Re: New VTA70 upgrade and a bias problem

    Post by Bob Latino on Thu Jun 18, 2015 8:27 am

    You should be OK now .. 55 VAC on one side of the diode usually equals about -75 VDC on the other side ..

    Bob

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    Re: New VTA70 upgrade and a bias problem

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