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    st70 power transformer question

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    wianieq

    Posts : 2
    Join date : 2011-05-17

    st70 power transformer question

    Post by wianieq on Fri May 20, 2011 12:09 pm

    Hi everyone. I'm in the process of rebuilding an st70 with the vta board and I ran into a little snag. Every manual or document I have found says that there is the red/black wire from the transformer that I believe goes to the vta board. I remember it being negative voltage, -50 I think. Anyway, my transformer doesn't have a red/blk wire. The one leftover wire is blk and yellow, or brown, it is hard to tell. By the length and curves in the wire it looks like it went to the selenium rectifier. I should have taken pictures before I started I know.

    I am about 95% sure this is the wire that should be the red/blk one, but I wanted to see if I could get an affirmative nod from someone who knows for sure. It will put my mind at ease as I hope to fire it up tonight.

    Thanks.


    Bob Latino
    Admin

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

    Re: st70 power transformer question

    Post by Bob Latino on Fri May 20, 2011 2:54 pm

    Turn the amp ON without any tubes in the amp while making sure that this wire in question does not touch the chassis. Measure the AC voltage from the end of that wire to chassis ground and see if you get about 50 to 60 volts AC on your meter. You should ..

    From the length of the original wire (it's pretty short), it appears that this wire could only connect to the original selenium rectifier. You will have to lengthen the wire (insulate the connection) to reach the "-50 VAC" eyelet on the VTA driver board.

    Bob

    wianieq

    Posts : 2
    Join date : 2011-05-17

    Re: st70 power transformer question

    Post by wianieq on Fri May 20, 2011 3:21 pm

    Thanks.

    I'll do that.

    I'm pretty certain now though, I found a picture on the web that seems to have the same cloth lead. But I'll do the test anyway.


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