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    ST-70 - Can Not Set Bias

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    Philagot

    Posts : 1
    Join date : 2017-01-22

    ST-70 - Can Not Set Bias Empty ST-70 - Can Not Set Bias

    Post by Philagot Mon Jul 12, 2021 5:09 pm

    I have an ST-70 that has been working well for a few years. Last week it started making a buzzing noise so I checked the Bias, It would not go over .3V using any of the VTA's pots, so I figured its the rectifier tube. I changed the rectifier tube still staying around .3V with the pots at max. There is still what I believe to be a transformer/amp buzz.

    Any help would be appreciated.

    Mark
    peterh
    peterh

    Posts : 1565
    Join date : 2012-12-25
    Location : gothenburg, sweden

    ST-70 - Can Not Set Bias Empty Re: ST-70 - Can Not Set Bias

    Post by peterh Mon Jul 12, 2021 5:17 pm

    Philagot wrote:I have an ST-70 that has been working well for a few years. Last week it started making a buzzing noise so I checked the Bias, It would not go over .3V using any of the VTA's pots, so I figured its the rectifier tube. I changed the rectifier tube still staying around .3V with the pots at max. There is still what I believe to be a transformer/amp buzz.

    Any help would be appreciated.

    Mark

    You could have a leaking / bad can cap. Check DC voltages, if they are low you could suspect the can cap.

    michael.samra likes this post

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    michael.samra

    Posts : 11
    Join date : 2015-05-16
    Age : 63
    Location : michigan

    ST-70 - Can Not Set Bias Empty To Peter and Philagot about your bias issues

    Post by michael.samra Sat Jul 24, 2021 10:01 pm

    I just looked at the OP again and realized it said your bias was .3v and not 3 volts. Yes, the twist loc cap will cause the drop in B+ and therefore you
    won't be able to bias it up to the 1.56v. You can also have a shorted EL34 or two that would load the B+. Check and see what your voltage is at pin 3 of each
    output tube. Now if the bias voltage was 3v and not .3vdc, then the selenium rect and the two 50uf axial lead caps need changing. I would change all those caps
    if they are original.


    Last edited by michael.samra on Sun Jul 25, 2021 1:41 am; edited 2 times in total (Reason for editing : I thought the bias was 3v and didn't see the decimal. I need reading glasses.)
    solderblob
    solderblob

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    Age : 75
    Location : Placerville, California

    ST-70 - Can Not Set Bias Empty Re: ST-70 - Can Not Set Bias

    Post by solderblob Sat Jul 24, 2021 10:59 pm

    Did you try swapping output tubes?

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    tuberadios69

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    Join date : 2012-09-02
    Age : 67

    ST-70 - Can Not Set Bias Empty Re: ST-70 - Can Not Set Bias

    Post by tuberadios69 Sat Jul 24, 2021 11:11 pm

    I had the same issue a few years ago and it was the multi-section twist-lok capacitor. If yours still has the original cap in it (on top of chassis), I'd replace it immediately. the two 50uv@75V bias caps should be replaced, too. I'd use 100uv@100v.

    michael.samra likes this post


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