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    ST-70 Restoration first Power-Up: Blowing fuses........any advice please??

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    Doctor Hugocat

    Posts : 92
    Join date : 2015-08-19
    Location : Los Angeles CA

    ST-70 Restoration first Power-Up: Blowing fuses........any advice please??

    Post by Doctor Hugocat on Sun Nov 20, 2016 1:49 am

    Hi Guys -

    Well I finished my St-70 (VTA PCB) project.....at last.

    I went to power it up tonight and I have a problem. Can anyone advise, please?

    I put a 1amp fast-blow fuse in, and have been following the instructions that came with my VTA Board. I put in JUST the driver tubes first, and they glowed. I powered off, removed the driver tubes and put in the rectifier and Right hand power tubes. The fuse blew. I put in another 1amp and that blew too. I changed the power tubes, tried once more. Blew.

    The rectifier is a NOS and tested good on my borrowed tube tester.

    Can anyone help?? I went over and over the wiring before powering up, and I don't think I've made a mistake. EVERYTHING except output trannys is brand new. THANKS!!
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    Tubes4ever

    Posts : 86
    Join date : 2015-07-14
    Location : Star, Idaho

    Re: ST-70 Restoration first Power-Up: Blowing fuses........any advice please??

    Post by Tubes4ever on Sun Nov 20, 2016 2:18 am

    The ST70 should have a 3 amp slow blow fuse. 1 amp is too small.

    Tim L.
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    Doctor Hugocat

    Posts : 92
    Join date : 2015-08-19
    Location : Los Angeles CA

    Re: ST-70 Restoration first Power-Up: Blowing fuses........any advice please??

    Post by Doctor Hugocat on Sun Nov 20, 2016 2:37 pm

    Thanks Tim!

    I replaced the 1a with a 3a slow-blow and I just successfully set the LH bias. Gotta go out now but with set RH later. Super-excited!!

    I read somewhere to do first power up with a 1a: I didn't know when to replace that with a 3a.

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    Peter W.

    Posts : 411
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: ST-70 Restoration first Power-Up: Blowing fuses........any advice please??

    Post by Peter W. on Sun Nov 20, 2016 3:53 pm

    Doctor Hugocat wrote:Thanks Tim!  

    I replaced the 1a with a 3a slow-blow and I just successfully set the LH bias. Gotta go out now but with set RH later. Super-excited!!

    I read somewhere to do first power up with a 1a: I didn't know when to replace that with a 3a.


    If one uses a metered variac and has a light touch, a 1A fuse is *just* feasible to start a 70. But, not for actually running it. And I use a very light fuse to test radios after restoration and/or repair - but without the tubes in it. This tests the most basic wiring, especially the transformer. But once the tubes are in, or the rectifier starts to pass B+, that is when the real load begins.

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    Re: ST-70 Restoration first Power-Up: Blowing fuses........any advice please??

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