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    Need help with my rebuilt ST-70

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    Awen

    Posts : 34
    Join date : 2015-01-21

    Need help with my rebuilt ST-70

    Post by Awen on Wed Jan 21, 2015 8:57 pm

    OK, so I bought a trashed Dynaco St-70 from this guy on craigslist and rebuilt it from the ground up. I have added new tube sockets, can cap, bias supply, tubes, choke, power transformer (Tomiko toroid), new output trannies (Tomiko), and a new board I got off the bay (China) that uses the 6GH8 tubes. With no input hooked up there is only a slight hum, but as soon as I hook up an input, there is a very very loud hum that is so loud that I quickly power off the amp out of fear of something bad happening. I have already tried two different new can caps, 3 new rectifier tubes, and checked and double checked all solder connections and made sure everything is wired correctly.With no input connected, if I as much as run my hand next to the input I get a loud hum. I am at my at the end of my rope with this thing and would really love to listen to it Mad

    GP49

    Posts : 732
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Need help with my rebuilt ST-70

    Post by GP49 on Thu Jan 22, 2015 12:35 am

    It sounds like you have not grounded the shields of the input jacks.

    And you really should post questions of this nature in the BASKET of the forum, not the PHOTOS section.
    Much more likely to get attention there...it's where most of the forum traffic is.

    MontanaWay

    Posts : 781
    Join date : 2014-02-06
    Age : 58
    Location : Brookings, Oregon

    Re: Need help with my rebuilt ST-70

    Post by MontanaWay on Thu Jan 22, 2015 12:48 am

    yes...Bob, can you 'transfer' this thread to the main section, thanks!

    Awen

    Posts : 34
    Join date : 2015-01-21

    Re: Need help with my rebuilt ST-70

    Post by Awen on Thu Jan 22, 2015 7:49 am

    GP49 wrote:It sounds like you have not grounded the shields of the input jacks.

    And you really should post questions of this nature in the BASKET of the forum, not the PHOTOS section.
    Much more likely to get attention there...it's where most of the forum traffic is.

    Oops, I didn't realize I posted in the photo section Embarassed I check and double checked the input grounds to be sure and they are grounded. I will go ahead and resolder them and see if that helps. I am really starting to suspect that one of the resistors on the board are bad. The left input reads 468K to ground and the right is 465K to ground, not sure if that makes a big difference or not. I should have spent the extra cash and got a decent board instead of the crap from China that I got installed.

    GP49

    Posts : 732
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Need help with my rebuilt ST-70

    Post by GP49 on Thu Jan 22, 2015 12:18 pm

    The input jacks may be grounded to the chassis; however in a stock Dynaco Stereo 70, each channel's signal input ground is connected from the circuit board to the input jacks via the untrimmed leads of two 10Ω resistors on the board. I've seen stock Dynaco boards where somebody thought the factory had forgotten to trim the leads, and cut them off. If your Chinese boards follow the same design, make sure those 10Ω resistor leads weren't trimmed, and that they are properly connected to the shields of the input jacks.

    tubes4hifi
    Admin

    Posts : 1282
    Join date : 2008-11-30

    Re: Need help with my rebuilt ST-70

    Post by tubes4hifi on Wed Jan 28, 2015 6:19 pm

    also make sure you have the polarity of the output transformers correct, if not, it will sound horrendous (positive feedback instead of negative feedback)

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