The Dynaco Tube Audio Forum

Would you like to react to this message? Create an account in a few clicks or log in to continue.
The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


4 posters

    ST70 chassis hum

    avatar
    breading


    Posts : 45
    Join date : 2010-01-08

    ST70 chassis hum Empty ST70 chassis hum

    Post by breading Tue Aug 27, 2019 1:44 am

    My ST70 has just recently developed a mechanical chassis hum (not from the speakers, but from the chassis itself).  I have tightened all transformer screws in addition to other socket screws, etc. to no avail.  I have swapped tubes with no improvement.  The hum appears to be 60 cycle in nature.

    Interestingly, when I pick the amp up and hold it a few inches off the cabinet shelf the hum disappears.

    The hum is very noticeable when the unit is muted, but masked when operating the system.  

    Is there a solution?   Isolation pads, etc.?

    Thanks for any suggestions.

    Bruce
    avatar
    mijohn


    Posts : 119
    Join date : 2013-06-19

    ST70 chassis hum Empty Re: ST70 chassis hum

    Post by mijohn Tue Aug 27, 2019 4:56 am

    It'll be mechanical hum from the power transformer being transmitted through the chassis.
    See Bob Latino's answer to this question in this link:

    https://dynacotubeaudio.forumotion.com/t3524-mechanical-hum-power-transformer
    avatar
    breading


    Posts : 45
    Join date : 2010-01-08

    ST70 chassis hum Empty Re: ST70 chassis hum

    Post by breading Tue Aug 27, 2019 12:45 pm

    mijohn wrote:It'll be mechanical hum from the power transformer being transmitted through the chassis.
    See Bob Latino's answer to this question in this link:

    https://dynacotubeaudio.forumotion.com/t3524-mechanical-hum-power-transformer

    The power transformer, which I purchased from Dynakitparts, has the isolation washers so that is not the source of the problem. Though I thank you for the suggestion.

    Bruce
    avatar
    Guest
    Guest


    ST70 chassis hum Empty Re: ST70 chassis hum

    Post by Guest Tue Aug 27, 2019 8:17 pm

    .


    Last edited by PeterCapo on Mon Nov 30, 2020 8:33 pm; edited 1 time in total
    avatar
    mijohn


    Posts : 119
    Join date : 2013-06-19

    ST70 chassis hum Empty Re: ST70 chassis hum

    Post by mijohn Wed Aug 28, 2019 3:57 am

    "Interestingly, when I pick the amp up and hold it a few inches off the cabinet shelf the hum disappears."

    Breading, you could try isolating your amp from the cabinet with the sort of thing in this link, others may have recommendations of similar products.

    https://www.audioadvisor.com/products.asp?dept=66
    deepee99
    deepee99


    Posts : 2244
    Join date : 2012-05-23
    Location : Wallace, Idaho

    ST70 chassis hum Empty Re: ST70 chassis hum

    Post by deepee99 Wed Aug 28, 2019 10:53 am

    Breading,
    I'm going to take a SWAG here, that your rubber mounting washers have shrunk a bit.  And when you're picking the amp up you're flexing it -- not much flex to one of Bob L's 10-gauge chassis, but it will flex, no matter minutely.
    One thing I would suggest is just retorquing the transformer's hold-down hardware. Tighter isn't always better; sometimes backing off turns the trick, let those isolation washers puff out a bit. I would loosen all four bolts and (assuming you're working from the top and not the bottom of the top deck), turn the bugger on and torque the bolts down in a pattern until the hum quits and when it does quit, you're noise-free. Don't tighten them any further; that big tranny ain't going anywhere.
    avatar
    Guest
    Guest


    ST70 chassis hum Empty Re: ST70 chassis hum

    Post by Guest Wed Aug 28, 2019 11:01 am

    .


    Last edited by PeterCapo on Mon Nov 30, 2020 8:32 pm; edited 1 time in total
    cci1492
    cci1492


    Posts : 331
    Join date : 2016-05-09
    Age : 64
    Location : NJ

    ST70 chassis hum Empty Re: ST70 chassis hum

    Post by cci1492 Wed Aug 28, 2019 12:09 pm

    What if you placed each leg of the amp on a small rag to stop the vibration from being amplified on the table and then with a mechanic's stethoscope (carefully!!!) poke around to locate the noise. But yeah, you don't want 500V going through your head so be careful.

    Sponsored content


    ST70 chassis hum Empty Re: ST70 chassis hum

    Post by Sponsored content


      Current date/time is Wed Dec 04, 2024 7:49 pm