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    st-70 rebuild hum

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    krebob

    Posts : 14
    Join date : 2011-01-08

    st-70 rebuild hum

    Post by krebob on Sat Jun 11, 2011 6:12 pm

    Hi Folks
    I rebuilt my st-70 about six weeks ago and have had a fairly loud hum since then that starts when the tubes warm up and doesn't vary with the volume.
    The rebuild consisted of:
    VTA board with k-40's and the cap upgrade, parts all installed inside the chassis
    new 80-40-30-20 quad cap w/80 oriented to the back of the amp
    removed front octals and mono switch
    put new bias tests near the OT's on top
    moved the power switch to the front, shielded the cable and drained it to the chassis grd.
    moved input jacks to the rear, shielded and drained to the rca grd

    After about two weeks the rectifier tube started to arc so I replaced it.
    voltage tests are good
    I have:
    gone through it and checked all the solder joints
    swapped tubes
    pulled the power switch and inputs out of the chassis.

    The only thing so far that stops the hum is pulling the center tube only vta board.
    Any help would be greatly appreciated.
    Thanks, Bob

    Sal

    Posts : 221
    Join date : 2009-02-05
    Location : Central New Jersey Dynaco-ST70.com

    Re: st-70 rebuild hum

    Post by Sal on Sat Jun 11, 2011 9:03 pm

    Bob,
    Did you use replace the 2 wire line cord with a 3 wire? Sometimes you will get a ground loop hum if connected to something with a ground wire. If you short the inputs do you still hear a hum?

    Take some clear photos of the top and bottom and post here, maybe one of us can figure out your problem.

    Sal

    krebob

    Posts : 14
    Join date : 2011-01-08

    Re: st-70 rebuild hum

    Post by krebob on Sun Jun 12, 2011 9:20 am

    Hi Sal
    When I short the inputs the hum stops!
    I still have a 2 wire line cord.
    I'm not able to post pictures at this point. I'll try to get that going later today.
    Thanks for your reply,
    Bob

    Bob Latino
    Admin

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

    Re: st-70 rebuild hum

    Post by Bob Latino on Sun Jun 12, 2011 11:19 am

    krebob wrote:Hi Sal
    When I short the inputs the hum stops!
    I still have a 2 wire line cord.
    I'm not able to post pictures at this point. I'll try to get that going later today.
    Thanks for your reply,
    Bob

    If you short the inputs and the hum stops that is usually an indication that the issue of the hum is not with the amp itself but some interaction of the amp with your associated equipment - be it cables, preamp etc.

    Bob

    krebob

    Posts : 14
    Join date : 2011-01-08

    Re: st-70 rebuild hum

    Post by krebob on Sun Jun 12, 2011 1:52 pm

    Hello
    I rearranged the cables and power cords and that helped alot.
    Then I remove the passive crossover for the Vandersteen subwoofer from between the preamp and the amp and that also helped quite a bit.
    Now all that's left is a minor hum that I can easily live with.
    Any ideas about the subwoofer crossover?
    I really like having it in the system.
    Thanks,
    Bob

    krebob

    Posts : 14
    Join date : 2011-01-08

    Re: st-70 rebuild hum

    Post by krebob on Sun Jun 12, 2011 4:24 pm

    Hey Folks
    I just blew my rectifier tube.
    Turned the amp off, then turned it back on before it had totally powered down.
    It arced then blew a fuse.
    Now it arcs every time I try to power it up.
    This is the second one in a month that I did this to.
    I'm learning some hard lessons here.
    Bob

    Bob Latino
    Admin

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

    Re: st-70 rebuild hum

    Post by Bob Latino on Sun Jun 12, 2011 4:36 pm

    Replace the quad cap
    Replace the fuse
    Replace the rectifier tube

    and ...

    You will probably have an amp with near zero hum that plays well ..

    Bob

    quad

    Posts : 14
    Join date : 2010-11-25

    Re: st-70 rebuild hum

    Post by quad on Tue Jun 14, 2011 7:59 am


    krebob

    Posts : 14
    Join date : 2011-01-08

    Re: st-70 rebuild hum

    Post by krebob on Tue Jun 14, 2011 9:03 am

    Thanks for the input.
    I've got a new gz34 and quad cap on the way and I'll put in a couple of in 4007's while I'm at it
    Bob

    sailor

    Posts : 269
    Join date : 2011-04-04

    diodes

    Post by sailor on Tue Jun 14, 2011 9:46 am

    You should install either diodes or the tube. But not both.

    Luddite

    Posts : 235
    Join date : 2009-02-04
    Age : 66
    Location : Texas

    Re: st-70 rebuild hum

    Post by Luddite on Tue Jun 14, 2011 11:34 am

    sailor wrote:You should install either diodes or the tube. But not both.

    Normally that's true, but in this application the diodes are intended to be used in conjunction with the rectifier tube.

    Best Regards,
    Charlie

    mantha3

    Posts : 298
    Join date : 2010-11-10

    Re: st-70 rebuild hum

    Post by mantha3 on Tue Jun 14, 2011 12:22 pm

    This Rectifier Mod is all over other forums. I actually got the lead on it from the guy at http://www.dynakitparts.com/. The main reason for this mod is the shorter life span of current production rectifiers (GZ34 in particular)

    You could run just the diodes... You want to run the rectifier tube as well I think to get the slow warm up. So the Tube just gets the pre-rectified volts. I'm running the mod and the amp sound great and is silent when idle.

    PS - Your issue with the hum. Did you try to simplify things? Just hook up a Right and Left speaker to the ST70. Then hook up a CD player to your amp. Don't turn the CD Player on or you will be blasting music! Just see with the speakers connected... And the CD Player connected direct to the amp but with the power OFF... Hum YES/NO?


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