The Dynaco Tube Audio Forum

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


    VTA ST-70 Quad Cap Options

    Share

    thegage

    Posts : 7
    Join date : 2011-10-04

    VTA ST-70 Quad Cap Options

    Post by thegage on Wed Nov 28, 2012 7:02 pm

    I opened up my Latino ST-70 to do the rectifier diode mod and noticed a stain on the bottom panel below the quad cap. It looks like it's leaking a bit. I guess I should replace it, and if so, what's the best replacement?

    John K.

    tubes4hifi
    Admin

    Posts : 1292
    Join date : 2008-11-30

    Re: VTA ST-70 Quad Cap Options

    Post by tubes4hifi on Wed Nov 28, 2012 7:50 pm

    if it's one of our VTA amps, not likely over 4-5 years old, so very unlikely the quad cap has gone bad, but possible.
    DynaKitParts.com has replacements

    thegage

    Posts : 7
    Join date : 2011-10-04

    Re: VTA ST-70 Quad Cap Options

    Post by thegage on Wed Nov 28, 2012 7:57 pm

    Yes, it's one of Bob's kits, a bit over a year old. It's definitely leaking some electrolyte; the stain's probably the size of a nickel, and there's some residue on one of the lugs where it's been dripping. I'll check the cap's values.

    John K.

    Bob Latino
    Admin

    Posts : 2414
    Join date : 2008-11-26
    Location : Massachusetts

    Re: VTA ST-70 Quad Cap Options

    Post by Bob Latino on Wed Nov 28, 2012 9:02 pm

    thegage wrote:Yes, it's one of Bob's kits, a bit over a year old. It's definitely leaking some electrolyte; the stain's probably the size of a nickel, and there's some residue on one of the lugs where it's been dripping. I'll check the cap's values.

    John K.

    Hi John,

    Can you post a photo on the forum here of what the underside of that quad cap looks like ? If you have trouble posting the photo just send me the photo as an attached file to my regular Email address > Bob01605(at)aol(dot)com and I will post it up here so we can all see it ..

    I have never seen one of those quad caps leak before but there is always a first time ..

    Bob

    thegage

    Posts : 7
    Join date : 2011-10-04

    Re: VTA ST-70 Quad Cap Options

    Post by thegage on Wed Nov 28, 2012 9:23 pm

    Well, some additional information: Before doing the diode mod I went to check the tube bias and found there's hum in both speakers with the inputs disconnected. Also, I'm unable to get to full bias current on the tubes (400 mV) even trying two different sets of power tubes and tube rectifiers. Is this a sign of a bad quad cap, or something else? The amp's just been sitting on its shelf playing normally, though a couple months back I did have the Weber cap fail and take out the fuse. Maybe I have to go back through everything.

    I'll see what I can do about the photo.

    John K.

    Bob Latino
    Admin

    Posts : 2414
    Join date : 2008-11-26
    Location : Massachusetts

    Re: VTA ST-70 Quad Cap Options

    Post by Bob Latino on Wed Nov 28, 2012 9:33 pm

    Hi John,

    While you have the amp open to take a photo, with the amp on with all tubes and the rectifier in place, measure the DC voltage to chassis ground off pin #8 of the rectifier tube socket. A good quad cap and rectifier should give 410 + VDC. If the voltage is in the mid to lower 300 VDC range then either the rectifier tube or the quad cap is bad. Because you see evidence of leakage, it's probably the quad cap that is bad ..

    Bob

    thegage

    Posts : 7
    Join date : 2011-10-04

    Re: VTA ST-70 Quad Cap Options

    Post by thegage on Wed Nov 28, 2012 10:02 pm

    I tried two different brand new rectifiers, both read the same 325 VDC at pin 8. Then I guess it's the quad....

    John K.

    tubes4hifi
    Admin

    Posts : 1292
    Join date : 2008-11-30

    Re: VTA ST-70 Quad Cap Options

    Post by tubes4hifi on Wed Nov 28, 2012 10:43 pm

    yeah, that's bad! Should be around 410-420v, so it's definitely leaking.

    Sponsored content

    Re: VTA ST-70 Quad Cap Options

    Post by Sponsored content Today at 4:45 am


      Current date/time is Sat Dec 10, 2016 4:45 am