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    ST-70 VTA-70 Standard 30-20-20-20 quad cap connections

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    richtp

    Posts : 31
    Join date : 2011-11-18
    Location : Sterling MI

    ST-70 VTA-70 Standard 30-20-20-20 quad cap connections

    Post by richtp on Wed Nov 26, 2014 10:40 am

    I am sure it's been asked before but I have an ST-70 project that been sitting unfinished for 3 years, that I have decided to finally finish.

    I have a VTA-70 driver board and do not have the documentation for it.I am installing it with a new 30/20/20/20 quad cap. I just want to make sure I am hooking it up properly. I am using Bob's VTA rebuild instructions (thank you!) The instructions have the upgrade quadcap. (Wish I would have ordered that instead)

    The 30mfd section D symbol should have 2 wires terminal 8 from rectifier and a lead from the choke?

    The 20mfd section with the square should have one wire going to b plus on the VTA board and one side of the 2.2k resistor?

    The 20mfd section with the triangle the other side of the 2.2k resistor and a jumper to the last unmarked section of the quadcap?

    The 20mfd with no mark should have 3 wires plus the jumper,one red from each output transformer and the remaining lead from the choke?
    Is this correct? Also is there and big disadvantage of using the 30/20/20/20 vs the higher capacity upgrade?

    Thanks in advance.

    Bob Latino
    Admin

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

    Re: ST-70 VTA-70 Standard 30-20-20-20 quad cap connections

    Post by Bob Latino on Wed Nov 26, 2014 11:14 am

    richtp wrote:I am sure it's been asked before but I have an ST-70 project that been sitting unfinished for 3 years, that I have decided to finally finish.

    I have a VTA-70 driver board and do not have the documentation for it.I am installing it with a new 30/20/20/20 quad cap. I just want to make sure I am hooking it up properly. I am using Bob's VTA rebuild instructions (thank you!) The instructions have the upgrade quadcap. (Wish I would have ordered that instead)

    The 30mfd section D symbol should have 2 wires terminal 8 from rectifier and a lead from the choke?

    The 20mfd section with the square should have one wire going to b plus on the VTA board and one side of the 2.2k resistor?

    The 20mfd section with the triangle the other side of the 2.2k resistor and a jumper to the last unmarked section of the quadcap?

    The 20mfd with no mark should have 3 wires plus the jumper,one red from each output transformer and the remaining lead from the choke?
    Is this correct? Also is there and big disadvantage of using the 30/20/20/20 vs the higher capacity upgrade?

    Thanks in advance.

    If you have an original type Dynaco 30, 20, 20, 20 quad cap - the 30 section should face the LEFT side of the amp towards the rectifier tube which leaves you with the three 20 sections facing the back, right side and the front of the amp. Set up for the VTA driver board you should have ....

    The 30 section facing left should have a wire coming from pin 8 of the rectifier tube socket and one choke lead
    The 20 section facing the back of the amp should have the other choke lead, two red wires - one from each output transformer and one end of a jumper wire.
    The 20 section facing the right side of the amp should have the other end of the jumper wire and one lead of the 2200 ohm resistor
    The 20 section facing the front of the amp should have the other end of the 2200 ohm resistor and one end of a wire that will eventually be connected to the "B+" eyelet on the driver board.

    The advantage of using the 80, 40, 30, 20 uF cap is that you can use the 80 section as the section that powers the two output transformers. (as the section that faces the rear of the amp). Using only a 20 section (as Dynaco did) there will cause the amp to "starve" for stored B+ when the amp is played loudly. The amp will play fine at lower and normal volume levels. The clue that the amp is starving for some extra B+ storage is that bass notes played at high volume levels will "crackle" a little.

    Bob

    richtp

    Posts : 31
    Join date : 2011-11-18
    Location : Sterling MI

    Re: ST-70 VTA-70 Standard 30-20-20-20 quad cap connections

    Post by richtp on Wed Nov 26, 2014 11:36 am

    Thanks Bob! That is how I have it. Just wanted to make sure before I soldered the leads.I guess I will be ordering the larger cap soon. I do have a tendency to play things a bit loud (:

    sailor

    Posts : 269
    Join date : 2011-04-04

    Re: ST-70 VTA-70 Standard 30-20-20-20 quad cap connections

    Post by sailor on Wed Nov 26, 2014 11:59 am

    Why not download a copy of the original owners manual for yourself. It's in the sticky section above. Full set of kit instructions and schematics.

    richtp

    Posts : 31
    Join date : 2011-11-18
    Location : Sterling MI

    Re: ST-70 VTA-70 Standard 30-20-20-20 quad cap connections

    Post by richtp on Wed Nov 26, 2014 1:17 pm

    I have the original Dynaco instructions. I just don't have the VTA-70 board instructions anymore. Just wanted to make sure there was no other changes to the quad cap hook up when using the VTA board with the standard quad cap.
    Thanks

    MexicoMike

    Posts : 28
    Join date : 2014-11-21

    Re: ST-70 VTA-70 Standard 30-20-20-20 quad cap connections

    Post by MexicoMike on Fri Nov 28, 2014 9:48 am

    I have a VTA board but have not yet assembled/installed it. I want to run the ST70 in original form for a while. I did purchase one of the 80/40/30/20 caps but found that the standard fast-blo fuse would blow sometimes. I tried for a couple of days that way and it would sometimes run just fine for several hours, then on the next turn-on, pop the fuse. A new fuse would be fine for a turn-on or two but then fail on a subsequent turn on. Perhaps a slo-blo would have worked OK and Dynaco even recommended that if you were having problems with blowing fuses regularly.

    I sent an email to Roy re that and he suggested that perhaps I had installed it incorrectly orientation-wise. But I checked it very carefully multiple times before powering up the first time. Also, I concluded - perhaps erroneously - that since the amp would run fine for hours if the fuse didn't blow on start-up, the current inrush for that capacitor was too much for the oem system to reliably accept.

    So I purchased a new oem cap from Dynakit which is in the amp now I will install the 80 when I decide to install the VTA board.

    richtp

    Posts : 31
    Join date : 2011-11-18
    Location : Sterling MI

    Re: ST-70 VTA-70 Standard 30-20-20-20 quad cap connections

    Post by richtp on Mon Dec 15, 2014 10:02 pm

    I ended up adding 160uf to the output transformer section and it made a noticeable difference at higher volume levels.Thanks for the advice!
    This site is a great resource. I have spent more time than I should reading through many posts and have learned a lot about my ST-70's.

    Bob Latino
    Admin

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

    Re: ST-70 VTA-70 Standard 30-20-20-20 quad cap connections

    Post by Bob Latino on Tue Dec 16, 2014 7:47 am

    MexicoMike wrote:I did purchase one of the 80/40/30/20 caps but found that the standard fast-blo fuse would blow sometimes.  I tried for a couple of days that way and it would sometimes run just fine for several hours, then on the next turn-on, pop the fuse.  A new fuse would be fine for a turn-on or two but then fail on a subsequent turn on.  Perhaps a slo-blo would have worked OK and Dynaco even recommended that if you were having problems with blowing fuses regularly.

    The STANDARD fuse on a Dynaco ST-70 or a VTA ST-70 is a 3 amp SLO-BLO fuse not a fast-blo fuse. A slo-blo fuse is designed to take the initial turn on surge which may go over 3 amps for a brief instant. A fast-blo fuse will blow as soon as the current drawn goes over 3 amps. Probably when you bought the amp, the previous owner had a fast-blo fuse in the amp because he ran out of slo-blo fuses. The 80, 40, 30, 20 quad cap will work fine if you use a slo-blo fuse AND make sure that the 40 section is the section that is connected to pin #8 of the rectifier tube.

    Bob

    MexicoMike

    Posts : 28
    Join date : 2014-11-21

    Re: ST-70 VTA-70 Standard 30-20-20-20 quad cap connections

    Post by MexicoMike on Tue Dec 16, 2014 10:17 am

    Thanks Bob,

    I was confused by the Dynaco ST70 instructions - in the trouble shooting section on page 9 under "Fuse Failure," it mentions that if the amp fuse blows regularly, to try a slo-blow. In reading that I ASSUMED that meant that a fast blow was the "normal" fuse. But the parts list shows that a slo-blow is supplied with the kit, and therefore, as you pointed out, is what should be in the amp.

    Seems odd that the troubleshooting instructions would suggest trying a slo blow when, in fact, that's what the amp came with.

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