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    Mod to ST70 - what's the use of extra capacitor?

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    maninmac771

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    Join date : 2011-03-07

    Mod to ST70 - what's the use of extra capacitor?

    Post by maninmac771 on Mon Mar 21, 2011 1:19 am

    Recently, I had the time to go through the assembly manual of my ST70 and also took a look of it's wiring under the chasis. I found that there is a capacitor (450V 100uF(m))(as shownn in the photo) connected between lug #1 of the filter cap and the chasis (ground). This capacitor does not appear in the wiring schematic of the manual. So, it appears that this is a modification by the previous owner. Anyone knows what is this capacitor for? Should it be disconnected? Thanks in advance for your help.
    [img][/img]

    Bob Latino
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    Re: Mod to ST70 - what's the use of extra capacitor?

    Post by Bob Latino on Mon Mar 21, 2011 6:01 am

    Hi,

    Your photo is pretty small and I can't see exactly which lug the cap is connected to ... Any "helper capacitor" on the ST-70 should be placed on the lug that faces the back of the amp. That is the lug that powers the two output capacitors. The general rule is that the lug that faces the rectifier tube should be no larger than 40 uF but the next cap in line (after the choke) can be made larger with the addition of another cap (or two). The only thing you must be aware of is that the ST-70 operates at 400 - 440 VDC on that section that faces the rear of the amp and the use of a 450 volt rated cap does not give much margin for safety. Dynaco's original quad cap was 525 volt rated.

    Bob

    maninmac771

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    Join date : 2011-03-07

    Re: Mod to ST70 - what's the use of extra capacitor?

    Post by maninmac771 on Tue Mar 22, 2011 1:46 am

    Hi Bob. I've been trying to include a larger picture for better view but so far not successful. Anyway, I'll try to describe it in more detail. The +ve thread of the capacitor in question is connected by a short wire to the quad cap's lug facing the back of the amp (this lug is shown as #1 in the ST70's assembly manual) and the -ve thread of the capacitor is connected directly to one of the two solder lugs - the main grounding point of the amp - next to the quad cap. Hope this would help to determine whether this 450V capacitor should stay connected or need to be discarded. Thanks.

    Bob Latino
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    Re: Mod to ST70 - what's the use of extra capacitor?

    Post by Bob Latino on Tue Mar 22, 2011 6:05 am

    From your description, that cap is connected to the correct lug and grounded correctly ... I would just leave it in there.

    Bob

    maninmac771

    Posts : 30
    Join date : 2011-03-07

    Re: Mod to ST70 - what's the use of extra capacitor?

    Post by maninmac771 on Wed Mar 23, 2011 3:05 am

    Thanks Bob. So, this "helper capacitor" is there to augment the two 50MFD output capacitors and hence improve performance. While disconnecting this capacitor will not diminish performance, will there be better results and larger margin for safety if this "helper" is replaced by one with higher voltage and larger uF? If yes, what would you recommend?
    BTW, on closer cross examination with the wiring schematic of ST70's assembly manual, I found that there are a few wrong connections, namely lugs #1 & #8 of V6 are not connected and the wire from point 16 of the PC driver board is connected to lug #2 and NOT lug #1 of the right power take off socket. I really look forward to hearing any difference when these are corrected. Since the majority of ST70 are not factory-wired, I guess it is necessary for a new owner to take a closer look at the wiring of his newly-acquired ST70.

    Bob Latino
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    Re: Mod to ST70 - what's the use of extra capacitor?

    Post by Bob Latino on Wed Mar 23, 2011 6:01 am

    JJ Electronics makes a 32 + 32 uF @ 500 volt capacitor that you could put in there as a helper capacitor. The capacitance is a little lower but the voltage rating is 50 volts higher than what you have in there now.

    Bob

    maninmac771

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    Join date : 2011-03-07

    Re: Mod to ST70 - what's the use of extra capacitor?

    Post by maninmac771 on Fri Mar 25, 2011 1:34 am

    I clicked into JJ Electronics www.jj-electronics.com and found the 500V 32 + 32 uF capacitor (model TC529) but couldn't find the price. Do you know how much? By the way, can I upgrade the existing two 50uF output capacitors to ones with higher voltage and do away with the "helper capacitor"? Thanks for your time Bob.

    Captain Coconut

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    Re: Mod to ST70 - what's the use of extra capacitor?

    Post by Captain Coconut on Fri Mar 25, 2011 9:12 am

    Parts Connexion has that cap for $9.95

    tubes4hifi
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    Re: Mod to ST70 - what's the use of extra capacitor?

    Post by tubes4hifi on Fri Mar 25, 2011 11:40 pm

    the JJ 50+50 cap is $8.50 at www.tubesandmore.com and shipping is probably less money also
    part number is C-EC32-32-500

    maninmac771

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    Re: Mod to ST70 - what's the use of extra capacitor?

    Post by maninmac771 on Sat Mar 26, 2011 12:46 am

    Thanks guys for the supply info of the cap. But I'd still like to know if in fact similar improvement can be obtained by upgrading the two 50uF output capacitors and then disconnecting the "helper capacitor". Thanks.

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