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    Newbie with questions

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    timsst70

    Posts : 7
    Join date : 2015-11-03

    Newbie with questions

    Post by timsst70 on Thu Nov 05, 2015 3:20 pm

    Hi All,

    I'm updating a unit that was stock. Have changed out selenium rectifier, 50 MFD caps and changed resistors to trim bias properly. Next step is to replace Quad cap, no motivation other than I have the part, it came with a small schematic that shows a 2.2K resistor and other connections, no mention of the 6800 ohm resistor in existing circuit, I don't know what to do?

    Tim

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Newbie with questions

    Post by Bob Latino on Thu Nov 05, 2015 3:46 pm



    If you are using the VTA driver board, then you do not need the 6800 and the 22,000 ohm resistors on the quad cap. Just wire the quad cap as seen in the photo above with a jumper wire between the second and third sections of the quad cap and the 2200 ohm resistor between the third and fourth sections.

    Bob

    timsst70

    Posts : 7
    Join date : 2015-11-03

    Re: Newbie with questions

    Post by timsst70 on Thu Nov 05, 2015 4:33 pm

    Hi Bob,
    Thanks, not upgrading to the VTA driver board yet, still stock board, so, I'll follow your drawing.

    Tim

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Newbie with questions

    Post by Bob Latino on Thu Nov 05, 2015 5:15 pm

    timsst70 wrote:Hi Bob,
    Thanks, not upgrading to the VTA driver board yet,  still stock board,  so, I'll follow your drawing.

    Tim

    Tim,

    If you have the stock board then you must use the 22,000 ohm and the 6800 ohm resistors on the quad cap. Disregard the diagram at my post above. That 2200 ohm resistor is to be used only if you are using a VTA driver board in place of the stock board.

    Bob

    PeterCapo

    Posts : 380
    Join date : 2008-12-05

    Re: Newbie with questions

    Post by PeterCapo on Thu Nov 05, 2015 7:29 pm

    timsst70 wrote:Hi All,

    I'm updating a unit that was stock.  Have changed out selenium rectifier, 50 MFD caps and changed resistors to trim bias properly.  Next step is to replace Quad cap,  no motivation other than I have the part,  it came with a small schematic that shows a 2.2K resistor and other connections,  no mention of the 6800 ohm resistor in existing circuit, I don't know what to do?

    Tim

    Download a copy of Dynaco's original assembly manual and pictorial diagrams. Not sure but you might have to Google around a bit to find the pictorials.

    timsst70

    Posts : 7
    Join date : 2015-11-03

    Re: Newbie with questions

    Post by timsst70 on Thu Nov 05, 2015 9:50 pm

    Ok, I understand now and I have the original manual and circuit diagrams.

    Thanks

    timsst70

    Posts : 7
    Join date : 2015-11-03

    Re: Newbie with questions

    Post by timsst70 on Wed Nov 11, 2015 4:04 pm

    Hi,

    Is there any benefit to replace the Quad cap if I'm not having any issues with the amp as is? I take it that the 80,40,30,20 cap can replace the original according the to original circuit diagram. Thanks in advance for advice.

    Tim

    PeterCapo

    Posts : 380
    Join date : 2008-12-05

    Re: Newbie with questions

    Post by PeterCapo on Wed Nov 11, 2015 4:19 pm

    The consensus seems to be that the quad cap should be replaced if it is an original.  However, I think I will be a bit contrarian on this and say that as long as your quad cap is functioning properly, then it should not be necessary to replace it, though there is a question of reliability.  But, I don’t recall ever reading much about damage to the amp when they do fail.  Just make sure the fuse is 3A.

    A few years ago, I ran an original Stereo 70 for a while with its original quad cap, until the quad cap failed (my fault for toggling the power switch too quickly).  I replaced it with the 80/40/30/20 unit, which did sound better but not a total night and day improvement.

    The 80/40/30/20 unit should not just be dropped-in and wired the same as the 30/20/20/20 unit.  It should be wired in this sequence: 40/80/30/20.  I think Bob might have a sticky or something somewhere with the wiring details?

    j beede

    Posts : 316
    Join date : 2011-02-07
    Location : California

    Re: Newbie with questions

    Post by j beede on Wed Nov 11, 2015 9:52 pm

    How do you know whether or not your amp is exhibiting issues related to the quad cap? E.g. if you had to re-center the bias divider to achieve proper bias this can indicate a failed segment in your quad cap. What do you hope to achieve by increasing the capacitive load on your rectifier and choke?

    Keep in mind that basic capacitance ranges on a DVM measure at low voltage. A "good" cap at low voltage may measure otherwise at high voltage.

    PeterCapo

    Posts : 380
    Join date : 2008-12-05

    Re: Newbie with questions

    Post by PeterCapo on Wed Nov 11, 2015 10:09 pm

    I've referred to the voltage chart in the Dynaco manual, which includes reference voltage levels for each section of the quad cap, to know if the circuit is working properly when powered up. One of the more common symptoms of quad cap failure I've seen reported is an inordinate amount of hum.

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