SX-828 pop when turning off the low filter and some interesting added switches

Discussion in 'Pioneer Audio' started by Blue Shadow, May 14, 2018.

  1. Blue Shadow

    Blue Shadow I gotta get me a new title

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    Picked up this 828 in rough shape and @dlucy dropped by and we gave it the once over. Since it has three extra switches added to the back panel we started by determining what they did.
    Back.jpg

    The first switch is above the preamp output/power amp input. This is what that switch controls. Takes the preamp signal and sends it either to the power amp or the preamp RCA jacks. Since this unit used a jumper bar with RCA pins, this mod makes those unnecessary since putting the signal on the Main Amp Input RCAs sends it to the amp section.

    The second and third switches, located between the ground and the A speaker outputs switch power. One DPST switch stops the power to the main caps, one side for - and one for +. The other switch interrupts power from the power supply board to the main board and to the protect board.

    The switch to the protect board interrupts PS Pin 7 to Protect Pin 11
    the other side of this switch interrupts PS Pin 17 to Main Amp Pin 23

    Sch 1.jpg

    Here is the Main amp circuit:
    Main Amp.jpg

    Here is the protect board:
    Protect Board.jpg

    Seems the amp and protect could be turned off along with the main PS caps. The unit runs on other power so when using it as a preamp only changes could be made.

    Anyone know why this might have been installed on this unit?


    But that is not why I'm writing this today. The unit has a pop when turning off the Low Filter and a bit when turning it on. Since it is not recapped, I'm thinking that is related. Maybe someone can point to the parts that might cause this even though the switch has been cleaned a couple/few times. The low filter is S4 a,b in the middle of this scat.

    AF board.jpg

    Thanks for any direction on this one.
     
    dlucy likes this.

     

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  2. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

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  3. gslikker

    gslikker Super Member

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    Any leakage of those 2.2 and 1 uF caps will result into a DC difference during moment of switching.
    Replace by film capacitors, no electrolytics.

    I presume it at least helps a bit.
     
  4. Blue Shadow

    Blue Shadow I gotta get me a new title

    Messages:
    20,375
    Location:
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    Well that worked. Now for the reason for those power supply disconnect switches.
     
  5. gslikker

    gslikker Super Member

    Messages:
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    Location:
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    just a guess, if it works as a preamp regardless of the switches, maybe the owner wanted the main amps not to consume power when in use as a preamp only? ....or.....or...who knows..;)
     

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