Xiang sheng 728a schematic

Discussion in 'Tube Audio' started by KenP, Jan 29, 2013.

  1. KenP

    KenP Well-Known Member

    Messages:
    622
    Location:
    Canada,Ontario
    I've gone through every scrap of info I could find, including the one here that shows a schematic but came up empty. The schematic that's shown here is really tiny and if you enlarge it can't see any info. Does someone have a better copy or perhaps a full copy showing layout on the pcb.

    Bought this thing used with a hum issue and it really sounds nasty and dirty, looked at the board and alot of solder splash and really poor components. If they are real then they probably didn't pass any type of QC approval, resistors look like they were made by a 5 here with playdow and so called mkp caps have such poor printing that I doubt if they are real.

    Any help is appreciated,thanks.
     
  2. gil41

    gil41 New Member

    Messages:
    3
    I have and been using this preamp for more than six months now and so far so good. For the schematic, I contacted the factory directly and was given an old schematic which they said is still applicable. I am attaching it here plus answer I got when I requested it. Contact info: xshn@xshn.com and Spencer was my contact. I've done only some tube rolling and I only hear some sort of hum when my speakers are so close (about a foot when testing some speakers) to it. I am a mechanical eng'r with very limited electrical/electronics background; thus, the schematic is only for a tech use in case I have an issue.

    I had an initial look when received and the boards and soldering in mine (I've done basic soldering only) looked ok. Please let me know what upgrade you will make on the components and could serve as a guide for me when I am ready for the component changes (caps, etc.).

    " Hi Gil,

    For the latest 728A preamp, we adopts the 12AU7, 12AT7 tubes. The schematic diagram is the old one, but could use as the reference for the latest preamp. Because we do not make so many differences.

    Have a nice day!

    Spencer"
     

    Attached Files:

    Last edited: Jan 29, 2013
  3. KenP

    KenP Well-Known Member

    Messages:
    622
    Location:
    Canada,Ontario
    I think mine is an older version, it doesn't have the large paper in oil russian caps instead it has the standard chinese mkp copies. I also noticed that it has a really big channel imballance. I got it cheap enough that I think I'll replace all the caps and all the resistors plus the volume pot, looks like an alps unit will fit right on the pcb.
     
  4. newbee123

    newbee123 New Member

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    20
    any one doing mods already?

    I just change the tube ....
     
  5. newbee123

    newbee123 New Member

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    20
    Here what I got from xiang sheng
     

    Attached Files:

    • 728a.jpg
      728a.jpg
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  6. KenP

    KenP Well-Known Member

    Messages:
    622
    Location:
    Canada,Ontario
    I changed all the caps panasonic for the big ones some silmic on the inputs and mundorfs for the bypass on the tubes, changed all the large resistors, some of the originals a way out of spec. changed the volume for an alps blue velvet, changed the wiring for the outputs to teflon coated silver wire. Thats about it for now, still no time to really listen to it.
     
  7. NOSValves

    NOSValves AK Subscriber Subscriber

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    I bet the changes you made will have a huge improvment if properly implemented...
     
  8. KenP

    KenP Well-Known Member

    Messages:
    622
    Location:
    Canada,Ontario
    DSC_2507.jpg Here's a pic of the internals.
     
  9. KenP

    KenP Well-Known Member

    Messages:
    622
    Location:
    Canada,Ontario
    Has anyone taken out the pcb for this pre? I need to know where the led for the power is connected on the bottom of it.
     
  10. newbee123

    newbee123 New Member

    Messages:
    20
    I think your and mine is little bit different
     
  11. KenP

    KenP Well-Known Member

    Messages:
    622
    Location:
    Canada,Ontario
    Mine is an older model before they used those large russian paper in oil caps. Look on the net for pics of the 728 and you'll see it. Aside from those caps only real difference is the pcb is split just behind the tone/volume/switching section.
     
  12. NOSValves

    NOSValves AK Subscriber Subscriber

    Messages:
    3,319
    I'd be upgrading every single part on that main circuit board carefully using parts for the Sonic benifits if it was mine. That parts on that board are like most Chinese made gear inexpensive crap. The nice thing about this piece is you have plenty of room to work with...
     
  13. newbee123

    newbee123 New Member

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    20
    let me know what is your upgrade and pls share it

    tks
     
  14. newbee123

    newbee123 New Member

    Messages:
    20
    It seems that what on the schematic is not the same with actual product.
    So many different after I trace out
     
  15. KenP

    KenP Well-Known Member

    Messages:
    622
    Location:
    Canada,Ontario
    There are many differences on the schematic, don't use it as a reference.
     
  16. newbee123

    newbee123 New Member

    Messages:
    20
    So far changing all electrolit cap with ELNA 2 and Nichicon
     

    Attached Files:

  17. KenP

    KenP Well-Known Member

    Messages:
    622
    Location:
    Canada,Ontario
    looks good so far, can you take a pic of the bottom of the board? I need to see where the led f.or the power connects
     
  18. newbee123

    newbee123 New Member

    Messages:
    20
    So far

    Change all capacitor and the cable
     

    Attached Files:

  19. KenP

    KenP Well-Known Member

    Messages:
    622
    Location:
    Canada,Ontario
    Looks good! you might consider changing some of the larger resistors too, I found the values sometimes differed greatly from what they should have been.

    I haven't looked at mine in a while now, once I get back to it it'll be to finish it off and sell it since I bought a shuguang s200mk pre and s845mk mono blocks.
     
  20. newbee123

    newbee123 New Member

    Messages:
    20
    What value you are referring with? Schematic?
     

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