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  #1  
Old 01-29-2013, 06:05 AM
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KenP KenP is offline
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Xiang sheng 728a schematic

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.
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Old 01-29-2013, 12:53 PM
gil41 gil41 is offline
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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
File Type: doc 728A schematic.doc (57.0 KB, 211 views)

Last edited by gil41; 01-29-2013 at 12:58 PM.
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  #3  
Old 01-29-2013, 05:41 PM
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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.
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Old 03-01-2013, 01:38 PM
newbee123 newbee123 is offline
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any one doing mods already?

I just change the tube ....
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  #5  
Old 03-01-2013, 01:54 PM
newbee123 newbee123 is offline
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Here what I got from xiang sheng
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File Type: jpg 728a.jpg (88.6 KB, 144 views)
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Old 03-01-2013, 02:42 PM
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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.
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Yamaha NS-1000M,Ls3/5a,Marantz HD-880
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Old 03-01-2013, 03:07 PM
NOSValves NOSValves is offline
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I bet the changes you made will have a huge improvment if properly implemented...
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http://www.nosvalves.com/images/engrave2.jpg
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Old 03-01-2013, 08:11 PM
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DSC_2507.jpgHere's a pic of the internals.
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Micromega stage1,phillips cd650, nexus adp32a
Yamaha NS-1000M,Ls3/5a,Marantz HD-880
Sony Ps-x70
Harmon Kardon HK400XM
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Old 03-01-2013, 08:15 PM
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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.
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Micromega stage1,phillips cd650, nexus adp32a
Yamaha NS-1000M,Ls3/5a,Marantz HD-880
Sony Ps-x70
Harmon Kardon HK400XM
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  #10  
Old 03-02-2013, 12:21 AM
newbee123 newbee123 is offline
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Quote:
Originally Posted by KenP View Post
Attachment 413985Here's a pic of the internals.
I think your and mine is little bit different
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Old 03-02-2013, 06:16 AM
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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.
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Micromega stage1,phillips cd650, nexus adp32a
Yamaha NS-1000M,Ls3/5a,Marantz HD-880
Sony Ps-x70
Harmon Kardon HK400XM
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  #12  
Old 03-02-2013, 07:30 AM
NOSValves NOSValves is offline
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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...
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NOS Valves NBS Preamplifier l NOS Valves VRD mono blocks l Sota Jewel w/RB-300 & Denon DL-103R l Sony SCD-333ES CD/SACD l Radio X Tuner modified Sony l Modified Lascala Baby (known world wide as Frankenscala)
http://www.nosvalves.com/images/engrave2.jpg
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Old 03-02-2013, 11:20 PM
newbee123 newbee123 is offline
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Quote:
Originally Posted by NOSValves View Post
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...
let me know what is your upgrade and pls share it

tks
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  #14  
Old 03-06-2013, 06:53 PM
newbee123 newbee123 is offline
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It seems that what on the schematic is not the same with actual product.
So many different after I trace out
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Old 03-06-2013, 07:01 PM
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There are many differences on the schematic, don't use it as a reference.
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Micromega stage1,phillips cd650, nexus adp32a
Yamaha NS-1000M,Ls3/5a,Marantz HD-880
Sony Ps-x70
Harmon Kardon HK400XM
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