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AU-7900 Restoration Question

Discussion in 'Exclusively Sansui' started by spark1, Nov 8, 2018 at 1:54 PM.

  1. spark1

    spark1 Super Member

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    I have an AU-7900 that I recapped last spring (along with new trimmers, new thermal compound, replacement of fuse resistors and a few other updates), but I did not do the "sandwiched" tone and filter boards at that time

    I'm getting around to them now, and given the relative difficulty of getting to them, I was thinking that perhaps I should just go ahead and replace the 2SC1313 and 2SA726 transistors at the same time.

    Any downsides to doing this? The unit sounds quite good, with no noticeable noise or distortion. My goal with the recap and other work was/is preventative maintenance, as I intend to keep this unit over the long run and then pass it on to one of my kids.

    Thank you!
     

     

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  2. slimecity

    slimecity Super Member

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    You will strike varying opinions on shotgunning those - but if it were me - I would do what you suggest.

    Its straighforward to replace these all with modern KSC1845 & KSA992 - observing correct pinout. I would do this especially if these boards are hard to access.
     
  3. spark1

    spark1 Super Member

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    I am confident in my ability to do the work correctly, so leaving that issue aside, there are no potential downsides to wholesale replacement on these two boards?
     
  4. lbcgav

    lbcgav AK Subscriber Subscriber

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    I ran into trouble on the flat amp of my AU-919 with the exact transistors you mention. I'd replace all of them with their modern equivalents.
     
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  5. spark1

    spark1 Super Member

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    Well, it appears that the Sansui gurus see no reason not to do it, so I'll go ahead and replace them all while I have access to the boards.

    Thanks! Hope to get it done this weekend, since I have all the parts on hand and the forecast is for crazy cold weather here.
     
  6. Hipocrates

    Hipocrates Anti-Muppet Subscriber

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    Kale and Hyperion talked about the 2SC1313 here
    http://audiokarma.org/forums/index....sansui-transistor-substitutions.583009/page-3
    They seems to agree that KSC1815 is a better option.
    I have used both and they works pretty good. But since I really like their work, I ended using KSC1815.
    More important and just in case (you probably know this already)
    Those old 2SC1313 are BCE, sometimes you find some datasheets on the web that states that they are ECB.
     
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  7. spark1

    spark1 Super Member

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    Thanks for taking the time to respond. The "Sansui transistor substitution" threads show the 1845 as the sub for the 1313, so it looks like I should do some additional homework on this one.

    I've got plenty of 1845s on hand, but will have to place another order to get enough 1815s.

    Similarly, is there not a consensus on the 2SA992 as the best sub for the 2SA726?

    Thanks!
     
  8. blhagstrom

    blhagstrom Mad Scientist, fixer. Subscriber

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    Any time you access a board in vintage for any reason, replacing e-caps and known/suspected problem maker transistors is a smart move.

    Modern manufacturing processes are so much better than the Stone Age of the 60s and 70s and even 80s that there is about no need to worry about “best” replacements.

    FWIW, much of electronics has a “close enough” factor. Digital stuff required much tighter tolorance and accuracy and from the improvements made there, the parts we have now are so much better.

    There was only one time where I hit a problem with replacements and that was because they were “too” good. The gain was high enough to cause the amp to start oscillating. They were replacements in the amp where things are a little more fussy.
     
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  9. Hipocrates

    Hipocrates Anti-Muppet Subscriber

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    KSC1845/KSA922 will do the job... just watch out for the pinout.
     
  10. spark1

    spark1 Super Member

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    Thanks. I tried to find outo why the 1815 would be recommended over the 1845, but could not find any threads with specifics.
     
  11. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    The '1815 is a little better current wise, so when I find a 2SC1313 in a PSU, I tend to use a '1815 to replace it - nothing more complicated than that. It is still a nice 'quiet' medium high gain transistor so it can be used in pre-amps for example, but for pre-amp use, overall I think the '1845 is better suited.
     

     

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  12. spark1

    spark1 Super Member

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    Good to know the basis of the recommendation of the 1815 over the 1845. Looks like the 1845 is fine for my application. As I'm pretty stocked up on parts right now, I'm glad I don't need to place another small order just for 1815's (altho I always find other stuff to buy when ordering!).
     
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  13. blhagstrom

    blhagstrom Mad Scientist, fixer. Subscriber

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    The 1845 was too high of gain in the old Kenwood I was thinking of before. The 1815 worked perfectly. Just a FYI
     
  14. Zekeman

    Zekeman Super Member

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    I have seen quite a few 7900's with noisy tone board transistors so I always replace them now.

    Cheers!
     
  15. spark1

    spark1 Super Member

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