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Sansui A-1100 Driver stage issue and distorted output

Discussion in 'Exclusively Sansui' started by Bassblaster, May 19, 2017.

  1. Bassblaster

    Bassblaster Super Member

    Messages:
    1,602
    Location:
    Cambridge, Ohio
    I think ill give it some new caps since those are cheap and if that fixes the issue then yay! if not then for all this amp is worth i can gamble with an eBay pack. at least the STK3102II isnt as common as say the 0050 so its less likely to be counterfeit. if that doesnt fix my issue then ill just part it out and most likely sell the other 3 pieces. this thing has a real meaty DC fan, heatsink and transformer.
     
  2. restorer-john

    restorer-john Super Member

    Messages:
    3,528
    Location:
    Australia
    Bassblaster, it isn't the caps.

    Check any ceramic emitter resistors, and if they are fine, the odds are still on the STK. Your oscilloscope shots prove that.

    I like your little DSO138 kit BTW. :)
     
  3. Bassblaster

    Bassblaster Super Member

    Messages:
    1,602
    Location:
    Cambridge, Ohio
    the FOUR emitter resistors test within 0.1 of their original 0.47 ohm rating. i dont know why they used 2 resistors per emitter. it looks like like this
    upload_2017-5-22_21-13-36.png
    IMG_0324[1].JPG
    haha thanks, you the first to know what it was.
     
  4. Bassblaster

    Bassblaster Super Member

    Messages:
    1,602
    Location:
    Cambridge, Ohio
    Well, what do you guys think about this piece of BPC? Anything else i should test or scope?
    EDITTTTT: I FORGOT the scope pics with the oscillation on post 1 was on pin ONE which in the INPUT pin. so the STK might be fine! we all kinda forgot about that...
     
    Last edited: May 25, 2017
  5. Bassblaster

    Bassblaster Super Member

    Messages:
    1,602
    Location:
    Cambridge, Ohio
    Well it magically works now. it was 3AM and i resoldered the entire driver board and swapped the outputs and it started working perfectly!

    Now since there's no way in hell i was going to get the emitter resistors back in under the heatsink, i just mounted them on the bottom. since this is just a piece of BPC and its not worth anything i didn't really care how it looks
    IMG_0361[1].JPG
    IMG_0358[1].JPG
    Plenty of room
    IMG_0359[1].JPG
    IMG_0362[1].JPG
     
  6. Bassblaster

    Bassblaster Super Member

    Messages:
    1,602
    Location:
    Cambridge, Ohio
    Okay need someone to look over and tell me if there is a way to have the fan run. this thing gets VERY hot after one song at around 50W output. i can only imagine if i was running it at full power. maybe i can stuff a 9V regulator somewhere :dunno:
     
  7. Bassblaster

    Bassblaster Super Member

    Messages:
    1,602
    Location:
    Cambridge, Ohio
    Maybe i should have ran it for more than 20 minutes to say its working perfectly. now the channel getting very distorted and going back to clear is now intermittent :/ it almost seems like a bad connection because SOMETIMES if i bang on it it will work, then go back to distorted. but ive already resoldered the entire PCB. if i push down on the heatsink right, it will work. then go back to distorted. ive never seen anything like this before. im lost with this. Maybe its the circuit glue?
    Man i wouldn't make a very good tech would i lol

    EDIT: The distortion IS affected by the speaker load if the power lights are anything to do by. when i shut the speaker button off the meters both indicate the same. but once i turn it back on it goes back down and the distortion resumes

    EDIT2: Headphones its working good, connect speakers i loose output and the distortion comes back. cant be the outputs as i swapped the left and right outputs and the right channel with the left outputs is working 100% no matter what.
     
    Last edited: May 28, 2017
  8. Bassblaster

    Bassblaster Super Member

    Messages:
    1,602
    Location:
    Cambridge, Ohio
    Okay i looked over the schematic again and see what appears to be discrete driver transistors after the STK pack and before the outputs. almost like its multistage
    upload_2017-5-30_2-43-14.png
    im going to swap those transistors with the other channel first thing tomorrow. 2:45AM for me right now
     
  9. llwhtt

    llwhtt Well-Known Member

    Messages:
    803
    Location:
    SoCal
    Although arranged a bit differently those transistors are the VI, voltage/current, limiters. When the voltage between the base and emitter is large enough they will turn on limiting the signal to the drivers. Most amplifiers will work fine without them for troubleshooting purposes.

    NEVERMIND, I was looking at lQ2/3 NOT kQ2/3. lQ2/3 are the limiters and kQ2/3 are indeed the drivers.

    Craig
     
    Last edited: Jun 15, 2017
  10. Bassblaster

    Bassblaster Super Member

    Messages:
    1,602
    Location:
    Cambridge, Ohio
    Last edited: Jun 14, 2017
  11. Bassblaster

    Bassblaster Super Member

    Messages:
    1,602
    Location:
    Cambridge, Ohio
    After looking at a photo i took it looks like on the drive board pad 4 had a cold joint. none of the other pads looks to be cold or shorted.
    i wonder if it took out anything else in the circuit
    upload_2017-6-14_21-49-37.png
     
    Last edited: Jun 15, 2017
  12. Bassblaster

    Bassblaster Super Member

    Messages:
    1,602
    Location:
    Cambridge, Ohio
    Bumperino...
     
    Last edited: Jun 17, 2017

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