Sansui A-1100 Driver stage issue and distorted output

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

  1. Bassblaster

    Bassblaster Super Member

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    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

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    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

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    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

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    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

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    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

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    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

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    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

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    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

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    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

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    Last edited: Jun 14, 2017
  11. Bassblaster

    Bassblaster Super Member

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    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

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    1,648
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    Bumperino...
     
    Last edited: Jun 17, 2017
  13. Bassblaster

    Bassblaster Super Member

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    since i go no replies i went ahead and ordered all my parts. driver board recap and new transistors and made sure all the stupid pads were soldered. found out the main 7A fuse blew when the driver board blew...only had a 3A that i put in. went to power it up for the first time on a DBT with a 100W bulb and it lit full brightness and NO lights on the Sansui lit. could really use some help with this now. going to pull the outputs again. maybe they shorted and thats what took out the driver board.
     
  14. Bassblaster

    Bassblaster Super Member

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    removed all 4 output transistors and was able to get it to power on, and more parts on the driver board went up in smoke. 100W bulb was about a quarter lit.
    EDIT: Tested all 4 outputs on my transistor tester and BOTH on the right channel are completely cooked. why me
     
    Last edited: Jul 7, 2017
  15. Overundr1

    Overundr1 AK Subscriber Subscriber

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    Been following along but not chiming in, would be VERY suspicious of the bias circuit, the power supply voltages (as shown by the missing negative portion of your scope trace) and then once that is checked the oscillation issue. Working with stk outputs is a real pita :(
     
  16. Bassblaster

    Bassblaster Super Member

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    It uses discrete transistors on the outputs, but an STK pack as the driver. What should I do with to attempt to get it working again. Could it be the exposed bias adjustment pots?:dunno:
     
  17. Bassblaster

    Bassblaster Super Member

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    1,648
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    So after testing more parts the right channels outputs A1106/C2581 are shorted. replace with NJW1302G/NJW3281G
    IMG_0798[1].JPG IMG_0799[1].JPG


    Right channel drivers are also completely shorted A1111/C2591 replace with MJE15033G/MJE15032G

    the 2 resistors that went into smoke were on the F-4762 driver board R12 47 Ohm and R13 47 Ohm.

    In the pic you can see the bias pots are those crappy open style ones. ill figure out the value and replace those as well.
    IMG_0789[1].JPG

    I Think everything is on the right track now.
     
    Last edited: Jul 8, 2017
    Overundr1 likes this.
  18. Bassblaster

    Bassblaster Super Member

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    Oh boy....found another fried transistor but thats not the bad part. its the thermal comp transistor mounted on the heatsink, bad thing is i have no idea how to get them out...so i cant see what they are and what to replace them with. but i found out the trimmer resistors are 1K
    IMG_0834[1].JPG

    Testing them gives me this
    LEFT
    IMG_0832[1].JPG

    RIGHT
    IMG_0833[1].JPG
     
    Last edited: Jul 10, 2017
  19. Bassblaster

    Bassblaster Super Member

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    1,648
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    i got it completely torn down to access those temp comp transistors mounted to the heatsink. Glad to say they are bone standard C1845's which i have almost 100 of. so i just replaced then right there

    Originals
    IMG_0870[1].JPG
    IMG_0872[1].JPG

    NEW, a little crooked but i straightened them up before i reinstalled the main PCB
    IMG_0873[1].JPG
     
    Last edited: Jul 13, 2017
  20. Bassblaster

    Bassblaster Super Member

    Messages:
    1,648
    Location:
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    More parts came in today.
    IMG_1024[1].JPG
    Ill get these installed and hope all is good, there's more than those transistors of course, but those are the main goodies
     

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