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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 decided to pull out the Sansui A-1100 then been sitting on my shelf for over a year.

    This model uses the STK3102II for the driver. whats going on it the right channel works perfectly no issues at all. Left channel is intermittently VERY distorted or doesn't work at all. I sat down with the scope and found the left channel is oscillating between 60 and 85ish kHz. i tested the input pins of the STK driver and the oscillation is on the left input so the issue has to be before the driver. when i tap the F-4773 control board (page 6 in the service manual) with a screw driver the signal cuts out, gets softer and louder, most likely a bad solder joint somewhere.

    SERVICE MANUAL - https://www.hifiengine.com/manual_library/sansui/a-1100.shtml
    STK3102II DATESHEET - http://pdf.datasheetcatalog.com/datasheet/sanyo/STK3122.pdf

    1kHz tone - pin 15 on the STK
    IMG_0292.JPG
    1kHz tone - pin 1 on STK
    IMG_0290.JPG
    no signal, pin 1 on STK
    IMG_0289.JPG
    another no signal, pin 1 on STK
    IMG_0294.JPG
     
  2. moefuzz

    moefuzz Active Member

    Messages:
    272
    Location:
    Peeyong Bangyou
    Interesting,
    Is this with a speaker/load?
    Speaker/driver impedance could be a factor.
    6 ohms or better at the speaker terminals help keep a Sansui happy.
    4 ohm loads could possibly be aiding or sending your Sansui into oscillation.

    Also, I have found that offshore power bars are dirty and you know what they say,
    garbage in garbage out. Powerbars with a neon 'on' light are especially bad and
    cheap powerbars may also allow other things on the circuit to interact.

    ..Or possibly dirty pots causing light arcing. I have had intermittent balance and tone control problems and when the signal passes thru the balance control the dirty contacts seems to cause arcing on a minute level which could possibly manifest itself further downstream. All pots are subject as well as input controls like tape/Aux/phone input switching as well.
    The fact that it effects one side makes me suspicious of balance and tape/input selectors whether rotary or push button type..
    Cold solder joints could cause the same *nth frequency arching which in tern could be tipping you into oscillation.

    Thin outside the box, -something, anything could be inducing the oscillation and that something could be anywhere on the inputs while shatty wall or powerbar voltage could be a factor that isn't helping your cause any..
    -the proverbial straw that broke the camels back as they say..

    Just thinking out loud here, hopefully someone with more knowledge than I will chime in
    as us welders tend to make poor lectricians often resorting to
    largest nearest hammer to bench test lectronics instead of those newfangled things that use knobs and probes.


    [​IMG]


    .
     
    Last edited: May 20, 2017
  3. Bassblaster

    Bassblaster Super Member

    Messages:
    1,602
    Location:
    Cambridge, Ohio
    its with an 8 ohm load and no load attached, it made no difference
    it was running on a separate outlet not a powerbar.
    all sliders and switches were cleaned. cool thing is the functions and tape switched are controlled by an IC so if the buttons were dirty it wouldn't hurt the signal path.
    my wall voltage is a bit high at 126V, but im failing to see how that would cause the issue at hand here.

    this thing isnt anything special. it would be nice to have as a beater test amp.
     
    Last edited: May 19, 2017
  4. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Messages:
    37,446
    Location:
    Hertfordshire, UK
    If you think you have pinned the issue down to the power amp stage including the STK - then I would be looking at C2, C5, C6, C10 & C11. And if you are going to replace them, replace with C0G/NP0 ceramic type.

    Don't be too dismissive of the intermittant fault you discovered, it could be important - I suggest trying to find that first.

    Good luck.
     
    Last edited: May 20, 2017
    AUD101 likes this.
  5. Bassblaster

    Bassblaster Super Member

    Messages:
    1,602
    Location:
    Cambridge, Ohio
    i'll pull the F-4773 control board and look for anything wrong there. The 5 caps you listed, i found C10L and C11L but i cant find C2, 5 and 6. where are they at?
    This thing is a royal SOB to work on because there's not room in it at all.
     

    Attached Files:

  6. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Messages:
    37,446
    Location:
    Hertfordshire, UK
    It looks like the ones you can't find, kC2, kC5 & kC6 are on the F-4762 Driver Amp PCB. ;)
     
  7. Bassblaster

    Bassblaster Super Member

    Messages:
    1,602
    Location:
    Cambridge, Ohio
    Ahh yes there they are!
    KC2 470pF
    KC5 6pF
    KC6 5pF
     
  8. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Messages:
    37,446
    Location:
    Hertfordshire, UK
    Yep, go for the low value ones first, kC2 last in other words.

    I am guessing they are all ceramic capacitors? (kC2 may not be) if they are, remember to replace with C0G/NP0 ceramic type.

    My thinking is that one or more of them has gone o/c or resistive, thus removing the anti-oscillation protection. It's your best bet to get the oscillation stopped - otherwise it may be the STK.
     
  9. Bassblaster

    Bassblaster Super Member

    Messages:
    1,602
    Location:
    Cambridge, Ohio
    i may aswell just recap that entire board. not very many caps on it. it even has some bi-polar caps on it. i have a thing of 470pF C0G caps but nothing else.
     
  10. moefuzz

    moefuzz Active Member

    Messages:
    272
    Location:
    Peeyong Bangyou
    Last edited: May 20, 2017
  11. Bassblaster

    Bassblaster Super Member

    Messages:
    1,602
    Location:
    Cambridge, Ohio
    okay lots to report now.
    after re-soldering the entire F-4773 control board AND the F-4774 board and the intermittent sounds is mostly gone. when i tapped the F-4773 the right channel became distorted but changing the input to something else then back solved it.
    the oscillation appears to be better, but i didn't fully test because i was sidetracked. the one time i checked i only saw a bunch of noise.

    the LEFT channel just got more interesting, with a scope the wave is perfect where the emitter resistors meet WITH NO LOAD ATTACHED, the oscillation was NOT affected by the load.
    IMG_0306.JPG
    IMG_0308[1].JPG

    NOW WHEN I ATTACH AN 8 OHM LOAD THIS HAPPENS
    IMG_0307.JPG
    IMG_0309.JPG
     
  12. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Messages:
    37,446
    Location:
    Hertfordshire, UK
    You have an open emitter resistor, or other resistor, or a blown driver/OP transistor.
     
    moefuzz and Bassblaster like this.
  13. Bassblaster

    Bassblaster Super Member

    Messages:
    1,602
    Location:
    Cambridge, Ohio
    Should i test the inputs and outputs of the STK driver with the load attached? and i already pulled all 4 outputs and they all test good in a transistor tester. Maybe its a rare case where it breaks down at voltage.
     
  14. Overundr1

    Overundr1 AK Subscriber Subscriber

    Messages:
    2,173
    Location:
    Mississippi
    I was thinking that without a load attached the outputs are seeing infinite resistance which would preclude an oscillation issue. Also, that trace makes me suspect as hyperion stated that you have lost half of the push-pull bjt output which points to a failed emitter, opt etc.
     
  15. moefuzz

    moefuzz Active Member

    Messages:
    272
    Location:
    Peeyong Bangyou


    If changing input back and forth "fixed" something then obviously there is a problem with switch /area.

    As others have stated, how did you gain access to internals of pots and switches to properly clean them?
    Spraying 'stuff' through the tiny holes or into their general direction is not cleaning them.

    The pots, push buttons and switches need to be activated or cycled multiple times during the actual cleaning stage as the action of the contacts swiping each other is what removes oxidization - if and when you can actually access and spray the contact surfaces with deoxy.



    .
     
  16. Bassblaster

    Bassblaster Super Member

    Messages:
    1,602
    Location:
    Cambridge, Ohio
    the changing inputs thing is funny because its not a problem with the button as the inputs are IC controlled, and its not arching and pressing buttons really fast because it has a little beeper that beeps on EVERY button press.

    I had to completely remove the front panel. and yes i know ive cleaned tons of sliders and switches.
     
  17. restorer-john

    restorer-john Super Member

    Messages:
    3,528
    Location:
    Australia
    Hyperion is correct. You are seeing the driver waveform which is fine, an OP or emitter resistor is toast.
     
  18. Bassblaster

    Bassblaster Super Member

    Messages:
    1,602
    Location:
    Cambridge, Ohio
    i haven't checked driver output WITH a load, stand by im going to do that right now
     
  19. Bassblaster

    Bassblaster Super Member

    Messages:
    1,602
    Location:
    Cambridge, Ohio
    LEFT OUTPUT of the STK WITH LOAD.
    IMG_0314[1].JPG
    IMG_0315[1].JPG
    Well, looks like its either the STK pack or something on its PCB. would this be a bad time to say the STK pack gets REEEEALLY hot in maybe 5 seconds.
     
    Last edited: May 20, 2017
  20. Bassblaster

    Bassblaster Super Member

    Messages:
    1,602
    Location:
    Cambridge, Ohio
    Meh, starting to wonder if its even worth to fix this, like its cool because it just SCREAMS the 80's. but the STK pack alone is worth more than the entire amplifier is.
     

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