Sansui AU-7900 - Big POP when Powered off ONLY with Tone Control ON

Discussion in 'Exclusively Sansui' started by Zekeman, Sep 29, 2017.

  1. restorer-john

    restorer-john Super Member

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    I really like your markup technique on the schematic. What did you use and how did you do it?

    I could really use that.
     
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  2. Zekeman

    Zekeman AK Subscriber Subscriber

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    I do it in PowerPoint. Take a screenshot of schematic, paste it in PowerPoint, and mark it up
     
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  3. restorer-john

    restorer-john Super Member

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    My power point must be too old- I've never seen those cool little markup speech bubble/balloon things.

    edit: I found them! Insert shapes, callouts. So cool. Thanks.
     
    Last edited: Oct 7, 2017
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  4. vigman

    vigman SUBSCRIBER

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    Zeke my only thought is the relay is being held on to long either mechanical or the coil hold voltage is being held high to long.
    The other thing that is disturbing in you see a flash when the relay opens...
    IS your scope dual trace, might be interesting to note the time between the pop and the coil voltage.

    The other thought is is it a teeny pop or are you really pushing the woofers ?
    I assume this is just 2 speakers hooked up to a stereo, no external sub stuff or any external processor ?
    ( yes reaching for straws here )

    This is a bit of a stumper....
     
  5. Zekeman

    Zekeman AK Subscriber Subscriber

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    I have a Tek 465B

    It's quite a pop and it moves the woofers. I'll take a video and post it to YouTube.
     
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  6. ManyMoonsAudio

    ManyMoonsAudio AKA paul79 Sponsor Subscriber

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    Replace the snubber diode across the relay coil? How about the VD1212's?
     
  7. Zekeman

    Zekeman AK Subscriber Subscriber

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    The VD1212's have all been replaced. I'll check the diode I think I may have replaced it...().
     
    Last edited: Oct 9, 2017
  8. Zekeman

    Zekeman AK Subscriber Subscriber

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    All diodes replaced in the protection circuit...still no change.
     
    Last edited: Oct 9, 2017
  9. Zekeman

    Zekeman AK Subscriber Subscriber

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    Tonight I am going to transplant a known-good tone and switch circuit board assembly into this unit to further troubleshoot...

    ...Result...!!! POP!!! The problem is NOT on the tone / switch board. So to recap...I have swapped out both the main driver motherboard AND the tone / switch board and I still have the problem. So the issue must be on the Secondary power supply/ phono equalizer board F-2599....this board was pretty filthy with much crud. It's been cleaned but could use more work. ...not sure if that may be a contributing factor. I have, by the way, installed new transistors in this secondary power supply already. Resistors have not been verified. Nonetheless...any thoughts on this latest development?
     
    Last edited: Oct 10, 2017
  10. Zekeman

    Zekeman AK Subscriber Subscriber

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    Man I've been poking around the F-2599 board for two days now-probing with my scope for signs of DC pop and NADA! Except for where the signal comes in from the switch board into the connected /separated switch and out to the power amp board. But that was already known. This one is a real challenge!

    :dunno:
     
  11. ManyMoonsAudio

    ManyMoonsAudio AKA paul79 Sponsor Subscriber

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    With no other symptom, you bet it is. I would explore the grounding. The input RCA Jacks I know, typically need some fixing anyways.
     
  12. Zekeman

    Zekeman AK Subscriber Subscriber

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    You mean just verifying, through continuity, that a ground is, in fact, grounded?
     
  13. llwhtt

    llwhtt Well-Known Member

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    Does it POP with the preamp and power amp separated? I haven't seen that mentioned yet.
     
  14. Zekeman

    Zekeman AK Subscriber Subscriber

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    No, it does not.
     
  15. llwhtt

    llwhtt Well-Known Member

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    That narrows it down some, gots to be in the preamp section "maybe".
     
  16. Zekeman

    Zekeman AK Subscriber Subscriber

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    F2599 secondary power supply. I decided to reconfirm voltages tonight. The voltages at the output of the board were close but off at tr-15. I replaced TR-13 and 15. The POP is now gone.
     
    Last edited: Oct 12, 2017 at 10:53 PM
  17. ManyMoonsAudio

    ManyMoonsAudio AKA paul79 Sponsor Subscriber

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  18. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

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    Nice find, well done :thumbsup:.
     
  19. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Well done Zeke. ;)
     
  20. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

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    Holy cow, nice work!
     

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