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

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

  1. Zekeman

    Zekeman AK Subscriber Subscriber

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    Gents, thanks for the replies. I'll take a look tonight. Regarding the suggestion of a reversed coupling cap...are you suggesting this is on the tone and or switch board?

    Thx
     
  2. ManyMoonsAudio

    ManyMoonsAudio AKA paul79 Sponsor Subscriber

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    Hmm... Seems strange to me that the relay is not opening up fast enough to keep the pop from happening. DC Burping at power down with a direct coupled amp is not uncommon. The relay opening too slow is uncommon.
     
  3. Zekeman

    Zekeman AK Subscriber Subscriber

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    The input coupling caps on F-2610 (tone board) are installed correctly.

    TR-08 had an incompletely soldered base which, when fixed, took care of the 0.6 mv on the emitter but the POP is still present.

    Paul - the relay opens just as soon (and as fast) as other 7900's I have.
     
  4. Zekeman

    Zekeman AK Subscriber Subscriber

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    AU-7900 POP Colored schema 2 with C37.png I connected my scope to points 18 and 19 on F2611 and when I power down I see a slow large drop in the DC at that point.


    Traced DC back to one side of ceramic cap (C37) .022uf that has 24v DC on one side of it.
     
    Last edited: Oct 2, 2017
  5. vigman

    vigman SUBSCRIBER

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    A slow DC move will be in the subaudible zone A POP will be a very fast transistion, well faster than 15 hz... BUT a coupling cap will let it pass thinking it's signal.
    Does 1 side of the PS that powers tone & filters collapse before the other side ? I assume the POP ( thud ) happens at the moment of power off.. How long does it take for the protection to drop out 1-2 -click ?
     
  6. Zekeman

    Zekeman AK Subscriber Subscriber

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    I'll check the two sides of the power supply drop out.

    The protection process occurs normally at two to 3 seconds on power up
     
  7. vigman

    vigman SUBSCRIBER

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    My question was meant to ask , how quick does the protect relay drop out on power off?
     
  8. ManyMoonsAudio

    ManyMoonsAudio AKA paul79 Sponsor Subscriber

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    No other symptoms?
     
  9. Zekeman

    Zekeman AK Subscriber Subscriber

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    On power off there is no delay in the relay dropping off. It's instantaneous.
     
  10. Zekeman

    Zekeman AK Subscriber Subscriber

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    Just to be sure the coupling caps on the tone board are C01 and C02, low value polarized electrolytics, correct?
     
  11. vigman

    vigman SUBSCRIBER

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    C-01 / 02 On the schemo .47 50 volt polarized WITH a .01 disc in parallel. Parts list only shows TR & VR values
     
  12. Zekeman

    Zekeman AK Subscriber Subscriber

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    They both collapse at the same/slow rate.


    No. Unit operates / sounds good.
     
  13. Zekeman

    Zekeman AK Subscriber Subscriber

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    Confirmed 0.47uf with a green chicklet cap in parallel.
     
  14. vigman

    vigman SUBSCRIBER

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    Ok so how does this work?
    How can you hear a POP when the protect relay drops out instantly ?
    Is it possible that the protect relay has arced? and is not releasing as fast as needed??
     
  15. vigman

    vigman SUBSCRIBER

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    Does it do the same in every input setting eg Aux VS tuner or phono ?
     
  16. Zekeman

    Zekeman AK Subscriber Subscriber

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    With the scope I traced the pop to 2611 at pins 18 and 19. I indicated where I am getting the pop with the red bursts on the image below. The pop does not exist where the blue "X" is shown.
    AU-7900 POP Pop locations on 2611.png
     
  17. Zekeman

    Zekeman AK Subscriber Subscriber

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    Yes, A loud pop through the speakers

    I can watch the relay release. (Instantaneously with no delay). As it is releasing I can see spark between the contacts
     
  18. vigman

    vigman SUBSCRIBER

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    A SPARK AT THE CONTACTS !!!!! WOAH....
    The blue X is NOT gnd so that's good , The print sez there is ~10 volts DC on the emitter of TR-01 / 02.. is that correct??
     
    Last edited: Oct 4, 2017
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  19. vigman

    vigman SUBSCRIBER

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    There is no DC offset pot in a 7900... tell me the DC voltage @ one side of R-73/74 to gnd with the amp @ idle , power on AND if there is a difference with filters on or off
     
  20. vigman

    vigman SUBSCRIBER

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    The amp is biased up properly ?
     

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