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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    Hello all!

    I have a restored AU-7900 that functions fully however when the unit is shut down I get a nasty POP through the speakers when the power switch is set to OFF...ONLY with the tone control switch set to ON. IF the tone switch is set to OFF then I get no POP.

    The tone control board has all new caps and transistors.

    Voltages measured on the tone board are below and all measure a tad higher than the schematic, but consistently relative. The emitter of TR 08 is supposed to be 0V but measures 60mV which is one that may be an issue.

    Any advice of how to proceed would be appreciated.

    Thanks!

    Zeke

    AU-7900 POP 1.png
     
    Last edited: Sep 30, 2017
  2. Zekeman

    Zekeman AK Subscriber Subscriber

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    New
    I replaced TR05 and 6 on the main power amp driver board because the DC at the speaker terminals was erratic. DC Measurements now are:

    With tone controls off: L-ch -22mV, Rch -18mV

    With tone controls ON: L-ch -17 to -30mV fluctuating, Rch -17 to -122mV fluctuating.
     
  3. Zekeman

    Zekeman AK Subscriber Subscriber

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    Checked out voltages on the -2611 board (switches - tone and filters) and all appears to check out OK.

    AU-7900 POP f2611 board.png
     
  4. Zekeman

    Zekeman AK Subscriber Subscriber

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    Additionally, I already tested the ceramic snubbing cap on the power switch by splicing in an additional cap. This is not the cause of the pop.
     
  5. ManyMoonsAudio

    ManyMoonsAudio AKA paul79 Sponsor Subscriber

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    Sounds like someone might have confused a 4.7uf with a 47uf in the protection circuit.
     
  6. Zekeman

    Zekeman AK Subscriber Subscriber

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    Paul - Nope...I do, in fact, have a 47uf 100V cap installed at C603.
     
  7. Zekeman

    Zekeman AK Subscriber Subscriber

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    Just to be sure the issue is NOT on the big power supply driver board I swapped it out with a known good one from another 7900 I have and the same problem is presented.

    So it appears I have DC somewhere in either the Tone control board or the switch board.

    Question - What is the proper procedure for beginning to track down the location of the DC on the board?
     
    Last edited: Oct 1, 2017
  8. Zekeman

    Zekeman AK Subscriber Subscriber

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    F- 2610 (tone control board) - All "green chicklets" test OK...

    All ceramic caps test OK...
    On to the switch board.
     
    Last edited: Oct 1, 2017
  9. Zekeman

    Zekeman AK Subscriber Subscriber

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    Whew...tediious work!

    F- 2611 (switch control board) - All "green chicklets" test OK...
    Two blue tantalums, two foil, and one ceramic caps all test OK...

    On to the resistors...
     
  10. Zekeman

    Zekeman AK Subscriber Subscriber

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    F- 2611 (switch control board) - All resistors test OK...
     
  11. Zekeman

    Zekeman AK Subscriber Subscriber

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    F- 2610 (Tone control board) - All resistors test OK...

    So ALL components test OK on both the tone and the switch boards F-2610 and 2611.

    Could the issue be somewhere in the power supply section on board F-2599?
     
  12. vigman

    vigman SUBSCRIBER

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    I assume it pops in tone AND filter Does it do it in filter only?
     
  13. vigman

    vigman SUBSCRIBER

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    Could be missing a ground on 2611 ?
     
  14. vigman

    vigman SUBSCRIBER

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    The out of TR-01/02 are cap coupled with a 100K to gnd so if there is any DC charge it would get bled off...
    Do you have a scope to poke about with, I assume it's on both channels ?
     
  15. Zekeman

    Zekeman AK Subscriber Subscriber

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    It pops in filters setting as well
     
  16. Zekeman

    Zekeman AK Subscriber Subscriber

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    I do have a scope...yes.
     
  17. vigman

    vigman SUBSCRIBER

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    You might want to look at the out of TR 01/02 with the scope ( DC coupling ) and watch the THUD.... working your way upstream to see where it goes away.
     
  18. vigman

    vigman SUBSCRIBER

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    The other thought is power for the filter / tone section is collapsing before the protect relay kicks out causing the THUD
     
  19. vigman

    vigman SUBSCRIBER

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    "The emitter of TR 08 is supposed to be 0V but measures 60mV which is one that may be an issue."
    HAve you looked at this with the scope could be HF oscillating as opposed to straight DC
     
  20. deltalight

    deltalight AK Subscriber Subscriber

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    I've had the same thing happen a couple times after restoration. You have a coupling cap in backwards.
    1) Determine which channel gives you the pop.
    2) start by comparing the voltage on the input cap to the Power amp section
    3) Work backwards towards the Tone Control. You're looking for an abnormally high DC voltage that's getting passed up the chain to the driver.
     

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