34V Offset on AU999

Discussion in 'Exclusively Sansui' started by Hipocrates, Aug 13, 2017.

  1. Hipocrates

    Hipocrates Anti-Muppet Subscriber

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    I took an AU-999 after several months of been rebuilt ( it was full of problems) to play it for a while, and I found that sounded really bad and have a nice thump when turn on.
    So I took it to the bench and test the offset, to my surprise the little bugger have 34 volts on RCH.
    Weird isn't?:dunno:
     
    Last edited: Aug 13, 2017
  2. LBPete

    LBPete Rolling Along Subscriber

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    It's a cap coupled amp. You can't measure offset by just measuring voltage at the speaker terminals. I haven't looked at the 999 procedure recently buy typically you need to have a dummy load connected, inject a 1k Hz sine wave and adjust it to clip the + and - apexes at the same time. It's sometimes called "balance."

    - Pete
     
  3. Hipocrates

    Hipocrates Anti-Muppet Subscriber

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    Hello Pete, all due respect, is not a coupled cap, I think you are referring to the AU-777, 505, and those.
     
  4. LBPete

    LBPete Rolling Along Subscriber

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    Yup, I just looked. Here's the service manual procedure.

    - Pete
     

    Attached Files:

  5. Hipocrates

    Hipocrates Anti-Muppet Subscriber

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    Thank you Pete.
    Curious thing, while the LCH adjust perfect, RCH stays 34v, even switching F-1159 sides.
    So far what I know is this:

    ./ Outputs are fine
    ./ Driver transistors are fine.
    So, it seems that I need to sharp my limited diagnostics skills :rolleyes:
     
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  6. petehall347

    petehall347 the brandy coffee man

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    maybe resistors .
     
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  7. Hipocrates

    Hipocrates Anti-Muppet Subscriber

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    Good idea... I'll start to test on F-1231, F-1277 ad F-1230 and see what I find
     
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  8. kevzep

    kevzep Its all about the Music Subscriber

    To clarify, the AU999 is not a Cap coupled amplifier. I should know I am working on one right now.

    Pull the F1209 boards out, or if you have one of the models where if you select "Speaker C" it also disconnects the pre-amp from the poweramp, do that.

    Then see what you have.

    If you still have huge offset, then pull the driver board out. Could be D801/809, at the front of the driver circuit, one or both might have shorted out...

    If you still have offset, you need to check the driver and output transistors, it could be possible with the Quasi design to lose a transistor without taking a rail fuse out...and now we are all ripping out the DC protection, it is possible to have this sort of DC on the outputs...
     
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  9. Hipocrates

    Hipocrates Anti-Muppet Subscriber

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    Thank you Kev,
    I'm on it right now... I'll report later.
    I did all what you said... right now I'm checking resistors too (as I'm there anyway I'll replace some old carbon with new vishay metal film that I have on hand)
     
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  10. kevzep

    kevzep Its all about the Music Subscriber

    There is one other thing to check, and that would be the filter block which the Emitter resistors are on, make sure the transistors on those are good.
    Apart from that it's just a bit of detective work and you'll find it.
     
  11. petehall347

    petehall347 the brandy coffee man

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    when i get such as this i first see what voltages at the outputs . if ok move to the drivers . but first noting whether offset is plus or minus .
    p.s output voltages wont be correct at this stage but might lend a clue as to where to look next .
     
  12. Hipocrates

    Hipocrates Anti-Muppet Subscriber

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    Great, thanks for pointers gentlemen, now that I'm finished of playing with dogs I'm back to the task!:thumbsup:
     
  13. Hipocrates

    Hipocrates Anti-Muppet Subscriber

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    Those damn emitter resistors are way out specs... I'll put some on my next mouser order.
    Thanks folks, your are gentlemen and scholars:beerchug:
     

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