990DB Project Thread

Discussion in 'Exclusively Sansui' started by dpcornell, May 10, 2018.

  1. avionic

    avionic " Black Knights " Subscriber

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    Sounds like your missing the signal..:biggrin: Stand-by....
     

     

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  2. avionic

    avionic " Black Knights " Subscriber

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    Verify that the dolby switch is indeed in the "off" ( center ) position.
     
  3. dpcornell

    dpcornell Member

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    Dolby switch is in the off position.

    Should Rec Level and Play Cal control the volume in addition to the main volume potentiometer?
     
  4. avionic

    avionic " Black Knights " Subscriber

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    40,584
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    Have you got speakers hooked up to this unit ?
     
  5. avionic

    avionic " Black Knights " Subscriber

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    Make sure your meter power level button is in the out position. ( non depressed)
     
  6. avionic

    avionic " Black Knights " Subscriber

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    Tape monitors in the out position as well.
     

     

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  7. dpcornell

    dpcornell Member

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    Speakers are connected.
    All buttons are in the out position.
    Bias and DC offset are within spec.
     
  8. avionic

    avionic " Black Knights " Subscriber

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    And your getting good sound . Right ?
     
    Last edited: May 14, 2018
  9. avionic

    avionic " Black Knights " Subscriber

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    I'm not sure.Its been quite a while since I've messed with one.I wouldn't think it would have any effect in the "off" position.
     
  10. dpcornell

    dpcornell Member

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    Not great sound, but sound. Sound comes in clear from the Main in, but the volume has to be more than half way before I get loud sound from the AUX in or FM.
     
  11. avionic

    avionic " Black Knights " Subscriber

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    Check F01 and F02 out of circuit with your meter and the solder connections to the same fuse clips.
     

     

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  12. dpcornell

    dpcornell Member

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    Both fuses test 0.00 on the Ohm setting.
     
  13. dpcornell

    dpcornell Member

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    With the fuses removed and the unit powered off I get 4.9 and 5 MOhms across the fuse clips for F01 and F02.
     
  14. DougBrewster

    DougBrewster AK Subscriber Subscriber

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    In the OFF position there should be no effect. In the Dolby Play position the play level does interact with the main volume control. I can't remember the impact of the REC control and can't check because I'm away from home...
     
  15. dpcornell

    dpcornell Member

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    On mine it slightly increases the volume, even with Dolby set to off. I will double check the solder joints to make sure there are not any connections being made where there shouldn't be. However, when I looked last night, they all appeared to be okay.

    I tested the fusistors on the other boards and they all tested okay. I'm sort of scratching my head on this. Short of some bad capacitors, I'm not all too sure where to go from here.

    Since I get sound from the main amp in, would that mean I have an issue with the driver board? I changed out all the fusistors and resistors that tested out of spec, but I have not touched the caps, diodes, or transistors.
     
  16. avionic

    avionic " Black Knights " Subscriber

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    This reminds me of a 9090DB I worked on years ago. Customer had incorrectly installed the dolby selector knob ... I really like easy fixes.
     
    Last edited: May 15, 2018
    eiraved and mgkfifty like this.

     

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  17. dpcornell

    dpcornell Member

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    Found part of my issue: the fuse holders were not making solid contact with the fuses. I now have consistent meter movement at max volume with the FM. Still only quivers when on Aux.
    I replaced the VD1212 diodes with paired 1N4148, since the diodes did not even read on my meter. I am going to test more components on the driver board and see if there are other issues.

    I wish it were that simple. I've reflowed the solder on everything on the power supply circuit. No changes.

    Could this be an issue of power? Where are the test points I should be looking at?
     
  18. avionic

    avionic " Black Knights " Subscriber

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    I've seen that issue as well. Had to give the clips a little squeeze with a pair of pliers.
     
  19. avionic

    avionic " Black Knights " Subscriber

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    These same issues also plague the fuse style lamps for the tuner...
     
  20. dpcornell

    dpcornell Member

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    After some late night (for me on the east coast) thread reading I came across a post about the metal shaft of the dolby switch becoming dislodged.

    I didn't think a fix could be so simple, but it was! I have both meters and normal audio! I guess I didn't notice when I had it off the dolby board to deal with the pass throughs.

    Where/how should I glue the shaft to prevent this from ever happening again?

    It still needs some tweaking. But, at least I can play some tunes while I decide whether to tear this thing down and do a full restoration.
     

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