990DB Project Thread

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

  1. dpcornell

    dpcornell Member

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    This is a new thread for the 990DB I recently purchased. It would not come out of protection mode unless the F2624 board was removed. Several of the fusistors tested out of spec.

    So I ordered replacements and finally got around to installing metal film resistors. No more blinking protection light!

    Unfortunately, I only have one channel and it is very faint. Tried FM, AM, Tape, and AUX inputs--they're all the same. No Dolby test sound. I get a light pop each time I change one of the Tone Control levers.

    All the fuses check good, except one so it will be replaced. I also have not had the opportunity to DeOxit any of the controls.

    I guess the next thing to do is test the Output Transistors (two are not original)?
    Should I order parts for a recap and redo the Dolby Board?

    I've attached some pictures for your viewing pleasure. (This thing is VERY dirty!)

    One more thing: Should this 990DB have a serial plate for a 9090DB?
     

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  2. avionic

    avionic " Black Knights " Subscriber

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    If it comes out of protection its probably not outputs.
     
    Last edited: May 10, 2018
  3. avionic

    avionic " Black Knights " Subscriber

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    Deoxiting the switches and controls may help.
     
    Last edited: May 10, 2018
  4. avionic

    avionic " Black Knights " Subscriber

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    I wouldn't think so.
     
  5. avionic

    avionic " Black Knights " Subscriber

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    Ohms check all your fuses with a multimeter out of circuit.
     
  6. dpcornell

    dpcornell Member

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    I did and only one fuse is bad.

    After my exam tomorrow, I will DeOxit all the controls and report back.
     

     

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  7. DougBrewster

    DougBrewster AK Subscriber Subscriber

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    If it's the 150/300Hz switch going from 150 to defeat to 300 it's characteristic of the unit. I had a thread going on this about three years ago. Happens on both of my 9090DBs. It is a very light pop though..
     
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  8. dpcornell

    dpcornell Member

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    Thank you, that's exactly the switch. I figured it was normal. My Marantz does the same thing.
     
  9. dpcornell

    dpcornell Member

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    Well I cleaned and lubed all the pots. I still get almost no sound unless the volume is cranked to max. It almost sounds like the music is playing through a drain pipe. Still no dolby test sound.

    So, I removed the Pre/Main jumpers and connected my aux input directly to the main amp. Nearly blew out my eardrums, but I now know the output transistors all work. It sounds pretty darn good!
    Once I finish cleaning the faceplate I will upload a picture. It sure does look better. The next step is to make it sound better.

    I want to fix the issue before I do a shotgun replacement of the capacitors and transistors. That being said, is this likely a dolby board issue or is it a preamp issue?
    If it is a dolby issue, I may as well do a recap of the boards when they come out for the repair.

    Hopefully I can dig out the preamp and do some testing this weekend in between studying.
     
  10. dpcornell

    dpcornell Member

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    Certainly cleaned up well. I don't feel like washing my hands every time I look at it. I think I will get some LEDs from Dwojo as well.
     

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  11. db SANSUI

    db SANSUI New Member

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    I recently purchased a 9090db thats blowing the 1 -10 amp fuse on the power supply board for the positve and negative output transistors.The unit powers on green protection light comes on steady. Are output transistors blown thats stemming back to the f2624 driver board as the cause looking for my multimeter now to check those output transistors.:dunno:
     

     

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  12. LBPete

    LBPete Rolling Along Subscriber

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    db Sansui, start a new thread for your 9090DB so we don't get confused between yours and the one in this thread. Also, welcome to Audiokarma!

    - Pete
     
  13. dpcornell

    dpcornell Member

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    I pulled and pinned the Dolby Board over the past five hours. It looks good and I actually get the test sound in both speakers! Still have some problems, though.
    Here is where I am at:
    I have to crank the volume almost to max to get sound out of it.
    The right side is a little weaker than the left.
    The power meters do not move, except sometimes they quiver.
    I get both speakers and proper amplification from the main amplifier when I connect directly to the main in at the rear of the receiver.

    Any suggestions?
     
  14. db SANSUI

    db SANSUI New Member

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  15. db SANSUI

    db SANSUI New Member

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    So instead on the 9090DB I proceeded to check out the rectifiers D03 & D04 with fuses out in power supply board first which they checked out no good.So know I have started on checking all the output transistor.I decided to stop and address this matter first.To reiterate my protection light comes on to steady green.And thanks Pete for the heads up. I got so caught up reading yours and R.I.P. Jim Transformers threads I wasn't paying any attention to anybody else's valuable threads too I am sorry
     
  16. dpcornell

    dpcornell Member

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    So I am still having issues with the meters and volume. I reflowed some of the solder on the other boards, specifically the regulated power supply where it was cracked.
    When I press lightly on the Dolby board, I get sound.
    Ideas?
     

     

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  17. avionic

    avionic " Black Knights " Subscriber

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    Still has pass through issues on the dolby board .. Or cold solder connections on the dolby board. Dirty/damaged dolby selector wafer switch to name a few..One or two screws need to be securely fastened to chassis ground to complete the circuit possibly loose screw ?

    Cracked copper trace on the dolby board...possible..
     
    Last edited: May 14, 2018
  18. eiraved

    eiraved AK Subscriber Subscriber

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    No...

    IMG_8765.JPG
     
  19. dpcornell

    dpcornell Member

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    It appears I missed two pass-throughs. I pinned those and now I have a constant volume.
    I used my meter to test the large and long traces across the board and they are okay.
    The dolby board is grounded. I double checked all the connections coming to or going from the board. They are solid and no changes when I move them with a probe.

    My meters still are not functional. What am I missing?
     
  20. dpcornell

    dpcornell Member

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    Thanks, I figured that was probably the case... I guess it was cheap for a reason....
     

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