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Home theatre system dead - repair

Discussion in 'Troubleshooting and Repair' started by petarg, Apr 12, 2012.

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  1. petarg

    petarg

    2
    0
    Apr 12, 2012
    Hi all,

    First post on the forum, so hope I get it all right - I'm trying to repair a Sony DAVS-800 home theatre system which went dead a while ago (12 months or so).

    How it happened - stopped all of a sudden, no power, no power lamp, nothing works. There was no smoke or sound, just didn't power on at some point. I checked the fuse (presumably AC, the usual see-through small glass cylinder one), it was OK (don't know if it has another (DC) fuse or not), so I left it for a while.

    Looking at fixing it today, so I checked a few things:

    - The unit still shows no sign of life when plugged in and powered on, except:
    - A small/short high pitch noise comes from it for the first few seconds after power button is pressed
    - AC current does get to the fuse, power switch is OK (AC voltmeter shows 223V)
    - Last thing I checked - one of the two DC power outputs shows a current (29V) so PSU is not entirely dead (this was my first thought, something in the PSU)
    - A large 400v capacitor on the power board has a very slight bulge in it, but did produce a big spark when I shorted it, isn't shorted when measured for resistance, and if Ohm-meter is held to it the resistance increases, making me think it's not the (obvious) cause

    Apart from this, I've managed to find a service manual for it (can't attach it due to size but will email it if anyone wants to/has the time to have a look), and took a few pics which might help (these are attached, will take more if needed).

    Would anyone know what I should look into next? (I have checked a few capacitors and diodes on the PS board and most seem OK, except for one diode and one capacitor which seemed to be shorted - but it was an on-circuit test so no idea if they are really faulty or not (both are OK visually).
    This sort of thing was a hobby of mine in early teenage years, so am quite happy to fiddle with it, can solder etc, but don't have many thoughts on what exactly to check next :(

    All help much appreciated,
    Best regards,
    Petar
     

    Attached Files:

    Last edited: Apr 12, 2012
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,419
    2,790
    Jan 21, 2010
    A shorted diode in the power supply is certainly an option. It would be one of those connected to a heatsink near the output of the power supply.
     
  3. davenn

    davenn Moderator

    13,676
    1,892
    Sep 5, 2009
    thats usually an indication that there is either a PSU fault or some other part of the system is loading the PSU down because of a fault in that other section

    Dave
     
  4. petarg

    petarg

    2
    0
    Apr 12, 2012
    Would it be safe to assume the fault is in the PSU somewhere - or should I be checking other parts as well?
     
  5. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    what about the other one? What is the voltage supposed to be at the output o the PSU?
     
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