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Thread: No sound from sony amp STR-DB930

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    Senior Member Moof's Avatar
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    Default No sound from sony amp STR-DB930

    I have a sony STR-DB930 amp that I have had for years. Last night I was listening to some music reasonably loud. The Amp overheated and the PROTECTOR cut in and started flashing on the display.
    This has happened a couple of times before over the years and when it cools the sound comes back. However I switched it off and went to turn it back on today and now there is no sound from any speakers. The protector is not flashing.
    Does anyone have any ideas or a schematic for one as I plan on having a look at it over the next weekend
    Thanks



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    Dry joints on the Audio output IC and the protect relay are a couple of things I have seen with that model
    When you do things right, people won't be sure that you have done anything at all

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    Senior Member Moof's Avatar
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    I have finally got arround to having a look and this is what I found
    Two large transistors on the main board Q506 (MN2488) and Q507 (MP1620) are shorted. These transistors are on the main heatsink and are for the front right speakers. I havn't found anthing else yet that may be faultly. Does anyone know if there may be any other suspect components I should check with this type of fault?
    Thanks
    Moof

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    Carefully check any diodes and resistors near the shorted transistors, there may be a pair (can be a single resistor with 3 legs) of emitter resistors which can go open if both devices were turned on at the same time. Carefully check the driver transistors connected directly to the output devices...you can also run A/B checks comparing the crook channel with the working channel, as in resistance to ground from various points etc. - but remove/disconnect the short-circuit transistors first.

    The protection circuit opens the speaker relay to prevent +/- rail getting across the speakers but the amplifier circuitry stays alive cooking things until a supply fuse opens. You can be lucky and have only the transistors crook - I have seen this many times - so after checking everything - twice - replace the devices protect your eyes and switch the juice on....or crank it up slowly using a VARIAC(tm) and watch/measure ....it will probably just work fine.

    The crook transistors are fairly unusual high-voltage Darlington pair devices which are generally harder to substitute so you probably need to get identical replacements.

    When it's fixed consider using some assisted cooling, a small computer fan running at decreased supply is quiet - any airflow is better than simple convection.

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    Senior Member Moof's Avatar
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    Thanks for all your help guys. It is all fixed now and only cost me about $15.00 in parts. I am looking into a fan for the heat sink that comes on when it warms up a bit.
    best4less the link you sent me really helped

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