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Sony STR-DB940 hum/bleed in, in SUB channel only

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    Sony STR-DB940 hum/bleed in, in SUB channel only

    Hello, been struggling with this receiver.

    It did some relay clicking some months ago (back channel lost for seconds), but clicking stopped. After that working fine.
    I purchased new speakers (Boston Acoustics), and for the first time used the SUB PRE OUT for an active sub. Worked fine. For one week.

    In the middle of a tv program suddenly came loud, audible hum from the sub woofer. As the sub was new, I thought I had a faulty unit.
    But after some troubleshooting, I narrowed the problem down to the old receiver. Please correct me if Im wrong, from the following symptoms:

    * Loud audible hum and little bleed in from the selected channel on tv

    * Rest of the receiver and sound from the subwoofer working fine (also sub channel)!

    * Most audible when active sub is turned all the way up

    * Still audible when volume on receiver is turned all the way down

    * Moving sub and receiver to a different room/outlet: Hum still audible

    * Different RCA cable: hum still audible

    * Removing all input cables from receiver/shutting tv, blu-ray off: hum still audible

    * Pulling cable from tv / shutting of tv: makes a difference in hum.

    * Changing RCA from SUB PRE OUT to CENTER/FRONT PRE OUT on the receiver clearly ELIMINATES HUM

    * Selecting "Sub woofer: NO" in setup (a relay clicking) ELIMINATES HUM


    Heres some more intensive testing:

    * Sources with DD-logo (Dolby) (eg. specific channel on tv) CLEARLY has more hum than channels with no DD logo

    * Shifting source on front on receiver makes kind of an echo in the hum, clearly audible in the sub woofer


    I tried open up the unit, but couldn't see any visible sign of bad caps or bad solders.
    I have fair solder capabilities, but no testing gear besides a battery driven hand multimeter.

    Repair manual for the unit HERE.

    The unit is worth much less than what repairs would cost in my country, and I dont want to toss an otherwise perfectly working unit.

    Really hopefull that anyone in here have the knowledge to point directions of something to exchange.. Thanks a lot, guys.
    Last edited by Jumme; 05-05-2015, 01:59 AM. Reason: missing information
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