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Old 12th October 2007, 01:17 PM   #1
baldy is offline baldy  United Kingdom
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Default diagnosing mains hum

Hello chaps,

My first post here and I'm asking for help already!

I have a Sony STR-DB795 7.1 channel amplifier that is suffering from mains hum. Its not loud but definitely detectable in a quiet room.

Its independent of volume and independent of input selected, or indeed all inputs disconnected.

It doesn't occur on line outs (tape loop, sub out , only speaker outs and headphone socket.

All this lead me to conclude thats its just a noisy psu, so I whizzed the lid off an measured (with my recently acquired oscilloscope) a 190mV ripple with only idle loading.

Unfortunately my last electronic class at college was over 20 years ago and I'm somewhat rusty so I'd value any input you guys can give me.

Is 190mV excessive? Should I be looking at the smoothing caps as the culprit here?


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Old 12th October 2007, 02:16 PM   #2
gni is offline gni  United States
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I would suspect the caps also with that much humm; the ripple seems
maybe higher than I would like. . . could also be a bad solder joint
in the ground path somewhere else in the circuit. Check the boards
and trace the ground paths. . .might even give them all a quick reheat
to the solder joints. . .just to make sure they are well connected.
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Old 17th October 2007, 08:01 AM   #3
baldy is offline baldy  United Kingdom
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Thanks for your reply, I'm glad I wasn't heading off in totally the wrong direction!

I've found a few forum posts which suggest that this model tends to do this, wish I'd found them before I bought it!

Given that the caps are probably the problem, can anyone recommend a cap to use to replace em? I need a 12000uF 63v PCB mount cap and they seem to be few and far between. I want to replace them with decent quality new ones, but some of the audio caps I found on google cost more than the amp! I need to be realistic here so no Black Gates thanks very much!

Is it worth replacing them with 15000uF caps or might that upset the rectifier? I've looked for specs on it but can't find any so I've no idea if its overspec'd enough to take the extra current, I'm guessing not?


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Old 17th October 2007, 01:16 PM   #4
gni is offline gni  United States
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This is where DIY is fun. You might need to build a new 'board'
with multiple caps in parallel and then attach that into the circuit
board. Special caps are used in amps all the time. . .special height
considerations. . .diameter, odd voltage, odd capacitance. Find what
you can: anywhere from 10,000 to 15000 uF will work fine - depends
if you use the unit with all channels blasting or just as a stereo unit.

For testing purposes. . .install a smaller value . . .4700 uF to begin
and see if the hummm is going down or up. . . with a good new cap
there should be a difference. . . don't drive too hard with the small
cap. . .but it should tell you if this is the cap in question.

Hopefully someone with Sony experience will respond to this thread.
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