diagnosing mains hum

baldy · 12th October 2007, 01:17 PM

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?

Cheers

Pete

gni · 12th October 2007, 02:16 PM

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.

baldy · 17th October 2007, 08:01 AM

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?

Cheers

Pete

gni · 17th October 2007, 01:16 PM

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.

12th October 2007, 01:17 PM	#1
baldy diyAudio Member Join Date: Oct 2007	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? Cheers Pete

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12th October 2007, 02:16 PM	#2
gni diyAudio Member Join Date: Sep 2006	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.

17th October 2007, 08:01 AM	#3
baldy diyAudio Member Join Date: Oct 2007	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? Cheers Pete

17th October 2007, 01:16 PM	#4
gni diyAudio Member Join Date: Sep 2006	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.