computer speaker amp hums loudly

Thread Starter

sdowney717

Joined Jul 18, 2012
648
It was working fine, then I get this 60 cycle like loud hum which the volume knob barely effects.
I checked the 4 diodes with diode test function and they are good. The AC transformer runs next to these 4 diodes.
There is a 1000uf electrolytic capacitor right above them. It looks like an AC filter cap?

Does it seem to you it is a bad cap?
I have a lot of 2200uf can I use those?

 

GopherT

Joined Nov 23, 2012
8,012
It was working fine, then I get this 60 cycle like loud hum which the volume knob barely effects.
I checked the 4 diodes with diode test function and they are good. The AC transformer runs next to these 4 diodes.
There is a 1000uf electrolytic capacitor right above them. It looks like an AC filter cap?

Does it seem to you it is a bad cap?
I have a lot of 2200uf can I use those?
Definitely worth trying if you have one in hand. Your existing capacitor may have crapped out. It does happen and the symptom indicates this.
 

Thread Starter

sdowney717

Joined Jul 18, 2012
648
well things are just not so simple.
Yes the cap was bad, when I unsoldered one of the leads fell out.
Plugged in and no hum so thought was fixed.
Plug into computer and silent no sound, the led still comes on.
So I put it together and it is still broken.

Would that bad cap have killed the board?
 

GopherT

Joined Nov 23, 2012
8,012
Something caused the pin to fall out of the capacitor. Usually too much heat. Check the solder joints on the diodes near that capacitor. Also, make sure you did not desolder anything near the bad cap.
 

Thread Starter

sdowney717

Joined Jul 18, 2012
648
I measured the output across that cap at 13.4 vdc. So the board is getting good power.
My guess is the IC under the heat sink has been destroyed by AC current?

There is not much too it, seems like a lot of various caps, pots, and only the one IC amplifier.
Right now it is good source for parts.

Unless anyone has ideas?

I also connected to our stereo and no sound. I verified the input wires are good. Speakers are good.
 

Thread Starter

sdowney717

Joined Jul 18, 2012
648
More bad news. My right channel sound coming from the PC MB is destroyed, left is working.
I plugged in a another PC speaker. Right side was all static, all the time even when no sound should play, static weird sounding.
So turned it off and reboot.
Now the right side is no static, quiet, but when testing the speakers, right side volume level is barely audible, left side normal.
So I think that failed speaker amp may have ruined the onboard sound. I dual boot ubuntu with windows and the sound is the same.
 

Thread Starter

sdowney717

Joined Jul 18, 2012
648
It is an Asus MB with 7.1 azalea sound ship.
I did manage to get a stereo sound working by moving from one green output plug to the other green outlet plug. But the MB will never be good for onboard 5.1 or 7.1 sound again. I could buy a new sound card or just live with it.
 

ian field

Joined Oct 27, 2012
6,539
well things are just not so simple.
Yes the cap was bad, when I unsoldered one of the leads fell out.
Plugged in and no hum so thought was fixed.
Plug into computer and silent no sound, the led still comes on.
So I put it together and it is still broken.

Would that bad cap have killed the board?
Did you check the voltage rating of that 2200uF cap before fitting it?

Check there's still voltage coming out of the rectifier after replacing the cap - once you've fixed it, you can inject some hum by touching the audio input terminal to check its going before putting it all back together.
 

Thread Starter

sdowney717

Joined Jul 18, 2012
648
Did you check the voltage rating of that 2200uF cap before fitting it?

Check there's still voltage coming out of the rectifier after replacing the cap - once you've fixed it, you can inject some hum by touching the audio input terminal to check its going before putting it all back together.
its rating was higher than 16v cap that came out.
I measured 13 vdc reading across the cap terminals
Touching the inputs with fingers - probes, I get no hum.
 
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