Repairing a speaker power supply

[Yamin]

Recently I got a speaker into repair. It died after it went through a power surge. I checked the power supply and found a dual diode package with one diode shorted out. The component was FCH20U10 which were schottky diodes - I replaced it with MUR2060CT which aren't schottky diodes but fast switching diodes(only what I could find). Question 1: Would this be a problem?
After I powered it on the control panel came back to life, but when I passed audio into it no sound was at the output. I investigated the power supply further. I noticed that output of the voltage regulator (+12V) was not very steady. I checked the input of the voltage regulator and it also seems to vary alot from 16V - 17V. Some one advised me to check if there was any AC at the input of the voltage regulator- I checked and there was about 1.5V AC at the input. What could be causing this and is this a problem?I checked the capacitors around couldn't find anything wrong. I should add that the components of the pcb are glued down so removing something takes quite a bit of effort.

I checked the output of the -12V regulator output it was very steady and the input was very steady as well and there was very little AC voltage present at the input (in the mV range).

No audio at the output could mean the amplifier IC is gone. Please advice on trouble shooting that. (The part number is scratched out so do not know which IC it is).

Thanks in advance

[coromonadalix]

You should give the model and brand name when posting,  and take some pictures to show us.

[Yamin]

Yep sorry here are some pictures. The model of the speaker (subwoofer) is Adam Sub 7
Picture 1 is the pre-amp section
Picture 2 is the amplifier IC which the markings are scratched out

Further development, I powered the unit up with the pre-amp board disconnected I could hear a loud switching sound from the power supply board

[coromonadalix]

Your 5 pins transistor is the smps suply driver controller,  I think your amplifier ic is under the pc board,  you have an metal bracket with two holding screws near the kt-3150 pcb markings

DEDUCTION  :  You see an shielded audio cable coming from the preamp board ... you see 220  marked inductors, you see you speaker cable near them ...

[Yamin]

@coromonadalix - ah I see thanks, you're right there is an unmarked IC at the bottom covered with a huge heat sink.
There has been some interesting development:
1)I was able to get the power supply fixed changed the fried components I measured the output of the voltage regulators I noticed that when I removed the board with volume knob the voltage stabilizes and there is no variance.(I didn't include a picture of that board before).
2)I got some sound out of the speaker and when I increase the volume above a certain point the speaker mutes at louder parts and the sounds come back again.
3)Here is the most dubious point - I wanted to check whether the sound problem resolved if I put the unit back together so I glued down the components and screwed the heat sinks etc.. There were some screws which tightens the heat sink of the amplifier IC to the outer chassis of the unit. (The two screws you mentioned secures the heat sink on to the IC and onto the board). Any way powered up the unit and there was no power! I removed the four screws which connected the outer chassis to the heat sink and tried powering it up and it powered up-  why might this be? I'll try and take a picture.

Still the sounds mutes  on louder parts momentarily, I tried a sine wave (100 Hz) and  the speaker did not mute even when I turned the volume all the way up.

[coromonadalix]

Maybe there is an isolation problems like an transistor mica, thermal pad  etc ...  ground body wire not re-placed correctly  ?   re check all your job

[Yamin]

Will do, just curious how could that stop the unit from powering up? (What's the "science" behind that?)