Need help repairing JBL amp

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I have a JBL amp which is working BUT there's something wrong with 2 of the outputs.
Front left output just buzzes and front right produces no sound at all.
I opened it yesterday and just from looking at the board I see nothing wrong with it so I'm not sure where to go from there.
I'm also a noob in electronics but willing to learn a lot!
I attached pics of both sides of the board (these are only until I charge my camera and learn how to take better pictures). Marked with red are those outputs.

EDIT: better pics.

 

[endecotp]

Do you own an oscilloscope?
Multimeter?

 

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I don't have an oscilloscope, but I do have a multimeter.

Do you own an oscilloscope?
Multimeter?

 

[endecotp]

With power applied, feed the same signal into all inputs and trace it from the input to the output. With a multimeter it's probably best on AC volts, but DC volts might also help in some places. The PCB layout is nice and symetrical so this shouldn't be too difficult. At some point, you'll find that the working channels and the non-working channels differ; that's the compoent that has failed.

The alternative approach is to guess which components are most likely to have failed and to probe them, again looking for differences between the working and the non-working channels. I'd start with the power semiconductors (what are they?) and the electrolytic capacitors.

 

[Cigaro]

I would never suggest that you simply throw parts at something, but this appears to be a very simple chip-amp based design with one per channel.

Where's the heatsinking - is it on the case? Presumably that little bit of aluminium on the front is just a heat spreader and bolts to something bigger?

This sort of design tends to be quite under-heatsinked and it's not unknown for the chips to overheat and fail as a result...

 

[mikeyd]

Is that out of a car? I know JBL amps of that sort of size used to be fitted in some cars (hi spec Citroen/peugeot). If so and you don't get anyware fixing it, I'm sure some will have outlived the cars so try some breakers. What is the JBL model number in case I'm talking rubbish?

 

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I should probably have mentioned it's a car amp.. So it's probably DC?
I'm also not sure about "feeding the same signal into all inputs". The signal (=voltage?) needed depends on overall design, chips used, I can't just select, say, 9 volts and use that?

With power applied, feed the same signal into all inputs and trace it from the input to the output. With a multimeter it's probably best on AC volts, but DC volts might also help in some places. The PCB layout is nice and symetrical so this shouldn't be too difficult. At some point, you'll find that the working channels and the non-working channels differ; that's the compoent that has failed.

The alternative approach is to guess which components are most likely to have failed and to probe them, again looking for differences between the working and the non-working channels. I'd start with the power semiconductors (what are they?) and the electrolytic capacitors.

Yep, you are correct, it attaches to the case:

Where's the heatsinking - is it on the case? Presumably that little bit of aluminium on the front is just a heat spreader and bolts to something bigger?

This sort of design tends to be quite under-heatsinked and it's not unknown for the chips to overheat and fail as a result...

Yes! You guessed right, it's from 2002 Peugeot 607. I know I can get it used for 20 euros or so, but it doesn't bother me that much that I'd justify spending money. I take it as an opportunity to learn about electronics and such.

Is that out of a car? I know JBL amps of that sort of size used to be fitted in some cars (hi spec Citroen/peugeot). If so and you don't get anyware fixing it, I'm sure some will have outlived the cars so try some breakers. What is the JBL model number in case I'm talking rubbish?

 

[endecotp]

I'm also not sure about "feeding the same signal into all inputs". The signal (=voltage?) needed depends on overall design, chips used, I can't just select, say, 9 volts and use that?

Audio signal. Similar to what it normally gets.