blown channel on Samson 308i class D

[bradbarron]

Hi guys

I have an expedition 308i PA system that i use for parties, playing music, and also as a home sound system (has an ipod dock - works great!).

Unfortunately, the left channel just quit working. It is definitely not the speaker or cables - have swapped all of these around. I also took the housing off and there is no visible failures (burnt resistors, popped electrolytic caps, ...).

I am not an audio electronic expert but have built a few small battery powered solid state amps, stomp boxes, ... as well as a bunch of other home electronic stuff. Also, I moved to Beijing China about a year ago so canot easily take it into a service center. Replacing is an option but electronics are crazy expensive over here and you never know if you are buying a counterfeit. It seems that I must be able to fix this thing. Any suggestions on how to track down the culprit?

There is a basic schematic here on the second to last page: http://www.samsontech.com/site_media/legacy_docs/XP308i_OM_v1_1.pdf

This is a class D 2 x 150 amp.

Glad to upload pictures as well if it helps.

thanks in advance

brad

 

[Markus2006]

This is not a schematic - it's only a block diagram. It will not help much. I would check power supply for the second channel and output transistors. Are there any fuses inside the amp?

Mark

 

[bradbarron]

agree mark - would be better to have the detailed schematic

i'll tear it down again tomorrow - didn't see any obvious fuses when i had it open poking around but i guess it is time to get serious with the multimeter

brad

 

[nigelwright7557]

I would check power supply for the second channel and output transistors. Are there any fuses inside the amp?

Mark

An output transistor(s) going sometimes takes the class d chip with it in my experience. Check gate resistors too.

 

[bradbarron]

Hey guys - i have reached the limits of my multimeter skills. Guess i should have studied electrical engineering instead of chemical.

There are photos of the teardown at sampson 308i teardown - a set on Flickr

I checked all the transistors on the board - the board is multilayer so i couldn't figure out which led to which channel. There are details at this picture in the set: top side of main board | Flickr - Photo Sharing!

Q5 is suspicious as all combos tested open with my miltimeter in diode mode

The fuse (bottom L - has a cover on it next to the yellow boxy transformer) is fine

Nigel - how do identify the gate resistors?

Also - any suggestion on how to check the voltage regulators without powering up? I get nervous around 220V

Nothing seems burnt, no busted caps, no visual signs of anything beyond a bit of dust.

Any more suggestions guys? I frankly had a blast today trying to sort this out but am out of ideas.

brad

 

[Markus2006]

Brad,

I would first identify where exactly are the power amps. The power amps are these two integrated circuits under the heatsink (covered with white grease). It means that there are no separate output transistors (as I thought), nor gate resistors to be checked. Everything is in the ICs. The components that you checked are most probably in the power supply. I seems to me that you don't have to check each of them separately, but I would just check whether there are proper voltages on the output. If they are correct, the chips are the most suspected. In similar cases (when you don't have schematic) you have to check the circuit on a "module" basis. I mean whether the power supply works, whether the power amp works, whether preamps work.
I hope you can fix it but in several similar cases I had to replace the power amp ICs. This is not an easy task since they are surface mounted components.

Mark

 

[bradbarron]

thank Mark

Unfortunately, while my circuit skills are fair to poor, my IC skills are nonexistent. Without a pin diagram, i have no idea where to check for the output voltages.

Oh well - it was worth a try anyway. Guess it is time to go shopping ....

thanks for all the quick help guys

brad

 

[Markus2006]

It's a pitty that you give up. It can be something very complex like failed output IC (requires some advanced soldering skills), or something very simple like open fuse or non-contacting plug. Don't throw the amp away .

Mark