Audison SRX5.1 in protection.

[marko]

Hi all, just trouble shooting this amp and it goes straight into protection, all outputs appear to be fine, the PS is using 75333p mosfets and one of them wasn't seated correctly on the heat sink when I initially stripped the amp down, someone else had already been in here and not done a too great a job! strangely all the PS fets also appear to be OK, I removed the suspicious fet but amp still goes into protect, gate resisters are 10ohms and seam fine.

..so where do I go from here? shall I pull the rectifiers to isolate the output section?

Mark.

[marko]

OK, I removed the rectifiers and the amp powers up fine, I think the sub section is where the problem is, the small drivers look a bit tired and also have small deposits of green stuff on them.. the problem is they are so small determining what value/type they are is a nightmare!

Mark.

[marko]

fitted rectifiers back in then pulled the smd driver on the suspect sub woofer output, the amp now powers up OK and the front and rear channels work fine now

so now I have to figure out the subwoofer channel and having a hard time identifying the smd driver I need, the markings are long gone! I really need a schematic but this would seam impossible to get hold of.. the front and rear outputs use different devices so I can't go by those ones.

any ideas?

[Perry Babin]

Sometimes, the drive circuit for the high and low side outputs are the same. Can you determine the components that way?

If they are using complementary driver components, you can look up one and the datasheet will give you the complementary part.

[marko]

I'll see if I can get a couple of good photos of the layout and post back..

[marko]

sorry about the poor resolution, must of had camera on lowest setting, doh! on the left you can see the green deposits I was talking about earlier.. you can see the basic layout of the amp and how the front/rear outputs differ from the sub outputs, at this point the actual outputs all seam fine

[Perry Babin]

Is the burned component connected to Q98 the same way that the components on Q100 and Q102 are connected to the center legs of those transistors?

The green stuff is corrosion. It's generally caused when the copper is exposed to moisture (water, electrolyte...).

[marko]

it's almost impossible to give a definite answer without scrapping all the blue film off, I really need to invest in a good magnifying glass to work on SMDs. but I think the answer is yes to your question..

[Perry Babin]

You don't have to scrape anything. You can see precisely where the traces are under the solder mask.

You need to use your meter (looking for 0 ohm continuity) to see if the connections are the same from the small transistor to the outputs in the front/rear/sub channels.

[marko]

just had chance to have a look at this, it seams they are connected in a similar way although I still don't quite understand how it all connects, much confusion with SMD technology and my eyes aren't getting any better with age!!