Need help with Yamaha ax900u amp

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So, I have blown a channel in my ax900u. I did some investigation and found that the four power transistors had shorted. I also found a couple of fusistors blown. I replaced these and powered up the amp and in a split second all four power transistors blew again. I have tried looking at the steady state dc voltages but can't seem to find the problem. What would cause all 4 output transistors q323, q325,q327, and q329 to blow so quick?
 

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Always use a DBT (dim bulb tester) when fault-finding. It will prevent such catastrophic failure modes.

You need to test the driver and pre driver transistors along with any low value resistors associated with them. Add a link to short out Q311 (the Vbe multiplier) and try again using the bulb tester.

Also beware of fake semiconductors. Always buy from reputable and authorised distributers.
 
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