Rockford Fosgate P300-1 Hum noise

[ss17]

This is my 1'st post, and 1'st amp to attempt repair. I have a Rockford Fosgate P300-1 with a hum, no music coming through that slowly increases in frequency from around 400hz. This occurs with or without the RCA's attached. It turns on, and there are no fault indicators, and the board looks perfect. Any idea what might cause this? I read that someone else had to replace caps, but the 3300uF 50V caps tested OK.
Any assistance is much appreciated.

 

[Perry Babin]

Does the position of any of the potentiometers or switches make a difference?

 

[ss17]

I have adjusted the gain, crossover, blue potentiometer on the board, and checked the RCA connection, and none of them made a difference.

 

[Perry Babin]

To clarify, the amp performs 'precisely' the same no matter what you've tried?

Do you have a scope?

 

[ss17]

yes I have tried adjusting all switches and all connections and it performs the same no matter what. I was able to hook the amp up to a sub that i can see when I turn it on the sub goes all the way One Direction and stays there while the humming noise continues. I tried reversing the polarity on the speaker wires and confirm the sub goes all the way the opposite direction and stays there while having the humming sound.

 

[Perry Babin]

That means that you have DC on the output. You may have a shorted output transistor. That's the first thing to check.