Crate GFX 30 solid state amp problem

[betweenthees]

I have a Crate GFX 30 solid state amp I'm having a problem with. After about 2-3 minutes after turning it on it gets a hissing sound that isn't effected by any of the controls. It may be in the effects knob because when the knob is turned nothing happens with the effects.

Im not an amp guy realy so im not sure where to start looking to fix this.

heres some info that might help
www.crateamps.com/pdf/manuals/GFX30_OM.pdf

Thanks