Troubleshooting a power section.

[GLUGSTER]

My friend owns an Hughes & Kettner Warp 7 combo. Recently, the power stage is starting to make weird "ring modulated" and other static type of noises, not right away, but after about 5 minutes or so of being used. The local repair guy "fixed" it once for this, but it started making these same noises a month later. Now he won't touch it with a 500' pole. I also cannot find any schematics for this amp anywhere. Any help on this subject would be awesome!!