Sunn Studio PA troubleshooting

[hijdra]

Hi, I recently acquired the solid state Studio PA (seen here http://sunn.ampage.org/site/museum/studiopa/studiopas.jpg) and am having some troubles with it. It powers up and gives off a very loud 60 hz cycle hum. None of the pots effect the volume of the hum and no signal goes through over the hum. Does anyone have a schematic for this amp (or one similar), or can anyone point me in the right direction of what to start checking?

Appreciate any help,
hijdra

[teemuk]

Check if the output transistors have blown.