Gallien Krueger Fusion 550 pops like a rifle when switched on

[John Lovell]

Hi there. My Gallien Krueger Fusion 550 pops like a rifle when switched on even with all the controls set to zero and the mute button engaged.

Any thoughts on fixing the fault or a copy of the schematic diagram or service manual out there?

[Enzo]

Does it otherwise work OK?