Examination of the 8552A schematics suggests that the blown fuse is connected with +100V only indirectly, because it is the reference controlling the -12.6V regulator.? The +20V rail won't?go above nominal on +100 overvoltage, but there is a 24V/14V crowbar on the -12.6V rail.? The fact that this crowbar is explicitly designed to pass a -24V spike for a few milliseconds after power-on indicates that HP was aware of the gas tube ignition problem.? But if the spike goes on too long, the crowbar setpoint drops to -14V, and poof.
