Which Hz, the horizontal or vertical? Either way, the deflection yokes are wound to be a particular value of inductance, which in is part of a tuned ramp oscillator circuit basically. When you try to force the oscillator to drive the yokes out of the range of frequencies they are tuned to operate, large magnetic fields can build up in the yoke and then dump all of that energy at once back into the circuit . Edit: Saw your other comment. You blew it up real good.