Tektronix 314 - floating power supply

Voltage between a USB GND line and the 'GND' of a floating power supply. There is a 10:1 probe, so the voltage is 200Vpp. It hurts to touch it.

This popped up because of a BAD RCA plug on the jyetech's probe wire while trying to measure a PWM signal on a device connected to my PC via USB. The probe never connects properly and no common ground was there. So the voltage chose my hand to equalize. Ouch. This is pretty annoying and I could have wrecked my controller with that ;-( I've cut off the RCA plug and tossed it into the trash to prevent further problems. I need a new ones, good ones, better ones.