Do you have another scope ? IDK how much digital stuff these have, like microprocessors and ADC/DAC's, but I'd try to trace a signal from the input. And also work back form some output drive transistors or op-amps. Some of that might have pretty high voltage, I'd skip most of the HV stuff, till last, if nothing else was wrong.
IDK what's in the troubleshooting flow chart, but the schematic's are usually full of test point voltages.
Then there's the power rails, have you checked any of that yet ?
Maybe some little part fried when you powered it up. Hopefully not some rare chip. These most be pretty old, I bet there's lots of old old caps in it. Did Tektronix use any carbon resistors back then ??