A variac allows you to slowly apply power, in steps, and also allows you to monitor the ammount of current the device draws as you slowly increase the voltage.I always use a variac when I power up a tv for the first time,this prevents damaging the set in the case that something is shorted.By watching how much current the chassis is pulling you can tell if there are problems.You should hear something from the speaker at around 90 volts.
BTW- Many older televisions did NOT have an isolated chassis.