Depends on the electronic switch...many IC logic circuits for instance operate only at a max of 5 volts. Too much voltage on an input and you'll damage the circuit. If the manufacturer used one of these with a 20% tolerance, of course anything higher than 6 volts can do damage.
Please review your EE-101, no engineer in their right mind would use 5V IC signal to control a sync output directly... regardless of 6V (or not)!
May be our resident EE expert KCan
can help you here... :G :G :G