Function generators have come down in price so much the past 10 years or so. In the past a good function gen was a very specialized expensive piece of kit. These days you can get a pretty decent unit for not much money at all.

When I first got this monitor powered up, the vertical deflection was totally collapsed. Turned out two transistors were acting weird. They weren't broken exactly, but not acting like they should either. Without a function gen I would not have caught that as quickly as I did.