This odd hissing noise: did it occur during the repair, or after you tried to turn it on? And was it fairly short-lived, or did it exist until the machine was turned off?

If it was short-lived and happened with the case off while you were fixing something, it's probably a broken CRT.

However, if it only occurs when it's turned on and disappears when turned off, it may be a bad component on the analog board.

Some more data would be useful.