the focus/g2 block is the technical term of the screen/focus controls that are part of the flyback, and in some instances are NOT.... but the color classic flyback, the focus/g2 block is integrated into it, and it can fail. Also if your HV starts going low, your focus and screen will drift as well, but the raster will also loose brightness, and start to "bloom" but the HV cannot go too far out of spec, as the X-ray protection circuitry will engage and shut down the deflection circuits. I know some of these terms as I am in the TV repair business. The G2 voltage is your screen voltage. the "screen" control is the voltage that goes into the G2 connection of the CRT.