This Apple II+, Rev 7, tagged (19)80(week)42. 48K plus Language Card and single disk drive and Apple Controller. Video seemed ok in all modes, until random white blocks started to appear here and there on the screen. I had replaced the power supply (new), but hadn't really cleaned up the computer. Pulled ALL the chips; de-ox'd the pins; and reseated them.
Computer is now functioning well .... with one exception: NO GRAPHICS.
I started with the Sam's™ "Troubleshooting and Repairing Apple II Plus" by replacing (with new) the 74LS194 at B4, 74LS194 at B9, 74LS257 at A8; then the 74LS257 at C12. They wanted to replace the 74LS08 at A11, but the 74LS08 was at B11; there was a 74LS74 at A11. I replaced them both.
Referencing the video circuits and other research on Applefritter, I replaced (with new):
74LS166 at A3 74S86 at B2 74LS04 at C11
74LS257 at A8 74LS194 at B4 74LS257 at C12
74LS151 at A9 74LS174 at B5 74LS51 at C13
74LS194 at A10 74LS174 at B8 74LS32 at C14
74LS74 at A11 74LS194 at B9
74LS02 at A12 74LS74 at B10
74LS02 at A14 74LS08 at B11
74LS11 at B12
74LS02 at B13
74LS02 at B14
The memory check (C050 C053 C054 C057 N 265:FF N 266<265.BFFEM 266<265.BFFEV 265:0 N 266<265.BFFEM 266<265.BFFEV 34:14 (Return)) had run successfully in the past, but now just produces a screen full of questions marks.
At this point, Sam's™ says, "Service Center Action", but there has to be something not soft switching. Any insight would be appreciated.