My Apple II Rev 0 Failure Problem

First check the values of all the resistors on the motherboard, I made a mistake with one when I built my replica and it really became a big headache. (:

https://youtu.be/NIRTCWGcBp4?si=FgIpu4uKv_MhtfEB

It looks like the  ENABLE  signal at the 74LS151 8-to-1 video multiplexor at A9 isn't switching-off promptly at the end of its  WINDOW  interval.  Some possible causes:

• A9 itself might be faulty.
• Pin 7 at A9 might have a poor connection to pin 13 at A10
• A10 might be faulty
• Pin 5 at A10 might have a poor connection to pin 6 at C14
• The gating logic at C14 or C13 might be faulty

If any of those faults occurred, then the video multiplexor could continue to transmit a signal after the  WINDOW  interval has ended.  On a Rev 0 motherboard, that would cause characters to be generated from BASIC's storage memory instead of the screen memory.  To test that, try typing this BASIC command to fill that region of memory with blank spaces and see if it erases the noise in the right margin:

    FOR A=4096 TO 8192:POKE A,160: NEXT 

That command takes about 25 seconds.  If the fault originates from one of the causes listed above, then this BASIC command will erase it from the screen.  (It doesn't really fix the problem, but it helps pinpoint where the noise is coming from.)