After reparing my Defender ( see this log: http://www.jammarcade.net/defender-repair-log/ ) I noticed that the screen had some horizontal lines of different brightness.
At first I didn’t give much attention but then I decided to look at this issue deeper.
Moving the screen left and right with the monitor commands I noticed these defects:
Having the experience of a similar problem on my Burnin Rubber ( http://www.jammarcade.net/burnin-rubber-repair-log/ ), to me it was clear there was a video blanking problem
With the schematics in hand I found this circuit:
Signals from 7420@5C were good so I started probing the 7400@4B
All signals were good except that pin 10 was floating.
Looking at the above mentioned schematics, pin 10 is tied to the nearby pin 11!
I checked the board and found out that actually the pcb has no trace connected the 2 pins togheter!
No broken traces, it is actually simply not there!
I connected the two pins and…..
Now I am wondering if my CPU board revision has a production error.
Searching on internet I haven’t found a similar problem therefore I would like to have a feedback from other owners of Defender if their pcbs has the trace or not.