The cause of the problem was a worn or broken trace in the RAS line between U26 pin 11 (74LS373) and the VIC pin 18. The symptoms with a cartridge were vertical bars with colors and patterns varying depending on the cartridge installed, and sometimes included patches of random pixels around the screen, and the computer was otherwise frozen up. The display shown in this picture is with the 586220 diagnostic cartridge installed without cartridges produced only a blank black screen at power up. – Bad socket contact on U1 (CIA) – bottom pin(s)Ī bad trace on the board (broken or causing short circuit) can be the cause of all sorts of problems. Blank screen if chip is shorted (remove to check) and chip may get hot to the touch. Partial failure: some keys or joystick positions don’t work, one character appears ahead of startup cursor or random characters appear at startup. No keyboard or control port access but cartridge works. Though in this case the problems were simply caused by a bad connection between the CIA1 chip and its socket, similar symptoms can likely be caused by a failing CIA chip. The computer seemed unresponsive on every boot. On one boot, screen was filled with white garbage characters (third picture). Sometimes the characters kept switching between upper and lower case. After about 10 seconds of waiting, most of the normal startup text appeared along with some garbage characters, and in some cases automatically typed load/run commands (second picture). ![]() The most common outcome was an empty blue screen with borders (first picture). Caused various abnormal startup screens, including a simple black screen on a couple of boots.
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