Vic 20 Issues

[craftsman1234]

Hey got another Vic-20 in bin of other Commodore stuff. Cleaned it up and tested it out. Worked great. Even hooked up to a test harness no issues. Ran for two days straight. Then the screen went blank. Well almost. See the below videos.

One is stock on a 1902. Running Avenger, I've also ran Radar Rat Race. Nothing is really visible on a stock monitor.

https://youtu.be/aFjCeIithYU
https://youtu.be/d2Ps4lhqJxc

The other is on an LCD. and you can see the screen. On games you can see the high scores but noting else. On the stock you can see the jiffydos banner. Both fill the screen. No graphics.

Any ideas on what can cause this? I've pulled the 2114 ram, Kernal, 555, c42, u11, u12. No difference.

[craftsman1234]

Follow up to this . I ended up removing the kernel character set basic and the 555. It turned out the biggest problem was the Vic chip. Now I can get a boot up screen but all the characters are the @ symbol. stays this way for about 30 seconds or until the computer starts to warm up . Check out the video

Also the replacement Vic chip gets hot super fast

https://youtu.be/pzuOAZVLU1k

[TheMightyMadman]

Hi,

I know this is a late reply, so my apologies!

Quick questions: is this an early VIC-20 with a two-prong PSU, or a later one with the C64 style PSU? Are you using an original PSU?

Garbage characters tend to be caused when a RAM chip holds a data line high or low when it shouldn't, hence the '@' issue - this fault could go away when the RAM chip warms up if the issue is temperature-sensitive, but this normally happens the other way around (chip malfunctions when warm).

Do any of the RAM chips get warm? This means it is shorted, though it is possible for RAM to fail open-circuit instead which you can't easily diagnose by its temperature. On a later VIC these are the two large chips under the VIAs (U14 & U15), and the group of three smaller chips to their immediate right (UE1 & UE2 are system RAM, UD2 is screen RAM).

RAM tends to fail before other chips when a PSU fails and outputs overvoltage, as they are more sensitive to it. The VIC chip normally gets hot to the touch but shouldn't be burning hot, and it shouldn't heat up really quickly - on a later VIC this could be due to a bad PSU feeding it too much power, so please check yours in case it ends up taking the chips out.

Cheers