Vic-20 display issues

[Meall]

I have a Vic 20 that displays a bunch of random characters on screen at boot up.

 

Im trying to find the issue, but cant find it. I tested all socketed chips in a working one, and they look fine.

 

Any idea?

Edited December 9, 2017 by Meall

[+OLD CS1]

I have no idea how the logic in a VIC-20 works so I cannot help, but I am very intrigued by the fact the characters appear to be up-side down and reversed... unless your picture needs to be rotated

[Meall]

I have no idea how the logic in a VIC-20 works so I cannot help, but I am very intrigued by the fact the characters appear to be up-side down and reversed... unless your picture needs to be rotated

They are not reversed on screen, that’s what happens when we take a picture of a monitor.

[carlsson]

I believe this is a RAM failure. Sometimes if you insert a RAM expansion that is faulty, you can get similar effects on screen, in particular if you insert one that maps into the RAM/ROM block at $A000 or if it is the lower RAM block at $0400.

 

As always, Ray Carlsen is a resource on troubleshooting. This is what he writes about the VIC-20 models:

http://personalpages.tds.net/~rcarlsen/cbm/vic20/vic20.txt