VIC20 no characters, blue squares only, PLEASE HELP - Solved - CPU error

[butjer1010]

Hi to all,
i managed to get another VIC20 in very bad condition, but there is a hope It turns on, but instead of characters, there are only few blue squares on the screen (pic in attachment)! I tried to change sockets on CPU and VIC chip, 'cause they were very low quality for better ones, tried to change both those CPU and VIC chip from working VIC20, and also both VIAs and 901460-03 (character ROM), but that didn't change anything!
There are no pictorial guide like for C64 that could make my life easier , so i'm asking if anybody knows where to start troubleshooting (don't have dead test cartridge for VIC20)?
Thanks in advance

[butjer1010]

Update:
Try to change Basic ROM from working one, no change!

[butjer1010]

Update:
Try to change Basic ROM from working one, no change!

Another update

tried to change UD8, UE8, UF8 and UD9 - no change!!!

[Vic20-Ian]

Try this article

https://sleepingelephant.com/ipw-web/bu ... sis#p75952

Looks like you tried changing Character Rom, try changing Basic Rom

[srowe]

Given that you have

• a screen of the right size and colour
• a steady character definition that looks right

I would say that both the KERNAL and character ROMs are ok. If you have an autostart cartridge does that boot correctly?

[butjer1010]

Try this article

https://sleepingelephant.com/ipw-web/bu ... sis#p75952

Looks like you tried changing Character Rom, try changing Basic Rom

Hi, thanks, but i mentioned in my second post that i tried that chip also. but no change! I have ordered diag cartridge, because i tried to change both RAMs ("small and big ones"), but the situation is same. All the chips are changed, with same result! Maybe the diag cartridge will point me in right direction!?!

[butjer1010]

Given that you have

• a screen of the right size and colour
• a steady character definition that looks right

I would say that both the KERNAL and character ROMs are ok. If you have an autostart cartridge does that boot correctly?

Hi,
i don't have any cartridge for VIC20, so i ordered diag cartridge (dead test), and i'm hoping this will guide me to some solution. I changed all the chips (only good thing is that i have all the chips socketed now), but this didn't changed anything. Still blue squares....

[butjer1010]

At the end, it was CPU that failed, but probably the first time i have changed CPU, i didn't change old, one sided, socket under CPU yet, so it gave me the same error. After a long time, socketing all the chips, and not finding the problem, i started again to exchange chips from working VC20! After changing CPU, everything worked!