Re: vic20 video problem

From: Marko Mäkelä (
Date: 2001-09-20 08:54:56

Hello Alessandro,

I'm sending a copy of this message to the cbm-hackers list; hence I'm
quoting much of your text.

> I switched on and off the machine and saw that the startup message was in
> red.
> After some simple test I have found out that colors 4-7 in the color ram
> are seen as 0-3 by the vic.

> It looks like one of the data lines from the vic to the color ram is
> stuck to 0.
> Actually, from time to time the characters start flickering and it is
> possible to see dots or lines in the correct color.
> I have also seen that reading back the value in the color ram gives the
> correct value.
> This seems to exclude a problem in the ram itself.
> To me it looks like a bad connection between the VIC and the RAM.
> Do you think that the 4066 can be the reason ?
> What happens if the relevant switch is always closed ? The vic will see
> the normal data bus while the processor is accessing it ?

Or the buffered data bus.  Yes, according to the file 251027l3.gif on
FUNET, the UD1 4066 (assuming that you have a VIC-20CR) is connected to
VD8..VD11 and BD0..BD3.  The buffer is UF8 74LS245, in the file

I don't know what failure modes are possible with the 4066, but I'd just
swap that chip.

> I really hope it is not the vic itself.
> Do you know if someone is still selling the 6561 ?

I have some 6560-101 (NTSC) chips lying around, but I don't know if anyone
has spare 6561 chips.  The only source for them seems to be scrapped


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