C64 random colors

Newsgroups: comp.sys.cbm
References: <4e4qrk$1cj@cc.joensuu.fi>
From: (William L. Mays)
Subject: Re: Please help: Random Colors

SvenDroll wrote:
: But then the colors start to change very fast and randomly. There is no disc-access possible.
: However, the keyboard-behavior is still ok.
: I have changed out the color-RAM and the VIC, but the same(!) error still occurs.

There is a type 4066 quad CMOS bidirectional switch 14 pin chip that connects the main data bus to the color bus (that goes from the color-ram to the VIC).
If data is leaking onto the color bus, that could give the symptoms.
If you remove that chip, then you can not write colors to color RAM, but the colors should not change.
You might also check the AND gate 74ls08 that feeds ChipSelect pin 8 of the color ram.

If you break the connection from PLA 13 GR/W-n to color ram 10 WE-N, by lifting a pin or even cut a trace, that will prevent writing the color ram and seperate it from the PLA. If your PLA is socketed, just lift pin 13 at that end.

I had the problem before that the PLA output went always low = write to the color ram and the colors went crazy. I know I changed one of those 4066 chips once too, not the one on the joy stick analog ports (U28?), the one in the color data circuit (U16?)

I am using my old 1982 schematic, in case some of your chips are different numbers. 

-- 
Speaking only for myself:  Bill Mays
\  /                       
 \/                        (=   <-I'm a mushroom...
 /\                        Same place, same job,
/  \  my mark.             fourth company.
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