Commodore 64 problems (garbage screen on startup)

[+thegoldenband]

A while back, my girlfriend and I brought her family's old C64 home with us. We finally got around to checking it out a couple of weeks ago, once we picked up a switch box (since we don't have the AV cable).

 

Unfortunately, something inside it seems to be fried. As soon as it's powered on, it fills the screen with either rapidly-changing garbage characters (75% of the time), or a blank, single-colored screen, sometimes with a border (25% of the time). In the former case, random low-level audio noises were also audible IIRC, but I'm not sure whether they were coming from the sound hardware.

 

I know working C64s are relatively easy to come by, but this one has sentimental value, so it'd be nice to get it working again. What's the likeliest culprit? I have very very little C64 experience, and I'm not a hardware expert by a long shot, but I remember reading that certain chips inside are sometimes seated poorly, or otherwise tend to fail. It's entirely possible this computer was retired because of a hardware failure; it certainly doesn't look like it was ever abused, and it was stored in a dry and safe place (attic). The junk I'm seeing on screen is a mix of semigraphics and letter characters, like what one sometimes sees from a CoCo gone bad.

Edited July 22, 2009 by thegoldenband

[+thegoldenband]

Hmmm, sorry, this one probably should be in Hardware now that I think about it.

[Rybags]

You could try swapping the CIA chips... although they're probably soldered in.

[+thegoldenband]

From what I'm seeing on Ray Carlsen's excellent page, I'm guessing that the 906114-01 PLA is the culprit. However, replacing it myself is probably beyond my ability. Hmmm.

[Herbarius]

I remember the first computer I was supposed to get at kid's age had that same problem. It was a used C64 as well, and before it was moved over to our place it worked. So I figure maybe it is something that has gone bad during transport?

 

I remember discovering the charcter patterns changing between powerups and trying powering it off and on repeatedly, each time hoping the next time I power it on, the characters would "behave properly", returning the computer to working order... but much to my dismay, that didn't work

 

I remember my father and my uncle trying out different power supplys with no luck, and discussing what could be wrong with the computer, but in the end it seems they had no idea how to fix it and eventually gave up.

Edited October 6, 2009 by Herbarius