Commodore 64 – Basic ROM, CIA replacement and Recap

This Commodore 64 is turning on and outputting a display but is not booting as expected. Instead of the usual 38911 basic bytes free message, it’s simply showing a blank screen with no cursor.

Inside the board was partially socketed but the Basic and Kernal ROMs were not so I started by socketing those as its helpful for whoever may own this board next and I suspect the Basic ROM may be gone here as that would cause this issue.

With the board socketed I start by switching out the Basic ROM and this somewhat fixes the problem. I now have it booting but there is still no cursor. That points to the CIA 6526 chips, they are responsible for handling keyboard, joystick, cursor, cassette and all manner of input/output functionality. Turns out its U1 and I don’t even need to test U2. I have the proper boot screen complete with flashing cursor. Just for good measure I load up Flimbo’s quest to test the joystick, keyboard, cartridge slot and sound. Everything works perfectly. Job done! Well, not quite…

This is a 250407 board and they have a lot (and I mean A LOT!) of electrolytic capacitors. I always like to replace the capcitors if they havent been done already, and this board definitely has not. A short while later and the board is completely recapped:

I’m going to clean up this board a little with some isopropyl alcohol and add heatsinks to the SID, CPU and PLA before using it this weekend to play some Sam’s Journey to ensure its running fine.

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