The SX-64, as it should be

Spot the difference.


Using the 901227-02 KERNAL chip


Using the 251104-04 KERNAL chip

The former owner replaced the original SX-64 KERNAL (251104-04) with a C64 one (901227-02), most probably to be 100% compatible with all the games.

But an SX-64 doesn’t feel like an SX-64 without its unique colors. That’s why I replaced the KERNAL chip again, this time with the original one. Actually I couldn’t find the original chip (the MOS 251104-04) anywhere, so instead, I installed an EPROM chip with the original SX-64 KERNAL programmed into it.

Fixing the Commodore SX-64: Replacing the MOS 6526

MOS 6526: The CIA chip

You turn on your beloved SX-64 and you don’t see the blinking cursor. Don’t panic. Most probably one of the CIA chips (MOS 6526) is not working correctly.

No blinking cursor

The Commodore 64 has two CIA chips. What you need to do is to replace the one that controls the keyboard, the CIA 1. But if you don’t know which one is CIA 1 and which is CIA 2, then you can either replace both, or do trial-and-error, or look at the IO schematics (hint: it is the one with the UB3 legend).

As far as I know any MOS 6526 should work:

  • MOS 6526 (found in the very first C64s)
  • MOS 6526 R4 (found in newer C64s)
  • MOS 6526A (the 2Mhz version, and I read somewhere that it works Ok)

So, in order to get the replacement chip, you can get it on eBay, or extract it from another another C64 or SX-64. The MOS 6526 chips on a regular C64, are located at the top-left corner.

I removed the 6526 from a regular C64

I suggest using a chip extractor to extract the chips, although a flat screwdriver can work as well.

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