Embedded Files
Commodore 64C Restoration - Part 3
April 21, 2026
So far we have cleaned up this old barn stored C64C, replaced some keyboard parts and addressed a failed MOS8701 clock generator. The system is outputting an image now but there is some static like artifacts going through the screen, and the colour fades in and out.
When on the bench I usually connect equipment up to a composite or S-Video to HDMI convertor to be able to see and capture output. The 8-pin DIN connector for the C64C A/V output has signals for Composite as well as separate Luminescence and Chrominance. Unfortunately with the convertor couldn't lock onto the composite signal so I hacked together an 8-pin DIN to S-Video cable to see if the Luminescence and Chrominance lines would provide any better output, but they did not. The only way I could get any image at all was to go directly into a TV, so unfortunately I didn't have the ability to capture any good screenshots of what I was seeing.
Let's take a look at the schematics for the 250469 short board.
Schematics show the color signal leaves the VIC II on Pin 14, goes into the RF modulator on Pin 4 and then comes out on Pin 7, going through a ferrite bead and then into the 8 pin DIN. So unfortunately with this design, the colour signal coming out of the VIC-II could be fine, but still get munged on it's way to the A/V output, even though we don't want to use RF.
For a quick and dirty test I decided to jump the colour signal from the VIC-II directly into the A/V output and lift the bead at FB4 to remove the color signal output from the RF modulator. This change cleaned up the signal and the colour stabilized on the display. So there was an issue somewhere in the RF stage that was causing the colour signal to get distorted.
Unfortunately I couldnt seem to find any schematics for the particular RF assembly that was on my board. There were some that were close, but nothing that was a match. Rather than spending time to trace the colour signal through the RF circuit, and then possibly encountering more issues that needed fixing in the RF stage I switched tactics and decided to pull the RF modulator out and replace it.
There are numerous RF replacement designs available, and I landed on one designed by Mark at The Retro Channel. He has a number of videos where he analyzes the video on the C64, produces versions of his solution and builds them. He was also kind enough to open source his design for the RF Replacement V2 on GitHub.
Desoldering the existing RF module is a bit of a pain as the structure wicks away heat very quickly. But with persistance I was able to pull the old module off the board. Using the gerber files from Mark's GitHub project I spun some boards, and ordered the necessary components. The design is well laid out and supplies an S-Video port in place of the existing RF out connector, as well as a multifunction TRRS jack which is capable of outputting composite video with SID audio, dual mono audio or it can route a signal from an Input pin located on the board.
Once the boards and components arrived I proceed with the build and installation.
RF Replacement V2 populated and installed.
A nice set of functional ports lined up in the shell.
After completing the installation of the replacement RF module I powered the system on and I was rewarded with the solid, crisp Commodore 64 Basic screen in colour through both the A/V output DIN, and the newly installed S-Video port. With a properly behaving video signal I was able to connect the C64 through my S-Video to HDMI convertor and begin to capture screenshots and video of the system operating.
The barn-stored C64C lives!
This isn't the end of the story quite yet. During a subsequent diagnostics test another issue was uncovered. See Part 4!
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