Commodore C64 Modifications
By: AlphA

Enhancing Video Output

If you are thinking about performing any of these modifications to your main board, keep in mind that this is a twenty year old computer which has a reputation for being less than forgiving when worked on. These old components are sensitive and WILL die on you if you are not careful. I recommend that you do not perform any work on the main board unless you feel completely comfortable with doing so and are confident in your skills.

With that said, it's time to wreak havoc on the main board.

I really hate this ugly little RF box. Unfortunately, it houses circuitry necessary for video luminance and chrominance output. It occupies the area I plan to use for the serial and S-Video outputs. Therefore, it needs to be relocated.

This is the solder side of the RF box. It is held in place by four metal tabs and the eight (two sets of four) pin headers. Desoldering this was a chore because the four metal tabs are larger than traditional solder joints. In addition, they are soldered to the large surface area of the ground traces which act like heat sinks. Both of these factors make the solder difficult to heat and it cools quickly. Another thing to remember is that electronic components are installed with zero force and therefore must be removed in the same manner. Any prying or pulling will break off solder pads and likely destroy conductive traces. This is especially true for these older printed circuit boards. If the part doesn't move freely after the solder has cooled, then you haven't finished desoldering.

Popping the top of the RF assembly and removing the plastic from the top of the pins allowed me to work on them one at a time.

After removing the RF daughterboard from the shielding, I added wires to reconnect it to the main board. Then, I surrounded the board with tape to keep the components from touching the main board.
It looks pretty sad, but it works.

S-video time! This mod is easy to do, and it greatly improves video output. Check out these examples.
video comparison

Images were captured directly from the capture device without any image manipulation. Now that we know S-Video is sweet, this is how it is accomplished.
three points to tack-solder

Keep in mind; we're looking at the later (and much smaller) C64c main board. Most of my images are extreme close ups of small portions of the main board. If you are unable to locate this area, you should not be attempting this mod. Simply tack-soldering to these points works very well. The solder point just above the FB3 marking is luma out. The solder point just below the FB4 marking is the chroma out. Alternately, you can get these signals directly from the RF modulator.
pinouts for wiring S-Video connector

C Ground and Y Ground are the same on a 64 and should be wired together. Therefore, it is only necessary to run three lines.
S-Video without chrominance signal

If you're using a black and white television/monitor, you can exclude the chrominance, which will make your video image even sharper. Using only the luminance will give you the crispest image possible and can be used as a regular composite out or with S-video. Adding chroma slightly degrades image sharpness and is not worth using if you will not need color.

Itís worth noting that the original C64 does not have all of the appropriate Audio/Video connector for using the S-video extension cable for the Commodore monitor cable. The original C64 connector did not have pin six (chroma). This isn't an issue if you only want enhanced video without color. Does this mean that the chroma signal does not exist on the older mainboards? Probably not. Most video circuits have the chroma and luma separated before merging them together to create component video output. It actually requires additional components (typically a 470pF capacitor) to produce composite video. So, the older C64 may be able to output S-video after all. This is in spite of claims that S-video output is not possible on the original C-64. I do not have an older C64 to test this theory. Try pin seven of the RF modulator.
S-Video with less saturation

One thing I've noticed about using the S-video out on a C64 is it tends to over saturate the image. The first S-video image probably looks fine because you are viewing a captured image on a computer monitor. Viewing those color levels on a normal television would cause some nasty color bleeding. While too much color may seem like a minor issue, my LCD monitor absolutely hates this and responds by displaying my video as crazy disco rainbow color-cycling madness. While crazy disco rainbow color-cycling madness looks cool, I would not want to stare at it while trying to work. Plus, it can't be good for the hardware. The solution: add a small amount of resistance between the chroma output and the S-video connector. A 300 ohm resistor did the job.

Click HERE for more samples.

I wanted to have easy access to the mainboard for future maintenance and/or additional modifications, so I didn't want to tie my lines directly to the enclosure. I've added connection pins to the back of my new S-video connector, then cut and filed a spare IDE connector to create a custom socket. Lodging a toothpick into the unused input of the connector will assure that it is oriented correctly.

I wrote a small C64 prog which outputs colorbars and a 1k tone.
You may download it here (along with other monitoring tools).





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