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This board intercepts the luma and chroma outputs from the VIC-II and runs them through a finely tuned custom circuit to generate new composite and luma/chroma (S-Video) outputs. These new outputs can be wired directly to the C64 AV DIN connector, bypassing the noisy mainboard traces and poor amplification circuit inside the RF modulator entirely.

Since the original RF modulator can remain in place, this mod is less invasive than installing an RF replacement and easily reversible. It will also work on all C64 board revisions including the 326298 with the 5-pin AV DIN connector, or if building a new board there is no need to even install an RF modulator or RF replacement.

Feature summary

• Compatible with both PAL and NTSC
• Works with all board and VIC-II revisions including the VIC-II Kawari
• Allows use of a shortboard VIC-II in a longboard machine (see install notes below)
• Vastly improved S-Video and composite output
• Chroma attenuated to 300mV as per the S-Video spec
• Optional S-Video (S.FIL) and composite (C.BST) filters for fine tuning the outputs
• Adjustable luma:sync ratio (contrast control)
• Easily reversible and does not rely on an RF modulator being installed
• Fully assembled and ready to install

Install

This guide assumes you have basic soldering skills, a desoldering pump or gun, and know how to open the C64 and remove any RF shields if present. If you prefer a video guide please see below.

Please note the RFON is intended to be installed in the original VIC-II socket - do not solder the RFON directly into the mainboard.

As a quick summary we need to remove the VIC-II, set the appropriate jumpers on the RFON, install the RFON and VIC-II, isolate the RF modulator outputs and wire the outputs from the RFON to the AV DIN. Please read on for more detailed info.

Setting the jumpers

Carefully remove the VIC-II from its socket and before installing the RFON you will need to set the jumpers on the underside with a blob of solder to match your VIC-II version.

For 6567 or 6569 longboard VIC-II's close the 656X jumper. For 8562 or 8565 shortboard VIC-II's close the 856X jumper. For the VIC-II Kawari close both the 656X, 856X jumpers. It is also recommended to close the C.BST jumper on the top side with the Kawari but you can always experiment with this once installed.

Do not touch the 5V jumper unless you wish to use a shortboard VIC-II in a longboard machine. Closing this jumper will send 5V to pin 13 of the VIC-II. You must isolate pin 13 of the RFON from the C64 VIC-II socket first, otherwise you will short the C64 12V and 5V rails and damage could occur to the machine, VIC-II or even SID. To do this you must cut off pin 13 of the RFON flush to the board (just like pins 14 and 15 next to it) be careful not to damage the RFON PCB while doing this. You may want to put a piece of tape over these pins to be extra safe. You can then close the 5V and 856X jumper - do not close the 656X jumper. Once installed be sure to check for any shorts between the 7812 and 7805 regulator outputs before powering on the machine.

Isolating the AV DIN

Once you've closed the appropriate jumpers for your setup install the RFON into the C64 VIC-II socket and the VIC-II into the RFON socket, make sure to double check the alignment of both! Next we need to isolate the outputs from the RF modulator to the AV DIN connector (see images above).

On all longboards (except the 326298) this requires removing the solder using a solder sucker around pins 5,6,7 of the RF modulator so they no longer make contact with the mainboard. Alternatively you can cut the traces running to those pins if you're not worried about trace cutting.

The 326298 longboard only uses the RF modulator to generate the RF output, so instead we need to remove a number of components on the mainboard and also isolate the composite video input going into the RF modulator. Also keep in mind as this board only uses a 5-pin DIN, we also need to repurpose the SID audio input to use as a chroma output for S-Video - see 5-pin DIN notes below.

For the 250469 shortboards just remove or lift the rear legs of FB2 and FB4, and the left leg of FB3.

Now the required pins on the DIN connector have been isolated you can proceed to wiring up the composite, luma and chroma wires from the RFON - again see images above.

It is highly recommended to add a new heatsink to the longboard VIC-II if you haven't already. These ones are the perfect size to cover the entire VIC-II, and any other 40 pin ICs like the CPU and CIAs if you want to heatsink those as well. You may also want to pick up some 35x14x7mm heatsinks from the same seller if you want to heatsink the SID and PLA, and 30x14x7mm for the ROMs - or just replace the ROMs with ones that don't get hot ;-)

Setup

Note the new chroma output is attenuated to 300mV - this conforms to the S-Video specification but will also work fine with Commodore monitors that use separate luma/chroma. Aftermarket C64 S-Video cables may have a 300ohm resistor in series with the chroma output to bring the original 1V output from the C64 down to 300mV, this resistor should be bypassed when using the RFON. Or if using Sven Petersen's AV adaptor the "CHROMA HI" jumper should be closed.

Contrast fine tune

Commodore used a fixed value resistor in their RF modulators which don't take into account variances in the video output of each and every VIC-II. I've added this option so the output can be tailored for your VIC-II and display combination.

To find the ideal value for your VIC-II and display combination, change the foreground colour to white (POKE 53281,1) and adjust the potentiometer on the RFON stopping when the foreground is pure white. Note the trimpot only has a 270° rotation, do not try and force it to turn 360°.

Note this adjustment may have little to no effect depending on your display or upscaler - in which case it is recommended just to leave the trimpot at the mid position.

C.BST and S.FIL jumpers

These two jumpers can be used to fine tune the composite and S-Video outputs. Depending on your VIC-II, board revision, cables, display, etc. you may find closing these jumpers with a blob of solder will result in a more pleasing image.

For S-Video, closing the S.FIL jumper places a low-pass filter on the luma output. This helps reduce high frequency noise, ringing and jailbars at the expense of sharpness.

For composite, closing the C.BST jumper brings the chroma portion of the composite signal up to the standard 300mV (it's around 150mV with the jumper open). Closing this jumper can help reduce colour fringing around high contrast areas like black text on a white background, but colour transisitons may look more jagged.

Optional 5-pin DIN mod

Should you wish to use a 5-pin DIN cable or have a 326298 board, you can make a simple modification to bring the chroma output (normally on the center pin of an 8-pin cable) to pin 5 of the AV DIN (normally SID audio in). Using pin 5 for chroma also matches the pinout of the Atari 8-bit computers and my VIC-20 video mod and may also help reduce background noise from the SID. Practially nothing uses this audio input on the SID and removing it from the AV port helps prevent accidental damage to the SID.

Doing this allows use of cables such as this one or 5-pin DIN to 4xRCA breakout cables such as this one (style 1). This breakout cable also makes it easier to connect to Commodore monitors that have seperate luma/chroma inputs as these use RCA jacks instead of the more standard mini DIN for S-Video. Alternatively for more modern devices 2xRCA to S-Video cables like this one can also be found but keep in mind it's not guaranteed the cable will be wired with luma on the white plug and chroma on red, you may need to swap them around if you don't get a stable image.

To perform this mod you must first disconnect the audio input to the SID, failure to do this may result in damage to the SID. The easiest way to do this is to remove the capacitor at C12 on the C64 mainboard. On 326298, KU-14194HB and 250407 boards this will be a 10uF electrolytic capacitor. On 250425, 250466 and 250469 boards this will be a 0.1uF ceramic capacitor, these later boards also have a 100K resistor to ground at R3 (R14 on the 250469) but removing this resistor isn't necessary and if left installed won't negatively affect the chroma output or pose any danger to the SID.

Once C12 has been removed you can now solder a tiny bodge wire or component leg between pin 6 and pin 5 of the DIN connector. When looking at the board from the underside, pin 6 is the middle pin of the DIN furthest away from the rear of the board and pin 5 is the closest pin to the left of it (around the 10 o'clock position).

For more details on this 5-pin DIN mod and comparisons of the SID noise with the audio input removed, see this video.

RF output and returning to stock

As the RFON cuts off the input to the RF modulator, the RF output will be disabled while the RFON is installed. Should you wish to use the RF output, all you need to do is unplug the connector from the RFON board, remove the RFON and reinstall the VIC-II in its original socket. There is no need to remove the RFON wiring or reconnect the outputs from the RF modulator to the DIN connector just to use the RF output unless you want to return completely to stock. On the 326298 boards you will also need to reinstall all the components that were removed and resolder the single RF modulator input pin in order to use the original RF output.

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