Fine‐Tuning the Video Signal

The following applies to both V3 and V4.

We assume the video cable and the terminations at the CRT monitor (or video upscaler + LCD) are both 75 ohms.

When using adjustable potentiometers in your build, turn the luma (Y) and chroma (C) pots counter-clockwise to decrease the output level, and clockwise to increase it.

Start with both pots fully counter-clockwise and adjust as needed, using a scope or visually.

The ideal chroma signal should be around 300mV peak-to-peak (AC coupled), with adjustments between approximately 250mV and 1.7V. This allows for optimization with S-video cables or compensating for cables with inline resistors. Some monitors tolerate higher signals, while others may display a checkered pattern if the signal is too strong. If you lack a scope, start with the chroma pot fully counter-clockwise and increase until the colors are saturated, avoiding a grainy appearance.

For luma, the target is 1V peak-to-peak (including sync pulse, DC coupled), but adjustments may range from approximately 575mV to 1.5V, depending on your TED chip. Original C16 and Plus/4 typically output around 1.2V, while later models are closer to the ideal 1V. If the output looks too dark, increase until light blue text is legible against a dark blue background. If text appears "white," you’ve gone too far. A good CRT test is white text on a black background; if it blooms, reduce the signal.

Note that composite video output remains fixed (1V peak-to-peak including sync pulse, DC coupled), regardless of Y and C adjustments.

If you still experience checkerboarding after adjusting the potentiometers, try increasing C84 from 100pF to 330pF (or experiment with other values) in parallel with R83. This combination creates an RC filter that can help eliminate the issue. C82 can also be something between 100pF, 120pF, and 150pF. Variability in A-to-D conversion circuitry across different LCD brands and devices like the RetroTink2X or OSSC may make some displays more prone to checkerboarding. In contrast, CRT monitors are generally not susceptible to this problem.

Ensure that your cable does not have a series resistor between 300 to 2K ohms on the CHROMA line, as some C64 cables do. If such a resistor is present, it will result in a very low CHROMA level without fine-tuning.