CRT RGB mod artifacts

RGB is the best picture you can get out of a consumer set. It is not free. A mux mod changes how sync, blanking, and OSD share the jungle, and a few artifacts show up. Most of them are expected. This page is the short list of what you can expect, why it happens, and what you can actually do about it.

Read RGB theory if you want the why. Use common CRT issues for failures that are not really "mod artifacts" (dark raster, foldover, shorted HOT etc.).

# Horizontal shift (mild)

Immediately after an RGB mod, you will notice the RGB picture is usually a bit left of the screen. You then have to walk it back to the right.

Service menu H-position, or a physical H-shift lever on some chassis, will recenter RGB. Those same controls also move composite and S-Video. You are picking a compromise.

If you want RGB centered and the other inputs left alone, shift RGB only:

1. An external RGB shifter
2. The sync shifteropen in new window on Sunthar mux boards 1.3 / 1.4 / 1.5

# Horizontal shift (excessive)

On some sets, taking sync from composite makes RGB jump much farther than a mild H-pos tweak can hide. This is due to composite sync going through extra filter and delay paths. That delay shows up as a large horizontal offset on RGB.

What helps:

1. Sync from S-Video luma or component luma instead. S-Video may need a switch (or a dummy plug in the S-Video jack) so the set actually enables luma.
2. Mux board 1.4 or newer — the transistor path does this for you when it sees external blanking.

# Scrolling picture on unused inputs

When muxed RGB / SCART or YPbPr is live, every input except the one carrying sync can roll. That is expected. The other inputs are looking at a sync they do not own.

There is no clean fix for this. You can build input-select blanking so external blanking enables only the video and audio input you are watching. That is a serious extra mod.

# Mono instead of stereo

SCART audio into a set that still has RCA jacks often comes out mono. The red RCA (right) has a mechanical lip that shorts left and right together when nothing is plugged in.

1. Dummy plug in the red RCA — works immediately
2. Recommended: cut the trace that connect right audio to left audio when nothing is plugged in to the right audio, or desolder the lip, typically found in the right audio port, so stereo stays on with nothing in the jack

Same idea shows up in common CRT issues.

# Faint OSD over RGB

If the jungle has a second RGB input, OSD can stay full strength. If you mux onto the existing OSD RGB lines, a washed out menu on top of RGB is normal.

RGB inline resistors on the mux board set that overlay. Higher resistance, brighter OSD — but do not guess. Use the RGB mux calculatoropen in new window so picture level and OSD land in the same ballpark.

# No RGB picture (blanking too low)

Raster is on. OSD may still work. RGB from the console is just not there. That is often blanking.

A mux mod expects something closer to 5V on the blanking / YS line (SCART pin 16) before the jungle will switch to the injected RGB. Several popular sources sit at 1.3V to 1.8V. That is still inside the SCART spec. It is usually not enough for a stock mux.

Typical offenders: original MiSTer FPGA, gscartsw.

gscartsw has a second habit. Pin 16 stays asserted the whole time the switch is powered, even if no console on that input is on. The set then sits on RGB blanking permanently, which also makes the scrolling unused inputs problem worse.

What helps:

1. Use a source that only puts 5V on pin 16 when that SCART input is actually live — MiSTer Pi, or a switch that behaves that way.
2. Use a 1.5 mux kit. It boosts a 1.3V-class blanking pulse up to the 5V the set wants.
3. Add the physical blanking switch (1.4 R2 and newer, and 1.5). That lets you force RGB on or off without trusting pin 16.

# RGB image is too dark after mod

There is a dedicated article for this. RGB brightness is the longer write-up when the whole picture is too dark or too hot.

# Noise and bars

This one is set-specific. Some chassis clean up. Some never go fully quiet. Shielding, diodes, blanking steering, and flux matter more than another service-menu pass.

That is covered in noise and bars.

# Irreversible cuts

If you care about keeping the set original, skip the full SCART window in the back shell. Once that rectangle is gone, it is gone. Prefer a no-cut or a cut you can actually hide later.

What I do instead:

1. Route the ribbon out of an existing gap and leave the mux board hanging outside. No new hole.
2. Use the micro mux kit in the RF tuner opening. You may have to desolder the tuner and open that hole a little so the board sits flush. Tuner can go back in later if you keep it.
3. If there is no tuner hole you want to use, a 5/8" round hole for the micro board is enough. A round hole patches cleanly. A SCART-sized cut does not.
4. Last resort: a clean, stock-looking port with no switches, on a ribbon (or similar) so the board unplugs from the chassis. You will always need an input. You may not want today's switch-and-blanking scheme glued into the plastic. A tidy detachable jack is fine. A clunky switch panel and a huge cut are not.