Video isolator

Video isolator

A video isolation circuit for adding inputs such as Y/C (S-Video) to hot-chassis TVs, including the Sony KV-1484, KV-2184, and KV-1442.

The primary use is adding a chroma (C) input to a TV that already has an isolated composite video input. Luma (Y) uses the existing video input path.

Safety

This modification is intended for people experienced in servicing hot-chassis TVs. A mistake can cause a fatal electric shock or damage connected equipment, including a console.

These notes describe the original modification, not a complete installation or safety procedure. Wiring and supply rails must be verified for the specific TV.

Do not connect the isolated and non-isolated grounds together. Incorrect power or ground connections can defeat isolation without any obvious sign of damage.

Compatibility and circuit reference

The TV models above were sold with and without AV inputs. This project primarily targets versions with AV inputs. A version without AV inputs may require one isolator for composite video or two for Y/C, plus an isolated power supply for the isolated side.

The circuit is based on the application circuit on page 10 of the HCPL-4562 datasheet. A local copy of the datasheet is included in case the external link becomes unavailable.

Project files

Open video-isolator.kicad_pro in KiCad. The design files were saved with KiCad 8.

Modification notes

Chroma input

The example board uses YCM301/YCM302 in the Y/C splitter and comb-filter circuitry. The original modification lifts pin 1 of the comb filter and injects the isolator output there, feeding chroma directly from the Y/C input. Luma continues through the normal video path.

The chroma input may need a voltage divider or termination. Use capacitive coupling, with the termination before the series capacitor: the example schematic shows the chroma input biased at approximately 3 V. Excessive chroma level can cause artifacts.

Power and isolation

Identify the input board's isolated and non-isolated supply and ground rails before connecting the circuit. Keep each side of the isolator connected to its corresponding supply and ground.

In the original installation, the RCA input ground identifies the isolated side, while chassis ground identifies the non-isolated side. The input board's small transformer and associated smoothing capacitor help locate the isolated supply. Verify the actual connections and supply voltages against the TV's schematic.

Bias adjustment

The onboard potentiometer adjusts the amplifier's bias, not its gain. Adjust it so the amplifier operates in its linear region. Too much bias can cause overheating; too little can cause nonlinearity.

Installed board

The isolator mounted on the TV's input board:

Results

The modified TV displaying a picture:

Luma/chroma timing mismatch

A small timing mismatch was observed after the modification. Chroma bypasses part of the original processing path while luma still passes through the Y/C circuitry. In the example below, the mouse cursor's chroma appears ahead of its luma, producing a blue shadow.

The effect is subtle and most noticeable under extreme conditions. Bypassing the Y/C filtering entirely was suggested as a possible fix, but has not been tested.