P3 Chassis S-Video Mod

This chassis, like many other TVs of the same generation, does not have an isolated power supply design - which makes traditional mod methods unsafe due to floating ground potentials. The way composite video is safely fed into this chassis is via opto-isolation, which uses a floating connection to transmit the video info without sharing a common ground point. This method is the safe way to add additional inputs to these types of TVs, and finally Kaz Packman aka KPackratt2k has designed a PCB to facilitate this for S-Video modding. Below are his instructions for the mod.

PCB Purchase

Schematic

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Notes

You can solder the wires for power and ground on the AV input board (or the U board) and run them to the Chroma isolation board via JST connectors. I forgot to take photos of the tap points, but the cold side shares the ground with the AV inputs whereas the hot side does not. The wires for Chroma go to the chassis (or the A board), you'll have to lift the Chroma coupling capacitor (C357) from the side that connects to pin 39 of the jungle chip (IC301).

To reduce the risk of interference, I strongly recommend using shielded wire for the ends of the Chroma connections that send it to the jungle chip. When I prototyped this mod, I was getting noise that sometimes affected color intensity on the S-Video input when I used unshielded wire. Using shielded wire instead eliminated the noise and kept the color intensity consistent with the stock Composite video input.

As for a place to mount the Chroma isolator board, there's an empty space in the cabinet where the chassis boards are mounted, the empty space is located near the input board (the U board), providing a convenient spot to mount the isolator board using 3M VHB double sided tape to hold it in place. It is recommended to place two layers of kapton tape on the board before putting the double sided tape on it.

Connector labels on the Chroma isolator board and their tapping points on the TV:

S-Video Chroma input (the 2-pin JST on the bottom-left of the isolator board):

Hot power and ground (the 3-pin JST on the top-right of the isolator board):

Chroma I/O from the TV chassis (the 2-pin JST near the 4-pin JST on the bottom-right of the isolator board):

Chroma I/O to an SPDT switch (the 4-pin JST on the bottom-right of the isolator board):

Luma (S-Video pin 3) simply goes to the Composite video input.

You'll have to be strategic for finding a place to mount the S-Video jack and the Chroma switch. I've decided to relocate the RF tuner I/O panel somewhere else and run a short Coax cable from the VHF/UHF input to a coupler mounted in the VHF/UHF input's original hole. The S-Video jack is mounted on the "Aux" input on the left, though the plastic on the back side of the jack panel plate had to be shaved to allow for the panel mount jack to fit without the nut coming off. I mounted a spare panel mount SPDT slide switch in the "To Converter" hole in the middle of the jack panel to serve as the switch to toggle between S-Video and Composite video/RF.

When/if I get ahold of more Sony TVs from the same era, I'll be sure to test the mod board on those models and document any specific instructions that apply to them. I've also been working on replicating the S-Video switching circuitry from the KV-27TS30 to implement on other models with the same chassis for either using a SPST switch or a completely switchless mod using an S-Video jack with its shield cut in half to connect one side to ground and the other side to a high/low logic signal.

Result