I needed a monitor for my CCTV system but didn't want to use a modern monitor that was needed for other purposes. I also didn't need a colour monitor, so an old Black and White TV would fit the bill. I chose a Philips 12B711 as it uses an isolated power supply and could work off a 12V dc input too. 

So the whole project relied on getting the set working first. I bought it off ebay and the seller said it was working but upon delivery, I discovered that the front of the inner cabinet was badly damaged, the CRT was loose and the main PCB had a huge crack across it. Plenty of glue and rewiring got the set back to a working condition. Input via the RF Aerial showed a good picture with minimal distortion. I then found the part of the video circuit where I planned to cut the RF signal out and introduce a composite video signal from the CCTV system.

Unmodified video circuit

The unmodified circuit. Inputting the CVBS into the base of TS350 should do the job and the output, from the emitter, feeds both the video amplifier and the synchronisation circuits.

Mod to Video CCT

I modified the circuit by cutting the print to the base of TX350 and introducing a 1µF 25V capacitor for the input isolation and a 10K resistor to the base to stabilise the image as the previos biasing components had been disconnected.

Here's the video of the modifications done. I did not video the repairs to the cabinet or PCB.