[Kevin Kuehn]

That composite video injection trick is a good thing for anyone working on old TV's to have in their toolbox. Once you get proficient at using an oscilloscope you can look at the video signal at 1st video input and compare it to the 2nd video output. According to RCA service data the in going signal should be about 1 volt peak to peak, and likewise about 30 volts peak to peak at the second video output to the CRT grid. The sync pulse needs to be negative going into the 1st video grid. Each amplification stage inverts the signal, which is why the polarity is important going in. Obviously it's working correctly the way you have it. As far as I know negative going sync is standard for most video component's, but it's not always the case where you're injecting into various TV circuitry. For instance when the video is feeding the cathode of the CRT in needs to be inverted compared to going to the grid.