Progress! I replaced R23 with a 56K resistor and replaced the .002 coupling cap with an .01 cap. The picture was nice but the vertical was still somewhat unstable. I doubled the value of the coupling cap to .02 and now the vertical is very stable.
Here's the video signal direct from the generator:
Here's the signal coming out of the preamp/inverter:
And here's a DVD using the preamp/inverter:
The screen image is so strong and bright that I had to turn it way down to avoid washing out in the photo.
I tried using a 9V battery in place of 12V, and it seems to work fine with 9V.
It would be nice to use this in a switchable A/V adapter like the one described in
http://antiqueradio.org/A-V_AdapterForVintageTVs.htm . Then you can just flip a switch to change from A/V input to ordinary input from the antenna. Changing the value of R23 complicates that scheme, but I suppose there's a way.
If I installed this permanently in the chassis (with or without switching), it would be nice to find (or generate) a +9V source somewhere in the chassis rather than rely on a battery.
Phil Nelson