I knew this TV thing would be a learning experience ;-) Somewhere along the line the problem stopped being intermittent - now it will not sync even from a cold start. So that speeds the troubleshooting process up at least.
By measurement and/or substitution (including a homebrewed K6 couplate), I think I've eliminated the AFC diodes and K6 as the problem. The Sams shows 3.5 volts (DC) on the grid but I still measure zero (although the AC voltage is somewhat greater). I think now the problem lies upstream from the AFC diodes.
I'm not real familiar with what the waveforms should look like, but I'm pretty sure that what I have is not right. Rather than trying to describe what I'm seeing verbally, I shot a short video off the oscilloscope screen and am hoping perhaps some of the experienced techs here might be willing to take a look. I didn't expect to see this much video and "jitter" at the output of the sync separator, yet the socket voltages aren't too far off. It seems clear that the horiz. oscillator can't sync, but I'm not sure where to look next.
The video runs only one minute and shows in sequence, 1) grid of sync sep tube, 2) plate of sync sep tube, 3) pin 2 of AFC diodes, 4) pin 1 of AFC diodes, and 5) a short clip off the screen showing the "tearing" and inability to achieve horiz. sync. All traces are with the scope set at 10 V/div and 200us/div.
Video:
http://smg.photobucket.com/albums/v6...ckel/Predicta/
I really appreciate the help. I haven't run into most of these circuits before but am trying to understand how they work.
Bob