Well, my televisor and I had a bit of a fight, and I got quite angry and we split up for a while.
But, after counselling, we've decided to give it another go and we're back together. I've agreed to be more tolerant, and she's agreed to stop smoking.
I ripped out the original power system because it was just loopy. I was feeding 22VDC into everything, with the 12V regulated going into the sync separator but the raw 22VDC going straight to the LED driver (NOT to just to the light box). That didn't seem right. In any case, the original was fed by a single AC in which I know now doesn't work - to rectify two voltages, must use two transformers. So, out came the original power stuff. Out with the old, in with the new.
With the new setup, I'm feeding either 9VAC or 12VAC into the rectifier and then the consequent voltage into the regulator. With 9VAC I get a post-regulator voltage of 11.2V which isn't terrible, but not quite 12V. With 12VAC I get a post-rectifier of about 17V and post-regulator voltage of 11.8V which is closer but I know there's ample voltage and the regulator must be trimming to about that. I have noticed NO heat from the regulator with the lower voltages - the heatsink is obviously more than sufficient. With that sorted/understood, I'm moving onto the sync separator diagnosis.
(added note: the sync separator and LED driver circuits are running just fine with the 9VAC inputs (11.2VDC).
I 'scoped the signal direct from CD player first and that looked OK. Then I scoped after the brightness potentiometer, and saw good-looking increase in height (voltage) of the signal as I adjusted it. Cuts out completely just before the dial is at "0" and increases nicely.
Next I set the sync circuit adjustment pot to zero, and scope leads on the output of that circuit. I adjusted the pot to see/understand the valid range of that, and found the setting where the output on the sync stage was consistent for all inputs and all brightnesses - except, of course, for 0 brightness. The resultant output is many +12V spikes, and the occasional wider -12V spike. I hope that's what I should see. I will go back through my original build soon and have a look at the notes there.
Here's my interpretation - the image shows a grab with one of the (infrequent) larger -12V pulses, and two (frequent) +12V pulses. Obviously the pulse heights are 12V. The larger -12V pulse is about 1.2 ms long and I need to understand what that is. The smaller +12V pulses are spaced at 2.5 ms apart and about 60 us duration. Let's see - 2.5 ms is 400 Hz and divide by 32 to get a frame count gives 12.5Hz - right on. Will do some more analysis of that larger negative pulse after some reading to understand what it is.
So we're getting along a bit better. She's stopped smoking, and I'm not so angry.
My little DSO Shell oscilloscope is just fantastic for this work.