First point of great interest: there's a 256x4 PROM involved. Has anybody ever dumped that? From a photo of the motherboard, it looks like IC23, the TBP24SA10 just below the modulator.
Inputs appear to be:
- pin 2: the R channel
- pin 3: the G channel
- pin 4: the B channel
- pin 7: the sync channel, inverted (presumably so that high = sync?)
- pin 1: the output of a sync-triggered 74LS123 (presumably a back porch timer, for the colour burst?)
- pin 15: the output of a sync-triggered 74LS74 (a flip-flop; to achieve the alternating part of PAL?)
- pins 5 and 6: the output of an independent 74LS74 (presumably a free-running colour subcarrier phase counter?)
Just from pin counting, that would appear to imply a very discrete approximation of the colour subcarrier (two input lines, so four values?), with only sixteen possible composite output levels?
Is there anybody that could dump the TBP24SA10?