Hello, I am trying to repair a framestore board from a for-A vps-500
vision mixer (late 80's, early 90's vintage)

I am unable to get any technical info on the board from For-A, but there
is a working second board for comparison. Unfortunately because they are
mounted side by side in a rack, it isn't possible to scope them both
running.

The fault is like a digital paint effect - looks like there are only 16
levels available in the output video. However it flickers at 25Hz (I am
in PAL 50Hz territory), and the problem only appears on odd fields not
even fields. So I went looking for an analogue-digital converter,
thinking maybe the problem was there. But there isn't one.

There is an area of RAM (8x sony CXK1206). There are some Sony CXK1202
chips (a delay line) and an SM5828 (shift register). There is a whole
load of PAL16CV8's. There are quite a few 74 series chips, mainly octal
latches.

If I could understand how it operates, I stand a good chance of locating
the fault. How would it digitise the video without an ADC?
--
Tim Mitchell