Followups set to sci.electronics.repair .

In sci.electronics.components DaveC wrote:
The board is populated with 80's technology, mainly 74LS' :: the
crystal pads connect to an 'LS04 inverter/driver and then to an 'LS96
parallel-to-serial converter.

Suggestion: Identify the "output" of the LS04, remove the LS04, hook up
a function generator to the "output" trace, and start turning the knob.
At some point, something resembling video should start coming out of the
output. Continue turning the knob until the period on the video is
correct. At a guess, the answer is probably somewhere between 1 and
20 MHz.

Another way to do it: Grab the first random crystal between 1 and 20 MHz
you can find and solder it in. Look at the video output with a scope.
You will probably see something resembling either PAL or NTSC video;
compare the period of what you see to the standard, and change frequency
accordingly.

An analog color TV will have a 3.579545 MHz crystal in it. An old PC
motherboard (286 and below) will probably have a couple of crystals on
it; one is often 14.31818 MHz.

http://en.wikipedia.org/wiki/Crystal...or_frequencies lists some
common PAL and NTSC crystal frequencies.

The board uses a 8275 CRT controller, and in the datasheet it says:
"CCLK is a multiple of the dot clock and an input to the 8275."

The "AC Characteristics" section gives the minimum CCLK period as
480 ns, which is 2.08 MHz. That doesn't mean you need a 2 MHz crystal
max; CCLK is one-eighth of the dot clock, so the crystal would be 16 Mhz
max. From the block diagrams on the data sheet, if the parallel bus on
the LS96 you found is hooked up to a couple of ROMs, then the clock
input to the LS96 is probably the dot clock.

Matt Roberds