13 Oct
2006
13 Oct
'06
2:36 p.m.
On 10/13/2006 at 12:36 AM ard at p850ug1.demon.co.uk wrote:
If you've got the scheamtics, it's obvious that the trimmer just 'pulls'
the crystal frequency a bit. And the CGA card scheamatics show how that
master clock is used to generate the NTSC colour subcarrier.
The TTL RGB monitors just take the 4 digital signals from the CGA card,
such a monitor doesn't use the NTSC encoder circuit. So it's clear the
clock freqeuncy doesn't alter the colour on said monitor.
IIRC, tweaking this trimmer is documetned in the Hardware Maintenance and
Service manuals, as a way to correct missing colour on a composite monitor.
-tony
But only if you're using a composite NTSC
monitor (the colour subcarrier
-- 3.58-ish MHz -- is 1/4 of the master clock freqwuncy, of course). On
a TTL RGB monitor, like the 5153, the trimemr does nothing.
Just relating what the notation is on the 5150 schematic. Most folks think
it's meant for correcting the time-of-day clock.