Re: Scan rate conversion?

[aw133@lafn.org (James Poore)]

In a previous article, pubpc1@library.ucla.edu () says:

>Is it possible for a qualified technician to change the scan rate
>of a Mac, VGA or SVGA monitor so that it will sync down to 15Khz
>for connection to an Apple IIGS?

Technically yes it is possible if you want to go thru all the trouble.
The Horiz freq is the easy part that involves changing a resistor value.
Then you have to change the hipass network in the sync section....wait 
we're not done yet...the timing capacitor for the sweep time has to be 
reduced otherwise you will have a 50% overscan (actually yoke timing not 
sweep timing) once thats done, then you hope that the centering of the 
horizontal sweep is still centered and finally pray that the flyback 
doesnt fry due to excessivensweep times and that the X-ray protection 
circuit doesnt trip prematurely.  Now we havent discussed voltage 
reduction in the supplies that get their power from the flyback nor the 
reduction of effectiveness of the supply filters.

I *Have* done a monitor that was originally set up for 30KHz horiz rate 
and set it down to 15.750KHz with 6 part-changes (4 resistors-2 
capacitors) and it worked.  But I swear that the picture was not as good 
as one factory-set at NTSC Freqs.  Mebbe it was jus me.
Note:  This was done on a monitor out of a commercial video game made by 
the Wells-Gardner Corp, not a computer monitor (yes they are a bit 
different).

Now if you have a monitor lyin around thats gonna go to the dump unless 
you put it good use, I'd say go for it, you got nothin to lose, but if 
its a $1000 20" unit off your new setup, I'd say hands off!
-- 
James M (Jim) Poore--Apple II's Forever--Especially IIgs's
Los Angeles Free Net---aw133@lafn.org
Here siggy, siggy...Here Boy
Where is that damn SIG?