Re: Book "Understanding the Apple IIe" - PAL encoder schematic?

["Linards Ticmanis" <ticmanis@gmx.de>]

Hello Wayne and thank you so much for taking the time to do this!

It turned out the solution was much easier than I expected. The circuit
is really quite different than that used on the Europlus PAL card, and
it turned out that the color carrier quartz in the //e is always
active, even in text mode. Only the color encoder's chip select signal
gets shut down, not enough to remove the subcarrier from the output to
a sufficient degree for sensitive TV sets.

The solution: Connect a diode (a simple 1N4148 will do) from Pin 1
(Subcarrier in) of the TCA 650 PAL-decoder-abused-as-PAL-encoder chip
at board position A13 to Pin 9 (Q2) of the 74LS74 Dual-D-Flip-Flop at
board position C15.  The diode's cathode (the end with the ring) goes
toward the 74LS74, the anode (the other end) goes toward the TCA 650.

Pin 9 of C15 goes high when graphics are visible. Its function in the
unmodified PAL //e is to engage the filter that removes the "false"
NTSC color subcarrier, created by the standard //e circuits inherited
from the NTSC model. This prevents strong vertical stripes from showing
up in plain, colored screen areas. Of course this means also, that the
pin goes low when graphics are not visible, shutting down the filter to
enhance the sharpness of the black-and-white letters. The modification
makes use of this fact to "sink" the subcarrier to GND when graphics
are off, preventing it from ever reaching the output. The 74LS74 can
"sink" up to 16mA on any output, far more than the subcarrier amperage
so there should be no danger of any harm. The diode prevents a signal
from flowing in the other direction, so that the subcarrier won't be
disturbed by the logically high Q2 Pin while graphics are enabled.

The result: The TV's color killer circuit kicks in, the screen is
absolutely black-and-white and extremely crisp (for a TV that is) in
text mode (or when you flip the little dip switch SW1 on the right side
of the mainboard to "mono"). When graphics are displayed, (and the
switch is not set to "mono"), there is no change in behaviour compared
to an unmodified board.

So thank you again, without the schematic I could never have come up
with this.

The usual legalese: If you do this modification, it's obviously at your
own risk.

Are any Apple sites still up that accept uploads? I'd like to make the
schematic available to the public if you don't mind.

Some keywords for the benefit of future Google users: Apple 2e, Apple
//e, Apple IIe, color smear, colour smear, color noise, colour noise,
text mode, textmode, PAL, encoder, schematic, black-and-white, smeary
text.

Thank you,
-- 
Linards Ticmanis