Book "Understanding the Apple IIe" - PAL encoder schematic?
Linards Ticmanis
I wonder if maybe somebody in this group owns the book by James Sather,
"Understanding the Apple IIe" - ISBN: 0-8359-8019-7.
This book (which unfortunately I do not own nor have access to) is
supposed to be the only source where a schematic of the PAL Apple //e's
mainboard circuit for PAL color encoding can be found. I'd really like
to get a hold of this one, so if you own the book and a scanner, you'd
be sure of my eternal gratitude if you could scan in that schematic and
mail it to me (the "From:" address works) or put it up somewhere on the
net.
I do have the schematic of the separate PAL encoder card for Slot 7, as
used in the IIeuroplus, but it seems the two are not exactly identical.
The reason I ask is because the TV set I use as a monitor has a color
detector that's overly sensitive, it detects the presence of a (very
weak) color signal even when the Apple is in text mode. The result is a
total smearing of the image in text mode (while it is fine and colorful
in graphics mode, and in mixed mode the four lines of text at the
bottom are also fine+dandy). So every time I use text mode I have to
zap through a row of menus on my remote to turn color saturation to
zero... then turn it back up when I use a graphical program.
I plan to modify the Apple so that the color carrier quartz is shut
down for good, e.g. with a little relay-based circuit, when the apple
is in text mode. Short-circuiting the quartz with the blade of a
screwdriver did remove the smear, so I figure I could automate that
with a small relay and a bit of relay-driving cicuitry.
I'd be grateful for any help.
Thank you,
--
Linards Ticmanis
Wayne Stewart
Wayne
Linards Ticmanis
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.
heuser...@freenet.de
//e, Apple IIe, color smear, colour smear, color noise, colour noise,
text mode, textmode, PAL, encoder, schematic, black-and-white, smeary
text.
Very thoughful of you!
And thanks for your insights, too!
bye
Marcus