I ran into a problem with my Apple II (a Taiwanese clone, my original Apple IIe still is in disrepair). The problem is that its color killer circuit is not good enough so I always get color artifacts in text screens. My Apple IIc does not have the problem with that same TV which appears to have a very sensitive color burst amplifier. Almost no burst amplitude, just a bit of wiggles, barely visible on the scope, are enough to get full color ! Alas, the phase lock on such a feeble signal is imperfect, and so the color in the text shifts, making the problem worse. Much like a LSD trip. Text with always shifting and moving colors all over the rainbow. No good for focused work on software development ! How people could have worked under these conditions on the Apple II back in the day eludes me. Must have been an ordeal. Apple IIc seems to have a 'color killer' circuit which works. Alas, it's a hybrid circuit in which everything is hidden under a layer of coating. And I don't want to destroy a perfectly good Apple IIc just to find out what is inside it.
So this question arises: did anyone analyze the 'color killer' circuit of the Apple IIc ? Are there any good circuits out there who can do the job ? --- Note that I tried the known mod to gate off the color burst in the digital logic, and it is gated off in text lines, but still the damn TV thinks there is a color burst. I can see in the oscilloscope that regular hires vertical stripe patterns can excite the LC tank in the Apple II color circuit during the 'live scan' portion of the TV scan lines, and at the moment I have the hypothesis that the whole machine is so polluted with harmonics and subharmonics of the 14.31818 MHz master clock that this LC tank may pick up some of that crap and turn it into a 'color burst' signal all the time, and the damn TV picks it up. So it's not a trivial solution which will bring good B&W text. At the moment I have lifted one leg of the inductor to disable the LC tank, and then I get B&W text, but this is not a solution as it also deprives me of color graphics.
Any ideas / links to good 'color killer' circuits ?
- Uncle Bernie