Retrace Lines: 2068 and RGB Monitor

Like several others, I wanted to use the nifty thrifty Skip Fisher RGB monitor with both my fascinating and formidable QL and also with my trusty old friendly TS-2068. The hookup to the QL or the 2068 both gave the crisp colors known to most, but the 2068 made retrace lines across the top half of the screen, unless I used BORDER 0 (black).

At the October CATS meeting George Rey, Duane Parker, Jonathan Vanderwall, myself, and several others brought their skills and resources to bear upon the problem; of course it had to yield once confronted with all that talent.

The circuit takes 3 IC chips, a couple of resistors, a variable resistor, 4 or 5 bypass capacitors, and the usual amount of bits of wire, solder, luck, sweat, and swears or tears, depending on your nature. The chips are: 74HC00 (as used on the Oliger board), 555 timer and 7408 Quad 2-input AND gate. I tried a 74LS08 at first but it didn’t do. The resistors are 47K, 1K, and a 10K pot that could be replaced with a fixed value (unless you enjoy tweaking things as much as I do). The capacitors I used were .1 uf bypasses that seem to be abundant on most of the surplus IC circuit boards found at hamfests and computerfests.

The idea is to use the 74HC00 as Oliger did to get nice horizontal and vertical sync pulses off the base of Q5 (via the 47K resistor), integrate (with the 1K resistor and a cap) a little of the vertical sync and invert that with a gate of the 74HC00 and then use that inverted pulse to trigger the 555 monostable. The output of the 555 (duration controlled by adjusting the variable resistor) is sent to the 7408 AND gates thus cutting off the RGB signals during retrace.

The schematics were published in “The Art Gallery,” in a later issue of the newsletter.