The easiest way to connect your computer to a monitor is to simply take the composite video signal from pin 16 of the ULA (“SCL” chip). Since this isn’t capable of much power delivery, it will only work in the “high-impedance” input setting. If your monitor doesn’t have a Hi/Lo-Z switch, open the monitor and trace where the input goes on the circuit board. Nearby, you’ll find a 75-ohm resistor (purple-green~black- gold or silver). Simply remove this resistor and you now have a Hi-Z monitor.
A better approach is to isolate the input using a single transistor “voltage follower” as in Figure 2a. Connect the base to ULA pin 16, the collector to +5V through a small resistor, and the emitter to the monitor. The monitor’s own 75-ohm terminator resistor acts as the return to ground. You can get away with a smaller transistor (2N3904, etc.) if you change this load resistor to a higher value, say 330 ohms.
If you want to keep the 75-ohm input, then use a medium~power transistor like the Radio Shack 276-2030 (same one we used for the motor mod last issue), The collector resistor is to limit the maximum current if you should short the output. Its value should be about 1/4 the value of the emitter load resistor, Alternately, omit the collector resistor and AC couple the output with a 47uF. cap. However, on some monitors this will degrade DC Restoration (brightness stability of background), so generally the DC-coupled approach is preferred.
Figure 2b shows a simple but very effective video cleanup and reverse circuit. It’s the fourth and simplest version of this that I’ve come up with, It gives you switchable normal and reverse, and cleans up the video signal in the process.
Although we get rid of the VHF fuzzies, the video signal itself usually has garbage riding along. This shows up as tenuous vertical lines on a white screen, and since they don’t move around they’re much more tolerable than the VHF harmonics. But while we’re at it, let’s get rid of them. The lower comparator does this for us; its reference voltage is set to the midpoint of the video signal, adjustable by the little trim pots. Depending which input the video goes into, we get either normal or inverted video output. Separate adjustments are provided, so you can trim up each mode separately for the best picture.
The Vref of the top comparator is set to a low value so it responds only to the syne pulses, (Note that we couldn’t just invert the entire signal to get reverse video, since the sync pulses must be negative-going.) So, this comparator is our “sync separator.” The outputs from the two are combined in a resistor chain to give about 2V black level and 1V video. This goes to the emitter-follower, thence to the monitor. It works great!
Build this up on as small a perf board as you can fit everything onto, and keep all lines as short as possible. Mount the switch (e.g. a miniature toggle) close by, preferably on the board itself. Open the anode end of diode D9 on the ZX81 board (goes to ULA pin 16). Connect pin 16 to the Vidrev input, and anode D9 to the output marked “T.” This way it will also provide reversing for the TV. Try bypassing D9 entirely; on some machines it looks a little better. Connect +5 and +9 at the nearest availble point. Keep the ground return (to modulator case) short! The LM311 comparator (an old standby) is very klutz-friendly, so if you have an error, it usually isn’t catastrophic.
When you get it working, you’ll probably find that your picture is small, with big wide borders. This is easy to fix. Vertically, simply adjust the “Vert. size” control on the back. You’ll have to touch up “Vert. Lin.(earity),” which adjusts out any line compression at top or bottom. These two are also interactive with “V. Hold,” so it might take a while till you get it just right. Horizontally, you usually have to remove the back cover, and locate a little coil marked “H. Width.” It has a ferrite slug in it. Adjusting this counterclockwise (out of the form) widens the picture. On every monitor I’ve played with, the best picture is the widest picture; i.e., with the slug completely removed. The reason you had to make these size adjustments is that most computers use much more of the available screen-time than NTSC TV; i.e., the syne pulses (retrace time) are narrower, and better resolution results. Remember, we’re only using 192 out of a (theoretical) maximum of 525 lines, whereas other computers may print up to 320 lines; the factory assumes you’ll change it as required, and there are usually no “warranty seals” to indicate that you’ve opened it to align it to your machine.
To center the picture, adjust the little tabs that rotate the metal rings on the “front” of the deflection yoke assembly on the neck of the CRT (picture tube). These are circular magnets which will allow you to center the picture, They don’t quite work the way you’d expect at first, but you’ll quickly get the hang of it. Try using a mirror to see the screen while you’re experimenting for “the ultimate picture” from the back.
