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CoCo-1 Monocrome video

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Home / Hardware - CoCo-1 Monocrome video



From: Hot CoCo Magazine - July 1983, pages 36-38 is the instructions for a Monochrome Monitor and instructions for audio on the following pages

At last there is a way to produce a clear, sharp black-and-white image with your Color Computer.

If you use your CoCo for long hours of text processing (programming or word processing) or for black-and-white graphics (PM ODE 4), you might want to construct the following circuit. This circuit grabs the video output before the color information is mixed in and produces the sharpest, cleanest text and black-and-white graphics that the Color Computer is capable of generating. When viewed on a monochrome monitor (green, amber, or black and white), the image is better than the images available from the normal (RF) output viewed on color or black-and-white TVs. It is even superior to the text I have seen on a friend's RGB monitor driver, which he built for his CoCo. The improvement is most noticeable when viewing 51-column text pages such as those produced by Telewriter.
Keep in mind that this circuit will not show colors, even if the output is displayed on a color monitor, and that multicolor graphics may look disappointing because the circuit assigns only one of three gray levels to the eight colors available. Artifact colors-the so-called extra PMODE 4 colors-appear as areas of fine stripes or dots. There is a way to modify this circuit to let it produce eight gray levels corresponding to the eight colors in the CoCo repertoire. I will describe this circuit in Part 2 of this series.
To use this circuit, you must have a monochrome monitor handy. I am typing this article on my CoCo using this driver circuit and my $170 Amdek green-screen monitor. New green-screen monitors can be had for less than $100. Paying a premium for ultra high-resolution monitors is not necessary, as the CoCo's signal is of limited resolution. Any monochrome monitor should do just fine. I am told that amber is preferable because it is easier on the eyes. My circuit will not have any detrimental effect on the color RF output signal.
Some folks have made or purchased color monitor driver circuits and are driving monochrome monitors with them. Although you do get a readable image, the image is smeared and contains annoying vertical stripes. This is the result of the color information messing up the black-and-white signal. The circuit described here provides a crystal-clear, sharp image on a monochrome screen.

Theory of Operation

The information for your video signal is created in the VDG chip (U7 MC6847). Pin 28 of this chip is the Y (synch + luminance) output, which carries the monochrome video signal in inverted form. Pins 10 and II (theta A and theta B) carry the color information. These three outputs are integrated into a color-composite video signal by U12, the video mixer IC.
This color-composite video signal is fed into that metal box with the RCA socket on it (U5, the ASTEC modulator). The ASTEC is actually a miniature TV station that produces a radio (RF) signal. This signal is then recognized by an ordinary color TV, which reverses the process to put an image on the screen. Thus, a black-and-white signal in a normal CoCo gets mixed with color data, modulated onto an RF carrier, demodulated in the TV set, separated into color and black-and-white data, and at long last put on your very low-resolution color TV screen.
It should not surprise you to learn that in this elaborate process a fair degree of clarity is lost. The system this article describes takes the signal directly out of the Y output of the VDG, buffers it (to preserve the integrity of the signal that contributes to the RF output), inverts and amplifies it, then buffers it again before sending it to your choice of socket for plugging into a 75-ohm impedance monochrome monitor.
The scheme I hinted at for achieving a gray scale output involves similarly (but not identically) buffering the theta A and theta B lines and bleeding part of those signals into the monochrome output through low-value resistors.

Construction and Mounting Hints

You can construct the entire circuit (Fig.1) on a piece of perfboard 1 by 1 1/2 inches in size. It is then mounted inside a small metal box the size and shape of the ASTEC using two small machine screws. The lid of the ASTEC lifts off easily to allow you to attach my box (be careful positioning the screw heads-it's dense in there) and thus allows for future adjustment of the ASTEC if necessary. More timid souls CoCo, opening the case will void your warranty. Some kind Radio Shack repair people will still repair modified computers (though not under warranty) as long as the modification clearly is not what is causing the problem. Others insist on ripping out the modification before they do anything. They do have the right to refuse to service anything they consider to be too messed up. I am assuming that you know your way around the inside of the Color Computer. For information on questions such as how to open your case and where to find the chips in question, I \lrge you to refer to the technical reference manual Radio Shack sells for $15 (cat. /6-3193) or to articles in this and other computer magazines. Seven screws hold the case together. Once the case is opened, you can find the chips discussed in this article under the removable metal shield (the RF shield). The tech reference manual and my schematic should be adequate to guide you in constructing and installing this circuit.

New CoCos and TDP lO

Since I wrote this article, Tandy has introduced a new board for the Color Computer and also uses this board in the TDP 100. Apart from changes in layout, made primarily in an attempt to comply with FCC regulations, Tandy made several changes in the video output circuitry. The part numbers of all components are different too. Although the MC 1372 chip now lies in a new location on the new board, it is even easier to find than it was on the old board. Tandy's engineers had to solder on a 56K resistor to the chip at the last minute, so look for that silly resistor soldered right to the pins. It is now called U6, not U12, and the VDG is now called U9, not U7. The resistor was added when Tandy found that the artifact colors wouldn't show up in the RF signal due to a too-weak color burst signal. The resistor bleeds a little extra VDG clock signal (colorburst) into the output of the video mixer, curing this problem.

An advantage new-board owners will immediately see is that their video-mixer chip is not covered by an RF shield. Thus, you have more freedom to construct a fancy plug-in adapter. When you take out the MC 1372 chip, don't worry about that resistor soldered to it: just leave it there. This circuit will work on a new board. If you have trouble that you think is related to circuit board changes, I would like ,o hear about them.
I will supply either the bare printed circuit board or the board and the necessary components. The board alone costs $10, and the board with components costs $15. You can purchase an assembled and tested unit for $35. Please include $1 for shipping. Order from Cheshire Cat Software, c/o Marty Goodman, 1529 Addison St., Berkeley, CA 94703 . • Marty Goodman is a physician and an active electronics hobbyist.


Photo 3. Back of Compuler Showing RCA Sockets