knudsen@ihnss.UUCP (06/02/84)
<boot it, boot it, take your OS-9 and shoot it> [Re-posted by request. Thanks to ihnp4!ihuxa!dafa] While improving my D-A resolution (earlier posting), I also improved my homebrew monochrome monitor circuit to provide unique grey-scale levels for all the colors. How many games use blue vs red men (chess, backgammon) that all look alike in B&W? Wonder where the cursor went in The Micro Works' MACRO-80C editor? I fixed all these and more. You should see the Radio Shack chessmen -- whiter than white, blacker than black! If I get a few requests, I'll post a verbal schematic of the whole thing. Has anyone else hacked their D-A yet? mike k OKAY, HERE IT IS.... This circuit will drive a hi-res, monochrome monitor with 75 ohm (more or less) input. Before telling how to build the Coco Grey-Scale Monitor Driver, a few words about what it tries to do. There are three possible objectives: (1) Duplicate the grey levels as seen on a B&W TV via the RF port. This cannot distinguish Red from Blue in color set 0 and is thus not worth pursuing. I doubt my circuit can be adjusted to do it. (2) Give each of the 9 colors its own unique grey level. This has obvious theoretical merit, and can be had by adjusting a SMALL amount of "A" drive (see below). However, Coco can't do much in the 8-color- plus-black modes (tho cross modes may be worth it). (3) Provide maximum contrast among the four colors in each of the two color sets (0=green, yel, blue, red; 1=cyan, buff(white), magenta, orange (may have order wrong)). This makes the best use of Coco's color abilities, and is how I tune my circuit by cranking in lots of "A" drive. Lets you tell the Red and Blue men apart in board games. General Notes: Parts values are not critical. Resistors should be 1/4, not 1/8, watt. C2 is 20 to 30 pf, and may be omitted depending on your monitor. Its purpose is to limit frequency response so that monitor does not "anticipate" grey-level transitions due to its own bandwidth limits. (Good rule: Don't feed a monitor more hi-freq than it can handle; it may abuse the excess). If your monitor has a hi-impedance input only, add a 75-100 ohm res across my output, preferably at the monitor end of cable. Subminiature trimpots are at Radio Shack. Q1, Q2 are any NPN transistor with good gain & freq response. "IC.n" below refers to pin n of the MC1372 14-pin chroma modulator. To connect, I recommend a 14-pin socket with stiff pins. Solder fine wires to top of its pins, plug this in place of the IC, plug IC into it. OK, heat up the iron & let's go: (1) R1 (1K) from IC.9 to Q1.base. (2) R6 (10K trimpot) from IC.5 to Q1.base also. This tunes the "B drive". (3) Connect R4(100 ohm) in series with R5(5K pot). One end of this pair to GND (available at IC.4), other to Q1.emitter. R5 is your gain (contrast) adjust. (4) Connect Q1.collector to Q2.base. Call this junction "J". (5) R7(50K pot) from J to IC.7. This tunes "A drive". (6) R2(2200 ohm) from J to +5 volts (available at IC.11). (7) C2(20 to 30 pf) in parallel with R2. (8) Q2.collector to +5. (9) C1 (0.1 to a few uf) from +5 to GND, as close to Q2.coll as you can. (a) R3 (22 to 27 ohm) from Q2.emitter to center hot pin of output jack. (b) GND to outer shell of jack. Use a BNC jack, bracket-mounted to the PC board or bottom of case. Cut hole in top half of case if needed for access. TUNE-UP instructions: (1) Set A and B pots to their maximum values (minimum drive) before turning on power, else IC may get hurt. Use ohmeter to be sure. (2) Power up, tune Gain pot for best picture. Recommend setting your monitor's contrast knob about 3/4 full up, then tune Gain. (3) Put up a display of all 4 Color Set 0 colors (RS "Chess" cartridge strongly recommended, or "Backgammon"). Advance "B" pot until Blue becomes blacker than Black (border); or until picture tears hor'ly, then back off a bit. Now, Red should be a little lighter than Blue but darker than Green. Yellow is still whitest. In Color Set 1, Orange will be whiter than white* (Buff). *Trademark of Proctor & Gamble. (4) You may omit pot "A" completely, but it greatly improves the red-blue separation. Advance A to make Red still lighter, but not as light as Green. Also too much A washes out the Buff-Orange distinction. (5) Trade off A & B to get both color sets well separated among themselves. A 'scope on the output is helpful. Best guide is the "Chess" cartridge: initial chessboard is Color Set 1. Above instructions will make the White pieces whiter than the white squares, and the Blacks darker than the black squares. Really beautiful. But in the other set, too much "A" will make the black squares as dark as the Black pieces . Enjoy, & let me know how it works out for you. "By the Power of Grayscale" ... mike k