@RUTGERS.ARPA:prindle@nadc (01/10/85)
From: prindle@NADC -------------------------------------------------------------------------------- Excerpt from Baron's MicroComputing Reports, May 1984: -------------------------------------------------------------------------------- * COMMODORE COLOR MONITORS Both of Commodore's color monitors, Models 1701 and 1702, significantly improve the display quality of the 64 computer. When properly connected, the sharpness, contrast and color rendition is equal to or better than any other monitor presently available in the same price category. (Note that RGB monitors with separate inputs for the primary video color signals offer no advantage in image quality since the Commodore 64 cannot generate the required signals.) The excellent performance of the color monitors is a result of the separate inputs for chrominance and luminance on their rear panel. Inputs contain the color and brightness information respectively. It is this separation of the video signals that optimizes the display. To the best of our knowledge, Commodore is the only manufacturer which privides this feature on a home computer and a companion video display unit. o CONNECTING THE 1702 COLOR MONITOR For most users, connecting the Commodore 64 to the 1702 color monitor is not difficult. You simply use the cable supplied with the monitor and follow the instructions provided. The cable terminates at a somewhat unusual eight- pin DIN connector at one end, and at three standard RCA-type phono plugs at the other end. This configuration does not match the five-pin diagram on Page 142 of the Commodore 64 User's Guide because there has been an undocumented improvement in the Commodore 64. The upgrade also causes some minor incompat- ibility problems with earlier models of the computer and the 1702 monitor. Fortunately, there is a simple method to correct these problems. Owners of earlier Commodore 64 models will note that thier computer's five-pin DIN connector cannot use the cable supplied with the model 1702. A replacement cable will have to be purchased or assembled from standard components. If you are buying a pre-assembled cable, ask for an "octopus" cable (approximate cost: $5-$10). It has a standard five-pin DIN connector at one end and four RCA-type phono plugs at the other. Only three of the RCA connectors will be used. The fourth should be taped out of the way. If you are building your own, you will need a five-pin DIN connector (Radio Shack No. 274-003), some shielded cable and three RCA-type phono plugs. The home- build version should cost less than $5. In both cases, referring to page 142 of the Commodore 64 User's Guide, pin 1 is the luminance connection. The video out signal on pin 4 should be used as the chrominance signal. Audio is obtained from pin 3. The cable shields are grounded to pin 2. o A BUG AND A FIX Under certain conditions, the Model 1702 will lose all color in the displayed image. This is due to a design change which, unfortunately, makes it slightly incompatible with the older Commodore 64s. The Model 1702 requires a pure chrominance signal for optimum operation. This is supplied at the center pin, or pin 8, on the current versions of the 64. Pin 6 (which is adjacent to pin 3) and pin 7 (which is adjacent to pin 1) are not used at this time. The older versions of the Commodore 64 do not have the center pin (pin 8). In addition, the pure chorminance signal is not available at the rear video con- nector on the computer. The signal supplied at pin 4 is a composite video signal containing both color and brightness information. While the Model 1701 was designed to function with the composite singal, the Model 1702 uses separate chrominance and luminance signals. A simple internal modification of the Commodore 64 will supply a pure chrominance singal at pin 4 of the video connector. A qualified service technician should be able to perform the operation. Refer to the second page of the Commodore 64 schmatic in the Commodore 64 Programmer's Reference Guide. Locate resistor R10, the 120 ohm resistor connected to the emitter of transistor Q4 (PN2222). If this resistor is disconnected or removed from the computer circuit board, the signal to pin 4 on the video connector will be changed to pure chrominance. The resistor is located under the metal shield at the approximate center of the Commodore 64 circuit board. Note that the signal to the RF modulator will be affected as well, making the computer unusable with a color television. o CONNECTING THE 1701 COLOR MONITOR The cable supplied with the Model 1701 color monitor terminates with a standard five-pin DIN connector at one end and two standard RCA-type phono plugs at the other. This cable is plugged into the video connection on the front of the monitor. Considerable image improvement can be obtained by replacing this cable with an octopus cable. Use the video connections on the back of the monitor. lThis will work equally well with both versions of the Commodore 64. Owners of the Commodore 64 with the eight-pin video connector and the Model 1701 may consider obtaining a matching video cable from a Commodore dealer. lWe have not tried this combination ourselves and cannot say whether it will offer any significant improvement over the octopus cable. Incidentally, either of the Commodore monitors will make an excellent display unit for a video cassette recorder. Simply connect the video and audio outputs from the recorder directly to the RCA jacks on the front of the Commodore monitor. The front inputs are selected by the switch on the back panel of the monitor. If the VCR and computer are in the same location, it will be unnecessary to pull out any cables when switching from one source to the other. -------------------------------------------------------------------------------- Additionally, I have found that, in order to center the "contrast" control on the 1702 monitor and have reasonable contrast (I have a modified "older" 64), it is necessary to put a 200 ohm resistor in series with the "luminance" cable (I do this with a short RCA-type plug/cable/jack combination which has the resistor buried in the jack). <FP>