prindle@nadc (05/12/86)
From: prindle@NADC Commodore engineers readily admitted in a C64 design article in IEEE spectrum about a year ago that early production C64s (about the first 1/3 or so made between 82 and 85, close as I can figure) had a video chip (VIC II) which divided by an even number (64 I think) instead of an odd number (65?) in deriving the line rate from one of the input clock rates, the result being the anomaly mentioned (chroma signal interference with the dots making up a character where such interference was the same in all frames displayed, thus making characters unreadable in many color combinations). The newer generation of C64s don't have this problem - the divisor was corrected and the chroma interference affects alternate dots on alternate frames, the net result being readable characters. This switchover seems to generally correspond to the switchover from a 5-pin to 8-pin DIN connector for video output, so if the C64 you are considering has the older 5-pin connector, chances are good that it also has the older VIC chip. Note that the chroma interference problem only applies to "composite video" output (including the RF modulated TV display) from the 64. If you use a monitor capable of "separated video" input (e.g. the Commodore 1701, 1702, 1802, 1902, and some others), where the luminance is input separately from the chroma signal, the problem does not exhibit itself, since the chroma is not present in the luminance signal, and therefore cannot interfere. One small problem: the older 64s (with 5-pin connectors) did not provide pure chroma output (luminance yes, chroma no). Two solutions: just use the composite video output as the chroma (works pretty well except when the luminance portion saturates the chroma amplifiers and the display goes black and white, or when single dot luminance signals cause some false color triggering), or open the 64, pop off the cover of the video circuitry shield, and lift the end of R10 (120 ohms) from the emitter of Q4 - this changes composite video into pure chroma (the RF modulater will become useless and the 64 must now always be used with a separated video monitor unless the resistor is re-connected). Don't trust your C64 schematic on this one - things are a bit different inside the real machine. I ran my 64 with this mod for the past several years and the picture on a 1702 is excellent (actually slightly better than a C128 which shows some vertical striping at high brightness). I've also run software generated 80 columns this way and the results are quite readable, though the dot pitch on the 1702 leaves a lot to be desired in this case. One more thing - the update to the VIC II chip (clock frequency change) made the C64 *incompatible* with it's own CP/M expansion interface!! So another way you can tell, is if it works with a CP/M card, it has the old chip... Frank Prindle Prindle@NADC.arpa