The Telstar Galaxy (model 6150) was released in 1977. It was the only model to use the General Instruments (GI) AY-3-8600 chip. It played ten games in color. Color was provided by the GI AY-3-8615 color encoder chip. Two very rare features make this model quite unique.
First, all games can be played by up to four players instead of the only two generated by the chip. Allowing up to four players with only two paddles is not very complicated, but requires a smart modification (multiplex) where a flip-flop enables two hand controls during odd frames (both paddles are instantly put at the position of those controls), and then enables the other two controls on even frames, thus instantly moving both paddles to their positions. The high frame rate makes the switching fast enough to display all paddles without flicker.
Second, this model features a quite unique Robot mode where the player(s) play against the machine. Usually, this feature is implemented on TTL/CMOS systems made with discrete components only. However, the GI chips never had a Robot mode implemented, so it is impossible to play against the machine with the chip itself. In order to do this, some additional circuitry is required to extract the vertical sync signal from the main video signal (game field), start a ramp generator (i.e a voltage that is increased time after time) when the vertical sync occurs, stop this ramp when the first "top" of the video signal of the ball is detected, and then send the voltage of the ramp generator to one of the players input, thus putting its paddle to the same vertical position as the ball. In other words, the circuit re-calculates the voltage corresponding to the vertical position of the ball in order to apply it to one of the players input in order to simulate a game against the machine. This feature is only possible because the chip outputs several individual video signals (game field, left paddle, right paddle, ball) in order to color them differently with the AY-3-8615 chip. If GI's AY-3-8500 and AY-3-8600 chips only generated a combined video signal, it would have never been possible to implement a robot mode (or at least, the circuitry do do it would have required so many components that the system would have been totally unsalable).
Another useful feature for young players is the possibility to disable the horizontal motion of the paddles.
This model was released in very limited amount, and very few specimens survive today. Two of them came from former Coleco employees, the few others were found by collectors or were sold to them by their original owners. Despite its rarety, please remember that many models used the same GI chips and therefore, such Ball and Paddle models cannot have a very high financial value. Loose units usually sell for $50. Complete and boxed ones may command up to twice that price if in excellent condition.
Click the pictures below (courtesy of Adrian Scheel) to view them in original size.
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