Wednesday, October 19, 2022

One more MOS 7600 Pong: Coleco Telstar Gemini

Oh, c'mon now, you're not sick of MOS Pong machines yet, are you? Of course you're not! Everybody loves Pong from the company that brought you the world-famous 6502 CPU! So here's another.

Briefly in the last entry but mostly in the one previous I discussed the unusual 1978 Coleco Telstar Arcade console in its unique triangular form factor, which had one of four variants of the MOS 7600/1 "Pong-in-a-chip" CPU in each cartridge, the specific variant containing its own mask ROM and shape table data. To maximize its investment and possibly to compete with Atari's Video Pinball which came out in 1977, Coleco essentially took their cartridge #3 with the MOS 7600-004 pinball-and-skeet variant and turned it into a standalone console of its own, the Coleco Telstar Gemini, also in 1978. (Although easily confused by modern collectors with the Coleco Gemini — no "Telstar" — which was their 1983 Atari VCS clone, neither was a sales contemporary of the other.)

This unit was an eBay find that was advertised as not working and priced accordingly. As expected, it works just fine with a Telstar AC adapter which I happen to have (I'll do a followup with some of the console wallwarts in my collection since the polarities and voltages are not always listed).

As received. Hey, eBay sellers, here's one weird trick you can do to get more money out of people: clean your damn merchandise. Just because its dirt patina has evolved consciousness sitting in your closet doesn't mean you couldn't put a little elbow grease in before hawking it, and people might actually pay you more for it.
I don't think paper towels and a gentle surface cleaner count as a Refurb Weekend, but in addition to wiping down the surfaces I also tried to pay attention to the grooves in that completely functionless grille in the middle and around the edges. For those I find an American dime works well for wedging the paper towel in, and a blunted penknife or small flathead screwdriver can (carefully! don't mar the craptastic plastic!) dig dirt out of the corners. I also cleaned around the label and controls.

The controls on the unit are slightly modified from the Telstar Arcade. The rotary game select switch is present, as are the power and player switches and reset and target buttons, just in different places. (The RF channel selector is not visible here but is a small switch in a cutout in the front edge.) But instead of the right and left slam buttons of the Telstar Arcade (pinball plunger and flippers, respectively) we get a big bright red launch button, and instead of the two dial pot paddles on the Arcade the Gemini gives us three slider paddles on the middle left.

The flipper buttons are on the sides. However, they are both wired to the left slam input, so either button controls both flippers as on the Arcade. The AC jack and RF cord are on the back.
Turning it over we see a big battery compartment with a removable cover, but there's nothing in it; this may have been a mould repurposed from something else. It doesn't fit the light gun but it does conveniently store the AC adapter. As with the Commodore TV Game 2000K the patents in the injection-moulded plastic aren't Coleco's but Ralph Baer's, the developer of the original Magnavox Odyssey in 1972. Among others the 3,829,095 patent was Baer's famous 1970 "METHOD OF EMPLOYING A TELEVISION RECEIVER FOR ACTIVE PARTICIPATION."

It also says "DO NOT OPEN." How cute! (gets out screwdriver)

The holes on the fake battery compartment side have deeply recessed screws, but only four total need to be removed to take the bottom off. Most of the interior is empty space. A small PCB is at the power jack, and the main board dominates the lower half.
Removing the dial and slider controls for the paddles and a single screw in the middle allows us to turn the main board over. The internal speaker is revealed, along with the RF modulator (in the sealed metal box), three very dusty slide paddles (canned air time) and a small number of ICs and discrete components.
The board is copyright 1977, revision A. I don't know of any other revisions but note that the board is not specifically labeled as any particular unit otherwise (I'll come back to this momentarily), and is more or less the multiple boards of the Arcade consolidated onto a single, cost-reduced PCB. The RF cord is just a cord; it has its own jack on the board, which makes replacing the cord easy if it gets frayed or damaged. A nice touch.
The MOS 7600-004 microcontroller is socketed. Combine this with the lack of any printed board identifiers I just mentioned and I think there's good evidence that Coleco intended this board to work with the other 7600 game variants (soccer-hockey-racquetball-skeet, racing-Pong-skeet, naval-speedball-spaceskeet) if the Telstar Gemini form factor was a success. It wasn't, so they never made any others. The date code is 38th week 1977 which is actually older than the chip in my Telstar Arcade cartridge #3, and proves that the Gemini and the Arcade were sold for at least some period of time simultaneously. Appropriately it is labeled "IC1" on the board. Next to it is the board's only oscillator, a 3.58MHz crystal (matching the NTSC colourburst frequency).
IC4 and IC5 are both NatSemi CD4015CNs, a four-stage double static shift register. These chips take four bits of serial data and emit them all at once in parallel, replacing the functionally equivalent Motorola MC14015B chips on the Arcade. IC3 on the left in this view is a Motorola MC14077B, a quad exclusive-NOR (not exclusive-OR).
IC2 is a TI CD4001AE (date code 1st week 1977), a set of four independent NOR gates with two inputs each. IC7 to the direct right of it is a TI CD4069AE hex inverter (31st week 1977), and IC6 below that is a National Semiconductor CD4016CN, a quad bilateral switch (four sets of in/out lines with a gate control each). These two are part of the colour select circuitry for the video output; as in all 7600 Pong games the microcontroller relies on the board to select colours for individual video elements, allowing sometimes wide variations in colour schemes for the same game. The hex inverter delays and inverts the colour signal, changing its phase to generate different colours, which is selected by the quad bilateral switch. The switch is set depending on what game is chosen with the rotary dial.
Thus, to distinguish it visually, the Telstar Gemini uses a different colour scheme from the Telstar Arcade playing what are otherwise exactly the same set of games by simply having different components on the board to generate the palette. This is Bonus Pinball, the first game. It has a royal blue background as opposed to the teal of cartridge #3, and a much more vibrant playfield.
Gameplay is otherwise the same, except that because the Gemini has three paddles, all three movable elements (olive here on the Arcade, yellow on the Gemini) can be positioned independently. On the Arcade, which only came with two dials (cartridge #2 was the only one that provided, and supported, a full complement of four paddles), two of the paddles are wired together onto the same dial. The front button (right slam on the Arcade) acts like a pinball launch plunger in the sense that the longer you hold it down, the more velocity the ball launches with. The flippers, however, are just flippers, and both flippers move together (left slam on the Arcade).
Both skeet-shooting modes (Shooting Gallery and Shoot The Bear) use the same misshapen bear sprite. On the Gemini it's a bright pink.
On the Arcade it's a dimmer purple. Notice that both match the colour of the flippers in the pinball game, suggesting they are encoded as the same sprite element.
And finally Deluxe Pinball, a minor variant with a slightly different playfield. The gameplay elements have the same colours as Bonus Pinball, but on the Gemini the background is a very dim burgundy red.
On the Arcade it's royal blue instead, which doesn't match the cartridge's own box.
Just to see if this was something about my video capture hardware (I use an LCDT600 connected to an INOGENI VGA2USB3), I plugged it into my actual Panasonic CRT TV/VCR combo. The colours were a bit brighter but they were otherwise the same. Note that I had to mute the TV because all audio comes through the console's internal speaker, not the RF output, and that the cat was taking an inordinate interest in the screen.
In fact, the "burgundy red" was even darker on the CRT; you could barely distinguish it. I once again managed to sandwich the ball between the paddles here for an infinite score. It's too easy to do, really.
I also attempted to shoot the cat with the light gun but only succeeded in getting glared at, and trying to shoot the bear instead was similarly unprofitable: I'm not sure if the photodetector is just old (and spraying the dust out didn't make much difference), but you really had to be right up on the screen to register a hit and it didn't give you much leeway. This was true of both my original Telstar Arcade gun and this one that came with the Telstar Gemini (they're the same and either works with either unit). Note that you can only play these skeet games on a true CRT television like this one: an LCD doesn't have the scanning raster beam the gun is watching for, meaning the photodetector will register a hit either everywhere or nowhere when you fire.

One thing that was also not a hit was this console. It sold very poorly and was not priced sufficiently low enough to attract the budget consumers Coleco appears to have been trying to entice. In fact, it seems likely that the Telstar Gemini was withdrawn from the market before the Telstar Arcade despite being descended from it, and was definitely not sold by 1979. Still, I like its approach to paddle pinball a bit more than the Arcade's because of the extra slider and the cheerier colours, so this has now become my 7600 Pong system of choice until I find another one to play with. Rest assured I'll torture you with pictures of that too.


  1. You dare to shoot your Cat Lord? You will pay for that! Cool how these old guys still work.

  2. It's colour! Don't know why I'm surprised, but I am. Also, thanks for reiterating mould Vs mold


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