Coleco Telstar Gemini Troubleshooting Guide
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Fault-finding for the Coleco Telstar Gemini, Coleco's 1978 pinball and target shooting console.[1] Coleco's trouble-shooting chart for the Gemini has not been found, and no schematic or repair report has been published. The checks below rest on three things that do survive: Arcade-History's description of the Gemini's controls, Coleco's rules for the same six games on Telstar Arcade Cartridge No. 3, and MOS Technology's specification for the MPS 7600 game chip series.[1][2][3][4] Cleaning and care are on Coleco Telstar Gemini General Maintenance.
How the Gemini works
[edit | edit source]MOS Technology's specification for the MPS 7600-001 describes the series.[4]
- One chip runs the games. A frame processor reads the control switches, moves the ball, checks it against the boundaries and the player shapes, keeps the score and makes the sound, all from a mask-programmed 512-word ROM.
- A crystal sets the clock and the colour timing. MOS says one of the two specified crystals is essential to stable operation even for a black-and-white picture.
- The chip runs from a single negative supply. Two VDD pins go to the negative rail and two VSS pins to the positive reference, and all four must be connected.
- The chip has four player-control inputs, P1 to P4, each fed from a potentiometer.[4] On the Arcade, Cartridge No. 3 takes its three playfield adjustments from the two rotary controls and the steering wheel; the Gemini has three sliders for the same jobs.[3][1]
- The sound is a single transistor driving the speaker, and the gun gives a trigger switch closure plus pulses from a photoelectric detector at the HIT pin.
The Centre for Computing History gives the chip on Arcade Cartridge No. 3, with the same games, as the MPS 7600-004.[5] The Gemini's own chip has not been photographed.
Reference values
[edit | edit source]| Point | Expected | Source |
|---|---|---|
| Game chip supply (MPS 7600-001 data) | -6.5 to -9.5 V, VDD measured from VSS | [4] |
| Crystal | 3.579545 MHz (full colour) or 3.595314 MHz ("rainbow" colour) | [4] |
| Battery | One 9 V battery (collector source) | [6] |
| Coleco Telstar adapter Model 6041 | 9 V DC, 200 mA (label) | [7] |
The chip figures are MOS Technology's for the -001 version, not measurements from a Gemini. Which adapter Coleco supplied for the Gemini is not recorded; the Model 6041 figures are given for comparison only.
Symptoms
[edit | edit source]| Symptom | Likely cause | Action |
|---|---|---|
| Nothing at all | Flat battery, corroded snap, adapter, power switch | Fit a fresh 9 V battery, clean the snap, and try the adapter separately.[6] See Coleco Telstar Gemini General Maintenance. |
| No picture, or a blank screen on the right channel | Television tuning; supply; clock | Retune the set. Then measure the chip supply against the range in the table, and check the crystal is oscillating.[4] |
| Picture disturbed or unstable on the adapter but steady on a battery | Poorly filtered adapter or a failed supply filter capacitor in the console | MOS warns that sharp transients on an AC-derived supply disturb the display. Try a known-good adapter, then see Coleco Telstar Gemini Capacitor Replacement Guide.[4] |
| Game will not start after power-on | RESET not pressed, or balls not yet served | Press RESET and wait for the balls to appear, as Coleco's rules for the same games direct.[3] |
| One flipper button does nothing | Dirty or broken button contact, or its lead | Clean the contact; check the lead back to the board.[1] |
| Every launch is fast, or every launch is slow | Launch button sticking or bouncing | The button gives a slow launch on a short tap and a fast one on a long press. Check that it returns fully and makes clean contact.[1][3] |
| A bumper or chute jumps, jitters or will not move over part of a slider's travel | Dirty or worn slider track | Clean the slider; see Coleco Telstar Gemini General Maintenance. MOS calls for the best potentiometers available because these controls take hard use.[4] |
| Target games: shots counted but no hits | Television too dark or contrast wrong; dirty barrel; detector or its lead | Set contrast for a grey background, as Coleco advises for the same games on the Arcade, and try firing from closer; Coleco notes that the firing range depends on the television. Then check the detector lead.[8][4] |
| Target games: trigger does nothing | Trigger switch or pistol lead | Check the trigger switch closes and the lead is continuous.[4] |
| No sound, games otherwise normal | Speaker, its lead, or the sound transistor | Check the speaker for continuity; then check the transistor that drives it.[4] |
Things that are not faults
[edit | edit source]- The balls take a moment to appear after RESET.[3]
- Points for special rollovers and bonus targets are added after the ball leaves the playfield, not when it hits them.[3]
- A short tap and a long press of the launch button give different ball speeds.[1]
- Video magazine, testing the Telstar Arcade in 1979, found that on a few of its games balls could hang up and circle endlessly, or pass through what should have been solid walls or bumpers.[9] The same thing on a Gemini is not on its own a sign of a hardware fault.
- On the MPS 7600-001, MOS built in logic to reject false hits from room lighting, reflections or aiming at a light source.[4] Whether the Gemini's chip has the same logic is not documented. If it does, a pistol aimed at a lamp will not score.
What is realistically repairable
[edit | edit source]| Part | Repairable? | Notes |
|---|---|---|
| Flipper and launch buttons, panel switches | Usually | Clean the contacts first. |
| Playfield sliders | Usually | Clean the tracks; a worn slider needs a replacement of the same value and travel, read from the part itself. |
| Pistol lead and plug | Yes | Ordinary wiring. |
| Crystal | Yes | 3.579545 MHz is the standard NTSC colour-burst frequency.[4] |
| Game chip | Only from a donor | Each MPS 7600 version holds its own games in mask ROM.[4] Arcade Cartridge No. 3 carries an MPS 7600-004 with the same games, but no one has reported fitting one in a Gemini.[5] |
Related pages
[edit | edit source]- Coleco Telstar Gemini
- Coleco Telstar Gemini General Maintenance
- Coleco Telstar Gemini Capacitor Replacement Guide
- Coleco Telstar Arcade Troubleshooting Guide, for the cartridge console that plays the same games
- Capacitor Failure Symptoms
References
[edit | edit source]- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Arcade-History, "Telstar Gemini" (retrieved 2026-10-19): Coleco, 1978; plays two shooting games and four pinball games instead of the common Pong games; games Bonus Pinball and Deluxe Pinball for one or two players, Shooting Gallery and Shoot the Bear; two flipper buttons, one on each side of the console; a big red button on top launches the ball, a short tap sending it out slowly and a long press fast; three sliding knobs on the left side move the playfield's bumpers and chutes up and down; controllers: the two pinball buttons and a rifle for the shooting games; CPU "Coleco MPS 7600"; colour; built-in beeper.
- ↑ David Winter, Pong-Story, "Coleco Telstar Arcade - Cartridge #3" (retrieved 2026-10-19): Cartridge #3 plays six games, four pinball games (Bonus Pinball and Deluxe Pinball, each for one or two players), Shooting Gallery (fixed targets) and Shoot the Bear (moving "bears"); it uses only the console's knobs and pistol; "The games of this cartridge are same as those of the Telstar Gemini system."
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Coleco Industries, Game Rules and Operation of Telstar Cartridge No. 3, Model No. 6113, Guide No. 76634, p. 1, photographed by Pong-Story, File:Coleco Telstar Arcade Cartridge No. 3 game rules, first page (Guide 76634).jpg: "Plays 6 video games in color": four pinball games with flipper control and two moving target games; flippers are worked by the LEFT SLAM button and the ball plunger by the RIGHT SLAM button, "a very short 'tap' will shoot the ball out slowly" and "a longer 'press' will shoot the ball out faster"; the steering wheel and the right and left rotary control knobs move the play field's bumpers and rollover chutes up and down (large chutes on the left rotary control, large side bumpers on the steering wheel, small chutes and small bumpers on the right rotary control); set the play field before starting; press RESET and wait for the balls to appear; three balls per game; scoring: regular bumper 1 point, chutes and alleys 1, hot shot bumper 1, special rollovers 2, bonus targets 3, with rollover and bonus target points added after the ball leaves the play field; turn the console off before changing cartridges.
- ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 4.11 4.12 4.13 MOS Technology, Specification for Video Game Array MPS 7600-001, preliminary, rev. 1, June 1976, pp. 7 and 9-12 and Figure 4.1A, File:MOS Technology MPS 7600-001 Video Game Array Specification (1976).pdf: a mask-programmed series (MPS 7600-XXX) built round a frame processor with a 512-word ROM; crystal clock of 3.579545 MHz (full colour) or 3.595314 MHz ("rainbow" colour), one of which is essential to stable operation; a single supply of -6.5 to -9.5 V, with two VDD and two VSS pins that must all be connected; most designs battery powered, and an AC supply needs adequate current and filtering because sharp transients "may result in perturbations of the game display"; four player-control inputs (P1 to P4 in Figure 4.1A), each a potentiometer, and the controls "receive considerable abuse"; sound through a single-transistor amplifier driving the speaker; the target games need a gun with a trigger switch in the control matrix and a photoelectric circuit sending pulses to the HIT pin; on the -001, special logic rejects false hits from room lighting, reflections or aiming at light sources.
- ↑ 5.0 5.1 Centre for Computing History, "Coleco Telstar Arcade", reference CH66264 (retrieved 2026-10-19): Telstar Arcade Cartridge 3 (Bonus Pinball, Shooting Gallery, Shoot the Bear and Deluxe Pinball) uses the chip MPS 7600-004.
- ↑ 6.0 6.1 Retro Gaming Consoles, "Coleco Telstar" (retrieved 2026-10-19): the Gemini, released in 1978, has two gun games and four pinball games, no paddles or joysticks, flipper buttons at the left and right sides and a large red button at the front to launch the ball; "powered by a 9 volt battery, it could also be powered using an ac adaptor, which handily fitted into the base of the unit".
- ↑ Photograph by Evan-Amos, File:Coleco-Telstar-AC-Adaptor.jpg (public domain, from Wikimedia Commons): label "TELSTAR AC ADAPTOR MODEL 6041 ... INPUT: 117V 60Hz 5W OUTPUT: 9 V DC 200mA".
- ↑ Coleco Industries, Telstar Arcade Installation Instructions and Game Rules, Model No. 6175, Guide No. 58024 C, 1977, p. 6, File:Coleco Telstar Arcade Installation Instructions (Guide 58024C).pdf: for the Arcade's pistol games, insert the pistol connector firmly, lined up with its key, and never force it; do not cock the pistol, fire with the trigger only; firing range depends on the television; set contrast so that the background is grey and colour so that the score is pink or white, and a brighter target area is easier to hit; Cartridge No. 3 has four pinball games and two target games.
- ↑ Berger-Braithwaite Labs, "Coleco Telstar Arcade Video Game", VideoTests, Video, Spring 1979, p. 55, Internet Archive scan (retrieved 2026-10-19): on the Arcade, "in other games, like pinball, the paddle knobs or the steering wheel modify the playing field"; on a few games the testers could get the balls hung up making endless rounds of the court, or watch them go through what should have been solid sidewalls or bumpers.
