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Coleco Telstar Gemini Capacitor Replacement Guide

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Coleco Telstar Gemini with pistol and box
A Telstar Gemini. No photograph of its circuit board has been published.

The Coleco Telstar Gemini dates from 1978, so any electrolytic capacitors inside it are more than 45 years old.[1] Coleco published no schematic or parts list for the Gemini, no photograph of its board has been published, and no capacitor values are known. Cleaning and opening are on Coleco Telstar Gemini General Maintenance; other faults on Coleco Telstar Gemini Troubleshooting Guide.

What is known

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  • Retro Gaming Consoles says the Gemini runs from a 9 V battery or an AC adapter.[2]
  • The game chip is an MPS 7600-series part.[1][3] MOS Technology's specification for the series calls for a single supply of -6.5 to -9.5 V. It says most designs will run from batteries, and that an AC-derived supply must have "adequate filtering to eliminate sharp spikes on the supply lines", because such transients "may result in perturbations of the game display".[4]
  • The same specification describes the sound circuit as a single transistor driving the speaker, and the player controls as potentiometers with their own passive components.[4] MOS gives the clock, player-control, sound and gun circuits in Figures 4.2A, 4.4, 4.5 and 4.9, but those figures are missing from the hosted scan, which has only the block diagrams, the control matrix and the RF figures.
  • The number, positions and values of the Gemini's electrolytics have not been recorded.

Signs of a failing capacitor

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  • A domed or split top, a capacitor leaning on its legs, or brown residue round its base.
  • A picture that wavers, rolls or shows interference on the adapter but is steady on a fresh battery. MOS links display disturbances to poor filtering of AC-derived supplies.[4]
  • Hum in the sound on the adapter that is absent on a battery.

See Capacitor Failure Symptoms.

Recording your board

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Telstar Gemini electrolytic capacitors: record your own console
Designator or position Capacitance Voltage Temperature Polarity mark faces Notes
  1. Photograph both sides of the board, and every lead to the sliders, buttons, control panel, pistol socket and battery snap, before removing anything.
  2. Note each electrolytic's designator, its value and voltage from the sleeve, and which way its negative stripe faces.
  3. Note which electrolytic sits nearest the adapter jack and the battery snap. That is the most likely supply filter.
  4. Keep the table with the photographs.

Replacement procedure

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  1. Remove the battery and unplug the adapter and the pistol.[2]
  2. Open the case as described in Coleco Telstar Gemini General Maintenance, supporting the top while you check for wiring between it and the board.
  3. Work on one capacitor at a time. Desolder from the solder side with a pump or braid, keeping the iron on the old pads only as long as needed.
  4. Fit a new electrolytic of the same capacitance and the same or a higher voltage rating, rated 105 °C, with the same lead spacing, and with its negative stripe where the old one was.
  5. Inspect for solder bridges before refitting the board.
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  • Temperature-controlled soldering iron, desoldering pump or braid, flux and isopropyl alcohol.
  • A multimeter, and a capacitance or ESR meter for checking the parts removed.
  • Radial electrolytic capacitors from a known maker (Nichicon, Panasonic, Rubycon or similar), rated 105 °C.
  • A current-limited bench supply set to 9 V for the first power-up.

See Recommended Tools.

Checks after recapping

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After recapping a Telstar Gemini
Check Expected Source
Supply in 9 V from a battery or adapter [2]
Game chip supply -6.5 to -9.5 V, VDD measured from VSS (MPS 7600-001 data) [4]
Picture Steady colour picture, the same on the adapter as on a battery [5][4]
Pinball Balls appear after RESET; both flippers, both launch speeds and all three sliders work [6][1]
Target games Shots and hits counted with the pistol [4]
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References

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  1. ↑ 1.0 1.1 1.2 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.
  2. ↑ 2.0 2.1 2.2 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".
  3. ↑ David Winter, Pong-Story, "Coleco Telstar Arcade" (retrieved 2026-10-18): each Arcade cartridge contains a chip made by MOS Technology, the MPS-7600; the four versions contain circuits customised for their types of game, all built round a basic frame processor driven by a small program in ROM.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 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. ↑ Box photograph on Pong-Story, File:Coleco Telstar Gemini box (Pong-Story).gif: TELSTAR GEMINI, "6 video games in color", "4 pinball games", "2 moving target games"; authentic flipper action, electronic pinball sounds, digital display scoring; the box art shows the console and a pistol.
  6. ↑ 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.


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