The Apple Macintosh Classic uses a 14-pin proprietary connector at position J12 on the logic board to link the main board to the CRT analogue board. Unlike a dedicated power connector, it combines DC power rails (+12V, +5V, and −12V), horizontal and vertical sync signals, composite video output, audio, and a PWM brightness control signal on a single harness — a design unique to the all-in-one compact Macintosh architecture. There is no +3.3V rail, no standby supply, and no power-good signal; the connector predates the ATX standard entirely, dating from the Macintosh Classic’s introduction in 1990.
| Pin | Name |
|---|---|
| 1 | +12 volts |
| 2 | + 5 volts |
| 3 | + 5 volts |
| 4 | /VSYNC |
| 5 | /HSYNC |
| 6 | VIDOUT |
| 7 | Sound |
| 8 | -12 volts |
| 9 | PWM (Brightness control signal) |
| 10 | Ground |
| 11 | Ground |
| 12 | Ground |
| 13 | Ground |
| 14 | Ground |
Signals
- +12 volts — positive 12 V DC supply rail, used by the CRT deflection circuitry on the analogue board.
- +5 volts — positive 5 V DC supply rail (two pins, 2 and 3), providing the primary logic supply to the analogue board.
- -12 volts — negative 12 V DC supply rail, required by the analogue board’s video and audio circuitry.
- /VSYNC — active-low vertical synchronisation signal from the logic board to the CRT controller; sets the vertical scan rate for the built-in display.
- /HSYNC — active-low horizontal synchronisation signal from the logic board to the CRT controller.
- VIDOUT — composite video output signal driving the CRT electron gun via the analogue board.
- Sound — analogue audio output signal routed to the built-in speaker amplifier on the analogue board.
- PWM — pulse-width modulated brightness control signal; adjusts the intensity of the CRT beam and therefore the screen brightness.
- Ground — DC return/common ground; five pins (10–14) provide the ground reference for all power and signal lines.
Notes
- The connector is located at position J12 on the Macintosh Classic logic board.
- This is not a dedicated power-only connector: it carries CRT video sync, video output, audio, and PWM brightness alongside the DC rails on a single 14-pin cable to the analogue board.
- Power rails present are +12 V, +5 V, and −12 V only. There is no +3.3 V supply, no standby rail, and no power-good signal.
- The connector and its pinout are specific to the Macintosh Classic (introduced 1990) and are not shared with other compact Macintosh models, which used different internal wiring arrangements.
- Mating connector details and wire colours are documented in Apple Tech Info Library article 6532.
References
- Apple Tech Info Library article 6532, “Macintosh Classic, Internal Power Connector Pinouts” — Apple Computer, Inc.