4 February 2018

Contents

1. Macintosh II does not power on
2. Macintosh II powers on but does not chime
3. Ethertalk board does not communicate using AUI port
4. Macintosh II does not power on (reprise)

Problem Log

Macintosh II does not power on

Even after replacing the system batteries, the Macintosh II does not power on. Nothing happens when the power switch is pressed.

The PCB trace between R3 and C6 has come open-circuit. Refer to schematic to repair.

Macintosh II powers on but does not chime

After repairing the trace between R3 and C6, the Macintosh II powers on, but it does not chime and nothing is displayed on the screen. A logic analyzer shows that reset is held high.

The PCB trace between C14+ and UI14 pin 10 has come open-circuit. Repair.

Ethertalk board does not communicate using AUI port

The Apple/3Com Ethertalk board (P/N 820-0207) is detected by the system and appears to operate normally from software, but if the board is configured to use the AUI port, no packets are sent or received. When a transceiver is plugged in, it does not receive power and all indicator LEDs remain dark. An open circuit is measured between pins 6 (signal ground) and 13 (+12 Vdc) of the AUI port.

Fuse F1 has failed. Replace. Component is rated 1A, Littelfuse Pico II 125V or similar.

Macintosh II does not power on (reprise)

After some period of disuse, the Macintosh II fails to respond to the power switch again. If the positive lead of B2 is momentarily shorted to pin 15 of J18, the system starts and operates normally. The power switch works to turn off the machine, indicating another fault in the power-on circuit.

UB2 (74HC132 SOIC) is making poor contact with the logic board, due to incomplete cleaning after having previously replaced leaked capacitors. Re-flow the solder joints on UB1 and UB2, and carefully clean the board in this area.