Amiga 4000 rev 2Black screen, won't boot

The 5 V rail

With the machine on, measure between a VCC pin and ground: it must read between 4.75 and 5.25 V. Below that, the power supply is usually to blame, not the board.

Reset

After power-on, _RESET must go high (close to 5 V). If it stays stuck low, the processor never starts: follow the line to the reset circuit and the keyboard connector.

The clock

With a scope, there must be a clock of about 7 MHz reaching the processor. Without a clock nothing moves: check the crystal and the chip that generates it (Agnus or Alice).

The processor at work

_AS pulses every time the processor accesses memory. If it pulses, the processor is alive and the fault is further along. If it is still, go back to reset and the clock.

DTACK

On every cycle, the glue chip (Gary or Gayle) pulls _DTACK low so the 68000 can close the access. If _AS pulses and _DTACK never drops, the processor is stuck waiting: check Gary or Gayle and the clocks reaching it.

OVL

At power-on OVL is high so the ROM appears at address zero; Kickstart pulls it low soon after, through the CIA. If it never changes, booting never got past that point.

The ROM

_ROMEN selects the Kickstart ROM. If it pulses and still no colour appears, suspect the ROM or its socket: clean the contacts and reseat it.