Amiga 500 · rev 8A
The last known production revision: a stripped-down A500+ board. The drawing shows the A500+ parts (Agnus 8375, Super Denise 8373 and the U9 clock); a stock A500 has the 8372A Agnus and the 8362 Denise, and no clock.
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Inside the Amiga 500: every chip explained, in 3D
Correction The video labels the Agnus 8375, the Super Denise 8373 and the clock, which are A500+ parts: a stock A500 has the Agnus 8370, 8371 or 8372A and the Denise 8362, and no clock. The boot colours also don't apply to every Kickstart.
See the boot colours guide →The video comes from YouTube (Google) and only loads when you click.
Guided diagnosis
Common faults and what to measure, step by step. Each step lights up the signal or chip it talks about on the board.
The power light comes on but the screen stays black, with no colour at all. Measure in order: each step is only worth it if the previous one is fine.
Open this guide as a page →- VCCShow on board →
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.
- _RESETShow on 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.
- 7MHZShow on board →
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).
- _ASShow on board →
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.
- _DTACKShow on board →
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.
- OVLShow on board →
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.
- _ROMENShow on board →
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.
Didn't fix it?
We repair these boards on our bench in Loures. The request already includes the board and the fault.
About this data
Where it comes from
Layouts and parts list from the Amiga PCB Explorer (amigapcb.org), used with the author's permission. amigapcb.org
How it was worked out
Signal names come from the chips' schematic. Continuity was worked out from the copper itself: traces and pads that touch, with both sides joined through the holes.
Before you solder
Always check with a multimeter before soldering a bodge wire. The layout may contain errors, and a repaired board may no longer match the factory one.
Rather not pick up the iron?
Traces eaten by battery acid or leaking capacitors are a common job on our bench.
Request a repair →