Editing Acorn Archimedes A3020 Troubleshooting Guide (section)

== Dead machine with power present ==

If the rails are good but the machine is dead, the fault is on the logic side of the board. Check the following, escalating from the simple to the scope-level:<ref name="sm" /><ref name="trm" />

* '''ROMs''': confirm the two RISC OS ROMs are seated with no lifted pins and links LK10/LK11/LK12/LK14 correct for the fitted ROM size.
* '''Battery corrosion''': inspect and buzz out the tracks around the battery and under the floppy connector — broken RAM control lines here give the 0x20000 POST fault.
* '''Clocks''': with an oscilloscope, confirm the oscillators are running (see the clock table below). No CPU clock means the machine cannot start.
* '''Reset''': the reset line should sit high in normal running and pulse low at power-on. A reset stuck low halts the ARM250.
* '''Bus activity''': with a healthy clock, reset released and rails present, the ARM250 should drive the address bus (address lines toggling as it fetches from ROM) and the DRAM should show RAS/CAS activity. If the clock, reset and rails are all present but the ARM250 produces no bus activity, the ARM250 itself is the suspect — a whole-device replacement.
* Do '''not''' leave the board powered with no clock: loss of DRAM refresh can corrupt or damage the RAM.

{| class="wikitable styled-table" style="width:90%; text-align:left;"
|+'''A3020 clocks — expected oscilloscope readings'''
! Signal !! Frequency !! Where
|-
| CPU / memory clock || 12 MHz || ARM250; derived from the master oscillator
|-
| Master oscillator || 72 MHz (crystal X4) || Divided down to the CPU, IOC and pixel clocks
|-
| IOC reference || 8 MHz || I/O timing
|-
| VIDC pixel clock || 24 / 25.175 / 36 MHz || Selected by monitor sync polarity (crystal X5 = 25.175 MHz)
|-
| Floppy controller clock || 24 MHz (crystal X7) || 82C711
|-
| RTC crystal || 32.768 kHz (crystal X2) || PCF8583 real-time clock
|}

Clock frequencies are from the TRM circuit diagram.<ref name="trm" />