Editing Atari ST Troubleshooting Guide (section)

== Power Supply Diagnosis ==

The PSU is the most common source of problems in ageing ST machines. The 1040ST has an internal switched-mode PSU; the 520ST/520STM uses an external supply.

=== Voltage Rails ===

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table"
! Rail !! Wire Colour (1040ST internal header) !! Nominal Voltage !! Acceptable Range !! Notes
|-
| +5V || Red || 5.00V || 4.75V – 5.25V || Primary logic supply; most critical rail. Below 4.75V causes instability; above 5.5V risks IC damage.
|-
| +12V || Blue || 12.0V || 11.0V – 13.0V || Floppy drive motor, RS-232 transceivers. The machine will operate without it but floppy and serial will not function.
|-
| GND || Black || 0V || — || Common ground reference.
|}

On the 520ST with external PSU, measure at the 7-pin DIN power connector: +5V (pins 1, 2), +12V (pin 6), GND (pins 3, 4, 5). Some external supplies also provide −12V (pin 7) for the RS-232 transceivers.

'''Testing procedure:'''
# '''Do NOT test a switched-mode PSU disconnected from the motherboard''' (or a dummy load). Without a load, output voltages may be dangerously high or the PSU may not start at all.<ref name="exxospsu">{{cite web |url=https://www.exxosforum.co.uk/atari/last/psu/index.htm |title=The LaST Upgrade - Atari PSU Repair |publisher=exxos |access-date=2026-03-27}}</ref>
# With the machine assembled and powered on, measure the +5V rail at the motherboard power header (red wires to black wires).
# If the voltage is below 4.9V under load, the PSU capacitors likely need replacement.
# Check for AC ripple on the +5V rail if you have an oscilloscope. A healthy recapped PSU should show less than 200mV of ripple. More than 500mV indicates failing capacitors and will cause instability.<ref name="exxospsu"/>

=== Common PSU Symptoms (Both 520ST and 1040ST) ===

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{| class="wikitable styled-table"
! Symptom !! Possible Cause !! Recommended Action
|-
| No power at all (no LED) || Blown mains fuse in cable or PSU; failed power switch (1040ST); open fusible resistor in PSU; failed external PSU brick (520ST) || Check mains fuse. On 1040ST: check PSU internal fuse and continuity through power switch. On 520ST: test external PSU output voltages with DMM.
|-
| Power LED flickers || PSU attempting to start but shutting down (overcurrent protection) || Likely shorted component on motherboard or PSU. Disconnect motherboard from PSU and test PSU with dummy load (1040ST) or try a different external PSU (520ST).
|-
| Random crashes during use || Low or noisy +5V rail || Measure +5V under load. Recap PSU if voltage below 4.9V or ripple exceeds 300mV.
|-
| Screen dims when floppy drive activates || Poor peak power delivery on +5V rail (1040ST internal PSU) || Recap PSU. On Mitsumi SR98 PSUs, the biasing resistors R201/R202 may need adjustment — replace R202 with 1KΩ and R201 with 1.2KΩ, then readjust the voltage trimmer to 5.00V.<ref name="exxospsu"/>
|-
| Audio noise/hum || Ripple on +5V rail coupling into YM2149 audio output || Recap PSU. Check ground connections.
|-
| Intermittent floppy read errors || Unstable +12V rail (floppy motor speed variation) or noisy +5V (logic errors) || Measure both rails under load. Recap PSU.
|}