Editing Atari ST Troubleshooting Guide (section)

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

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! Symptom !! Possible Cause !! Recommended Action
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| 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.
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| Intermittent floppy read errors || Unstable +12V rail (floppy motor speed variation) or noisy +5V (logic errors) || Measure both rails under load. Recap PSU.
|}