Editing Atari Mega ST Troubleshooting (section)

=== PSU Fault Table ===

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|+ '''PSU Symptom Diagnosis'''
|-
! Symptom !! Possible Cause !! Recommended Action
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| Completely dead — no LED, no fan || Blown mains fuse; open power switch; broken IEC lead || Check continuity from plug through switch to PCB
|-
| LED on but erratic behaviour || +5 V rail out of spec (ripple > 500 mV) || Measure with oscilloscope. Recap all electrolytics; check output diode
|-
| Screen dims 30–50 % during floppy access || Poor peak current delivery from aged capacitors || Recap PSU. On SR98: replace R201 (→ 1.2 kΩ), R202 (→ 1.0 kΩ); replace 1 µF opto capacitor<ref name="exxos-psu"/>
|-
| +5 V rail reads > 5.5 V or < 4.5 V || Failed voltage regulation; shorted/open feedback resistors || Check opto-coupler, TL431 reference (if fitted), feedback resistor network. Adjust trim pot if present
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| Audible ticking (4–5 Hz) from PSU || PSU attempting to start but shutting down (overcurrent or no load) || Verify load is connected. Check for shorted output diodes or capacitors
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| PSU runs but +12 V absent || Failed 12 V rectifier or filter cap || Check 12 V rail diode and associated capacitors
|}