Editing Atari ST Troubleshooting Guide (section)

=== No Display at All ===

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! Symptom !! Possible Cause !! Recommended Action
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| No display, no power LED || PSU failure || See Power Supply Diagnosis section above.
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| Power LED on, completely blank screen || CPU not running; no clock signal; or monitor cable/connection fault || Verify monitor cable is connected and monitor is set to correct input. Check 32 MHz crystal oscillator with oscilloscope. Reseat CPU, Shifter, GLUE.
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| Power LED on, white screen only (colour monitor) || CPU running but failing during early TOS initialisation. Common causes: RAM fault, TOS ROM fault, MMU failure, GLUE failure. || Reseat all socketed ICs. Try known-good TOS ROMs. Try with diagnostic cartridge — if cartridge menu appears, motherboard is partially functional and fault is likely in RAM or TOS ROMs. If no cartridge output, fault is in ROM address space decoding (GLUE/MMU).<ref name="diagcart"/>
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| Power LED on, white screen only (monochrome monitor) || As above, plus: mono detect pin not being grounded (faulty cable/connector) causing Shifter to output colour mode to mono monitor || Check pin 4 on the 13-pin DIN connector. It should be grounded for monochrome mode.
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| Display appears briefly then goes blank || PSU voltage droop under full load; thermal fault in Shifter or GLUE || Measure +5V rail under load. If voltage is stable, suspect thermal failure in Shifter — use freeze spray to isolate.
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