Editing Atari ST Troubleshooting Guide (section)

== Floppy Drive Troubleshooting ==

The floppy subsystem is the same across 520ST (with external drive) and 1040ST (internal drive). The WD1772 controller and DMA chip are on the motherboard; drive select and motor control are handled by the YM2149.

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{| class="wikitable styled-table"
! Symptom !! Possible Cause !! Recommended Action
|-
| Drive motor does not spin || YM2149 not asserting drive select/motor on; bad cable; drive motor failure || Check YM2149 Port A bits for drive select and motor on. Check ribbon cable (1040ST) or DIN cable (520ST external). Try known-good drive.
|-
| Drive seeks but cannot read || Dirty heads; misaligned heads; failing WD1772; bad data cable || Clean heads with IPA. Try multiple known-good disks. Try known-good drive. If external drive also fails, suspect WD1772 or DMA.
|-
| Drive seeks to track 0 repeatedly (clicking) || Drive cannot find track 0 sensor; head mechanism stuck; belt failure || Check track 0 sensor. Clean and lubricate head carriage rails. Replace drive belt if stretched.
|-
| Drive reads but cannot write || Write-protect switch stuck or misread; WD1772 write circuit fault || Check write-protect sensor. Try a different disk with the tab in the correct position.
|-
| Intermittent read errors || Dirty heads; ageing media; PSU instability; thermal WD1772 issue || Clean heads. Try fresh disk. Check PSU voltages. If errors appear only after warm-up, suspect thermal WD1772 failure.
|-
| Very slow boot (30+ seconds) || TOS searching for bootable disk on drive A with no disk inserted || Not a fault. Insert a non-bootable disk or upgrade to TOS 1.04 for improved boot behaviour.
|}