Editing Atari Mega ST (section)

== Known issues ==

Common issues affecting the Mega ST after decades of use include:

* '''Power supply degradation''' — The Mitsumi SR98/SR118 PSU contains electrolytic capacitors that degrade over time, causing voltage ripple, random crashes, floppy read errors, video noise, and in severe cases, damage to the motherboard. PSU recapping is considered mandatory maintenance.<ref name="exxospsu">{{cite web |url=https://www.exxosforum.co.uk/atari/last/psu/index.htm |title=Atari PSU Repair — The LaST Upgrade |publisher=exxos |access-date=2026-03-27}}</ref>
* '''Reset circuit malfunction''' — The reset circuit on ST/Mega ST boards can fail due to aged capacitors, causing the machine to require multiple power cycles or a manual reset button press to start. A modification involving the reset capacitor addresses this issue.<ref name="mandatory">{{cite web |url=https://www.exxosforum.co.uk/forum/viewtopic.php?t=1585 |title=Mandatory Fixes |publisher=exxos Forum |access-date=2026-03-27}}</ref>
* '''DMA bus pull-up resistors''' — The DMA data bus lacks pull-up resistors in the original design, which can cause intermittent hard drive and floppy drive failures. Adding 10K pull-up resistors to the DMA data bus is a recommended preventive modification.<ref name="mandatory"/>
* '''1772 floppy controller pull-ups''' — Replacement floppy drives lacking internal pull-up resistors (such as the MPF920) may exhibit unreliable operation. Pull-up resistors should be added to the 1772 FDC output lines.<ref name="mandatory"/>
* '''RTC battery leakage''' — The CR2032 battery can leak over time, causing corrosion on nearby PCB traces and components. Regular battery replacement is advised.
* '''Bus resistor values''' — Stock 10K bus resistors on the address and data bus can cause instability when cartridges or certain CPU/ROM combinations are used. Replacing with 2.2K resistors improves reliability.<ref name="mandatory"/>