A501 - 512K Ram Expansion

Real Time Clock

The RTC chip used in the A501 is the OKI M6242B. The data sheet can be found at:

https://datasheetspdf.com/datasheet/M6242B.html

https://www.amigawiki.org/doku.php?id=de:parts:m6242b

The operating voltage of OKI M6242B is 2V to 7V.

The RTC chip is powered by the 12V rail. However, the voltage is dropped by several resistors and diodes. It is also pulled down further by the presence of a battery.

If the battery is not present, the overall pulled down voltage getting to the RTC chip may exceed the 7V maximum operating voltage. Thus, the RTC chip will not work, and will not be detected by the system.

Button Cell Battery Mod

As per: https://eab.abime.net/showthread.php?t=98373

If adding a CR2032 button cell battery, replace the 470 ohm resistor at R911 with a 1N4148 diode.

For an alkaline rechargeable battery, R911 limits the charging current from the 12V line. Replacing with a diode with stop the charging current but will allow the non-rechargable button cell to still power the RTC chip.