> the 12V rail is also used for external devices, eg harddisk, cd drive, expansion cards, etc

External drives should have their own power supply and indeed the A590, A570, GVP drives etc all do.

The A500/A2000 Technical Reference (page 227) shows that the 12V current limit on the expansion connector is 50mA

The A600/1200 don't even have the expansion connector

Page 5 of the TRM specs the -12V supply at 100mA

> Too low current on the -12V rail can cause crackling sound.

Maybe if the current is zero

TRM: https://archive.org/details/CommodoreAmigaA500A2000Technical...