Editing Macintosh SE/30 Troubleshooting (section)

=== Power Rails ===
Attempting to power up the motherboard on its own with just a bench power supply and minimum rails, using an oscilloscope to see video output.

12V Rail (J12 pin 14) goes to external and internal drives, external PDS connector and sound, '''so will boot without 12V to some degree, however the sound chip UB11 will hold the system in reset so you will only get to horizontal lines/simasimac screen at most'''.

* J6 (External 3.5" Floppy Drive)
* J8 (Internal Floppy Drive)
* J13 (PDS video/expansion slot)
* UB10/'''UB11'''/UA9 (Sound)

-12V Rail (J12 pin 7) goes to the PDS slot only, so should definitely boot without it.

* J13 (PDS video/expansion slot)

-5V Rail (J12 pin 6) goes to the line drivers for the printer and modem, and the PDS port, so again should at least boot without it.

* UE12 / UD12 (Line drivers for printer and modem)
* J13 (PDS video/expansion slot)

This leaves the 5V rail (J12 pin 12/13) which goes everywhere else to loads of logic. This is absolutely needed.

Based on that, the system should at least boot with some video output on J12 (pins 9/10/11) you can observe with an oscilloscope with just the motherboard and 5V rail powered. However, due to UB11 sound chip not having power it will hold the !RESET line at 2.5V getting to horizontal lines/simasimac screen at most.

To get to a further boot, you need to also apply the 12V rail.