It's a good thing that these machines don't even need -5 volts. With just the positive voltages provided, RS-422 still works, including LocalTalk.
I think the -5 volts is only there in case an expansion card needs it.
I think the -5 volts is only there in case an expansion card needs it.
Most RS-232 receivers of that era had a fair amount of gain with a transition point close to 0V. So only a little minus voltage would be required in practice, not the entire -5V. So just as long as the reversed capacitor was not entirely shorting out the rail things would have worked.
These days, a charge pump device costs just a few dollars from Ali-express, so even though things work fine this way for many things, it wouldn't hurt to make them work for most if not all things.
When a replacement power supply does provide negative voltage, I try to use it, even if it does mean adapting -12 volts, which is what most ATX power supplies provide, to -5 volts, which is what, it seems, many 68030 Macs want.
Funny - I just did an ATX power supply for a Quadra 630, and it seems that most of the 68040 machines were using -12 volts instead of -5 volts (except the Quadra 605, which happened to use the same form factor and power supply as the LC / LC II / LC III).