[bilegeek]

> the main bit ran off something like 400hz power, for some reason the CPU needed faster mains volts going in.

Aerospace originally did that to reduce component size, CDC and IBM took advantage of the standard in the early 60's.

Strangely, it seems mainframes didn't adopt switching power supplies until the end of the 70's, despite the tech being around since the end of the 60's.