Thanks, TheChitterneFlyer and Wodrick,
That visit to the garage seems less urgent now. Your info backs up dixi188's point about the 2 AC Main Buses on the One-Eleven being separate and autonomous, except in the single genny case; very different from the 707. From what I remember, it was a very nice system that even we pilots could get our heads round - and rather like the A320.
If memory serves, you can also isolate the AC generator field windings with a GCR switch (?) and, of course, disconnect the generator's CSD (constant-speed drive). The latter is actually a CSD-S, doubling as an engine starter (using air from the APU or an external air truck, just like aircraft today) The CSD-S is connected to the N2 (HP) accessory gearbox of the Spey engine by a somewhat fragile shaft. During engine start, this is what produces the classic shriek, its pitch rising with N2 rpm, until the starter dog(?) disengages at a certain rpm. At this point, there is often a momentary hesitation in the scream before it mercifully dies away. [Particularly welcome if you are having to stand under the engine to hold the start (air) valve open manually, in the event of a faulty solenoid.]
For the uninitiated, it is the CSD that controls the generator speed (regardless of engine N2 rpm); hopefully giving a steady 400Hz for the AC electrical system. This is the note (pitch) of the hum you are constantly aware of in an airliner cockpit. One of these days I’ll find out what note it equates to on a musical scale, but it is slightly below (flatter than) the “A” that an orchestra tunes to. If the note falls in pitch (i.e., below 400Hz), one of your gennies may be about to drop off line.
As for the B707, maybe 411A didn't recall the fact that − unlike the One-Eleven − the BTBs are closed in normal operation, effectively de-segregating the different parts of the AC system into a big daisy chain. After my post late last night, disjointed bits of it - including those flashing lights - were floating through my mind as I dozed off. The "enj" certainly works for his keep. The Seven-oh has so many other evolved idiosyncrasies for the successors of Wodrick and NG Kaptain to play with, including splitting the inner and outer flaps electrically to cope with a jammed-stabiliser landing. Phew! Don't the VC10 F/Es have it easy by comparison? [Can't comment on the A300.]