As useful as electronics are, the need to have some source of power for them can be a bit of an issue, especially for small, portable devices. One of the most low-tech but universally applicable source is human mechanical power, as demonstrated by the rugged 1980s-era Messenger II tape player in a recent [TechMoan] video. Without beating around the bush, this is indeed a device created by an evangelical organization (GRN) that missionaries would take with them to wherever their mission took them. Naturally this put the availability of power from a wall outlet in question, especially in the 1980s when this tape player was produced.
Per the specifications for the device, it uses about 1.2 Watt at full power, with said power coming from a 6 VDC barrel jack input (AC adapter or 4 D-cell battery pack), an AC adapter, or from the hard to miss crank handle. This crank handle connects to a set of gears that drive a generator via a belt, though with no buffering in terms of a spring, capacitor or the like. On the same specifications page for the device you can find the service manual (also on Archive.org), which includes full schematics. In the service manual on page 19 you can see the main schematics, including how the 6 VDC, transformer and generator inputs are handled.
Internally, the circuitry operates off a 5 V rail (after diode drop of the approximately 6 VDC input), with playback and other operations disabled once the input voltage drops below a certain point. Which can happen when the batteries run low, or your cranking arm gets tired after at most a few minutes. Fortunately, the playback speed is regulated so that cranking speed (above a certain RPM) is always sufficient to get decent sounding mono audio output.
Although the crank handle is noted by [TechMoan] as being easy enough to handle, it’s also incredibly tiring so that you can only use it for short sections. For a proper doomsday device, it might be advisable to use something like a pedal-based generator as one’s leg muscles are much better at duration trials, such as pedaling through the entirety of Queen’s Bohemian Rhapsody.