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.
Inside of the GRC Messenger II tape player, with the generator and transformer visible. (Credit: TechMoan)
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 ( ..
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