The Global Recordings Network “Messenger II” cassette player, intended for distribution of religious pre-recorded cassettes in regions without electrical power or ready access to batteries, had a built-in generator and hand crank to power it. Unlike many survival radios, there was no rechargeable battery—you had to continuously turn the crank in order to make it play, rewind, or fast forward. There is a voltage regulator, so as long as you crank at or above the minimum speed, the tape will run at the correct speed. A mains power and six volt DC battery power sockets allow running from those sources if available.
The service manual [PDF] that accompanied the device is stunning—even the old Hewlett-Packard would have been impressed. The introduction states:
The technical descriptions have been kept as simple as possible and have been prepared with the nontechnical person in mind. As well as providing this Service Manual and any necessary spare parts, we believe in the concept that “prevention is better than cure”, and we strongly recommend that where any “Messenger-II” players are put into use that those using them should be given careful instructions in the basic care and maintenance of those players.
An indeed, a bag of spare parts is included inside the player case.
Further encouragement is in the “Troubleshooting Procedure” section:
The novice or layman in the area of electronic repair should not be overawed by the apparent complexity of the service manual. The manual has been prepared with the non-expert in mind and we trust that the instructions given will be clear enough for all to follow. Regular experience in maintenance and repair of the Messenger-II will soon help the novice to become familiar with the parts of the machine most likely to give problems. Those who only carry out repairs on odd occasions or in emergencies need to take special care and be aware that hasty action or diagnosis of the problems may lead to a lot more expense and time expended than is really necessary.