Hammond Model A, Manual, Page: 281 / 298

STAGE DATA AND FINAL TESTING
5-1. DETAILED THEORY OF OPERATION.
5-2. MAIN TONE GENERATOR ASSEMBLY.
5-3. The main tone generator assembly consists principally of 48 rotating
sub-assemblies (each subassembly consists of a shaft, 2 disks called tone wheels,
and a bakelite gear), and a drive shaft which extends the entire length of the
generator. This drive shaft is resiliently coupled at one end to a starting motor
and at the other end to a synchronous run motor (reference paragraph 5-12), and is
divided into several sections connected by semi-flexible couplings. A series of 24
driving gears, 2 each of 12 sizes, is mounted on this shaft.
5-4. Twenty-four of the 48 rotating subassemblies are mounted on each side of the
drive shaft so that each of the driving gears engages 2 bakelite gears associated
with opposite rotating subassemblies. These bakelite gears rotate freely with the
tone wheels on separate shafts and are connected to their respective assemblies by a
pair of compression-type springs. The bakelite gears are provided in 12 different
sizes corresponding to the 12 driving gears of different sizes. Consequently, 4 of
the tone wheel subassemblies, each containing 2 tone wheels, operate at each of 12
different speeds. Each driving gear, with its associated bakelite gears and 4 tone
wheels, is contained in a separate compartment, magnetically shielded from the rest
by steel plates which divide the generator into a series of bins. (See figure 5-2
on next page). All four tone wheels in anyone compartment run at the same speed.
5-5. Each tone wheel is a steel disk about 2 inches in diameter and contains a
predetermined number of high and low points on its outer edge. (See figures 2 & 3 in
Main Generator section.) Each high point is called a tooth. There are 12 wheels with
2 teeth, 1 wheel to operate at each of the 12 speeds (reference paragraph 5-4);
similarly 12 wheels each have 4 teeth, 8 teeth, 16 teeth, 32 teeth, 64 teeth, and
128 teeth; also 7 tone wheels have 192 teeth. A 2-tooth wheel and a 32-tooth wheel
form an assembly, giving 2 frequencies, 4 octaves apart. The 4- and 64-tooth wheels
are assembled together, as are the 8- and 128- tooth wheels and the 16- and
192-tooth wheels. Five 16-tooth wheels are mounted with blanks to maintain the
balance of the rotating unit. (See figure 5-2 on next page.) Only 91 frequencies are
required for the organ; for identification purposes these frequencies are numbered 1
to 91 inclusive.
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