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Technical Information for Hammond Tone Wheel Organ
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-l2), and is divided into several sections connected by semi-flexible couplings. (See figure 1-5.) 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 sub assemblies. 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.) All four tone wheels in any one 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 figure 5-l .) 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 54); 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 3Ztooth wheel form an assembly, giving 2 frequencies, 4 octaves apart. The 4- and 64-tooth wheels are assembled together, as are the 8- and 12-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.) Only 91 frequencies are required for the organ; for identification purposes these frequencies are numbered 1 to 91 inclusive.
5-6. A magnetized rod, about 4 inches long and l/4 inch in diameter, is mounted near each tone wheel. (See figures 5-l and 5-2.) A small coil of wire is wound near one end of the magnet. The tip of the magnet at the coil end is ground to a sharp edge and mounted near the edge of the associated tone wheel. Each time that a tooth of the wheel passes the rod, the magnetic circuit changes and a cycle of voltage is induced in the coil. The voltage is very small and is of known frequency. The frequency is predetermined by
the number of teeth and the speed of the rotating tone wheel. Larger coils are used with tone wheels of lower frequencies to provide good low frequency output, but smaller coils are used with tone wheels of higher frequency to prevent excessive losses.
5-7. Copper rings are mounted on certain low frequency coils for the purpose of reducing
harmonics. The eddy current loss in such a ring is small for the fundamental frequency of the coil, but is high for its harmonics. As a result, the relative intensities of any harmonics which may be produced by irregularities in the tone wheels are reduced
5-8. The edge of each tone wheel and the tip of each magnet are coated with lacquer to prevent corrosion, for, should oxidation set in, the change in tooth shape would introduce undesirable frequencies.
5-9. Filters for eliminating spurious harmonics from the generated simple tones are located on the top of the main tone generator, and consist of filter capacitors and reactors. (See figure 3-4.) (These capacitors and reactors are tuned units and are called tone generator filters.)
5-10. The tone generator filters have a single tapped winding. This tap is grounded and one side, which is connected to the associated coil assembly through a capacitor, forms a resonant circuit for the fundamental frequency of that coil. Harmonics are supressed. The capacitors for frequencies 49 to 54 inclusive are 0.255 mf, and the capacitors for frequencies 55 to 91 inclusive are 0.105 mf. Both capacitors and re-actors are used with frequencies numbered 49 to 91 inclusive. On frequencies 44 to 48 inclusive, the capacitors are omitted, but the reactors used have a greater number of turns. Below frequency 44, neither capacitors nor reactors are used; a length of resistance wire shunts each generator output. This resistance wire is wound on the appropriate magnet coil.
5-l1. The tone generator filters are mounted on top of the generator at an angle to minimize reaction between them. Wires connect the filters to the coil assemblies and to the terminal strip on the generator. Ninety-six terminals are provided on this strip; 3 terminals are grounded to the generator frame and serve to ground the manuals and pedals, and 91 terminals carry the various frequencies.
5-l2. The start motor is a shaded-pole induction motor. The synchronous run motor (used on 60 cycles) has a 2-pole field and 6-pole armature, and a synchronous speed of 1,200 rpm (revolutions per minute). For 50 cycles, a 4-pole armature is used which has a speed of 1,500 rpm. When the organ is placed into operation, the start switch is first operated to apply power to the start motor. The rotor of the start motor slides endwise and engages a pinion on its shaft which a gear on the generator drive shaft. (See figure 5-3.) When the "RUN" switch is operated, while the start switch is held in "ON" position, power is applied to the synchronous run motor and a 250&m resistor (1,000 ohm for 234 volts) is connected in series with the start motor, thus reducing the driving power of the start motor. Because of the braking action and the loss of power of the start motor, the system slows down to, and locks into, synchronous speed; the run motor then begins to carry the load. When the "START" switch is released and springs back into position, the start motor disengages from the drive shaft by action of a spring assembly, and stops.
5-13. The spring couplings of the motor shaft, the flexible couplings between the sections of the drive shaft, and the tone wheel spring couplings are provided to absorb the variations in motor speed. The synchronous motor operates with a series of pulsations, one each half-cycle. If the tone wheels were coupled rigidly to the motor, this irregularity would carry extra frequencies into each tone wheel. The spring suspension system for supporting the main tone generator minimizes the transmission of mechanical vibration between the console cabinet and the main generator.
5-14. VIBRATO EQUIPMENT.
5-l5. The vibrato effect is created by a periodic raising and lowering of pitch, and thus is fundamentally different from a tremolo or loudness variation. It is comparable to the effect produced when a violinist moves his finger back and forth on a string while playing, varying the frequency while maintaining constant volume.
5-l6. The Hammond Organ vibrato equipment, as shown in simplified block diagram, figure 5-4 varies the frequency of all tones by continuously shifting their phase. It includes a phase shift network or electrical time delay line, composed of a number of low pass filter sections, and a capacity type pickup or scanner, which is motor-driven so that it scans back and forth along the line.
5-17. Electrical waves fed into the line are shifted in phase by each line section (the amount per section being proportional to frequency), so that at any tap on the line, the phase is retarded relative to the previous tap.
5-l 8. The scanning pick-up traveling along the line will thus encounter waves increasingly retarded in phase at each successive tap, and the signal it picks up will continuously change in phase. The rate at which this phase shift occurs will depend on how many line sections are scanned each second.
5-19. Since a cycle is equivalent to 360 electrical degrees, a frequency shift of 1 cycle occurs for each 360 electrical degrees scanned per second. For example, if the scanner passes over the line at such a rate that 3,600 electrical degrees are scanned each second, there will be a frequency change of 10 cycles.
5-20 For the widest vibrato, the whole line is scanned from beginning to end in about l/14
second and this rate of change of phase causes about 1-1 /2 percent decrease in frequency. Note that the frequency remains constantly l-l /2 percent low as long as the moving pick-up retards the phase at a constant rate.
5-21. Since the pick-up sweeps from start to end of the line and then back, it increases the
frequency by an equal percentage on its return trip the average output frequency remaining equal to the input frequency. The exact amount of frequency shift depends not only on the amount of phase shift in the line but also on the scanning rate. This rate, however, is constant because the scanner is driven by the synchronous running motor of the organ.
5-22. The degree of vibrato (or amount of frequency shift) may be varied by a switch (not
shown in figure 5-4) which causes the whole line to be scanned for No. 3 (wide) vibrato,
about half of it for No. 2, and about one-third for No. 1.