Servicing the Hammond Tone Generator

Technical Description

From Service Manual

Each Hammond Organ Console has a main generator within it, and in some cases, depending on the model, a chorus generator.

The main generator consist of the generator proper, a shaded pole induction motor for starting, a non-self-starting synchronous motor for driving the unit after starting, and a vibrato scanner mounted on the run motor for creating the vibrato effect.

A drive shaft, resiliently coupled to the synchronous running motor, extends the entire length of the generator. Twenty-four driving gears, two each of twelve sizes, are mounted on this shaft, and the drive shaft itself is divided into several sections connected by flexible couplings. The starting motor is mounted at the end of this drive shaft, opposite the synchronous motor.

There are 48 rotating assemblies, each consisting of a shaft and two discs known as tone or phonic wheels. These assemblies are coupled resiliently to the drive shaft. Each of the driving gears engages two bakelite gears associated with opposite rotating assemblies.

Tone Wheel

The bakelite gears rotate freely on the shafts with their tone wheels, and are coupled with their respective assemblies by a pair of coil springs. There are twelve sizes of bakelite gears. Thus 4 of the tone wheel assemblies, each with two tone wheels, run at each of twelve speeds.

Each tone wheels is a steel disc about two inches in diameter, accurately machined with a definite number of high and low points on its edge.

Figure 3 – Tone Generator

Each high point on a tone wheel is called a tooth. the number of teeth on each of these tone wheels, in conjunction with the speed at which the tone wheel is revolving, determines the frequency of the tone generated.

Each driving gear, with its two bakelite gears and four tone wheels, runs in a separate compartment magnetically shielded from the rest by steel plates which divide the generator into a series of bins.

All four tone wheels in any one compartment run at the same speed. The individual tone wheel shafts are mounted in bearings made of a special porous bronze and each of these bearings is connected to the oiling system by a cotton thread from the oil trough. Thus, oil from the trough is carried by capillary action to all bearings, penetrating them and lubricating the actual bearing surface. The drive shaft and both motors are lubricated in a similar manner. It is very important to use the recommended grade of oil regularly, as it is essential to the proper operation of the organ that the generator be well lubricated. If oil of varying grades is used, it is likely that the generator may be sluggish in starting, and in time the threads may gum up and prevent the proper flow of oil.

The two spring couplings on the motor shaft, the flexible couplings between sections of the drive shaft, and the tone wheel spring couplings all contribute to absorbing variations in motor speed. The synchronous motor does not deliver absolutely steady power, but rather operates with a series of pulsations, one with each half cycle. If the tone wheels were rigidly coupled to the motor, this slight irregularity would carry extra frequencies into each tone wheel. In addition, “hunting” is suppressed by the resilient couplings and inertia members of the synchronous motor proper.

Associated with each tone wheel is a magnetized rod about 1/2 inch in diameter and 4 inches in length, with a coil of wire wound near one end. The tip of the magnet at the coil end is ground to a sharp edge and mounted near the edge of the tone wheel. Each time a tooth passes this rod it causes a change in the magnetic field which induces a small voltage in the coil, the frequency being determined by the number of teeth and the wheel speed.

Figure 4 – Hammond Tone Generator

Small coils are used on the higher frequency magnets and larger coils on the lower frequencies. It is found that large pole pieces are needed on the low frequency magnets to give good frequency output, but it is necessary to use smaller ones on the high frequencies to prevent excessive iron losses.

Some of the coils have copper rings mounted on them for the purpose of reducing harmonics. As these are used only on fairly low frequency coils, the eddy current loss in such a ring is small for the fundamental frequency of that coil, but high for its harmonics. This has the effect of reducing the relative intensities of any harmonics which may be produced by irregularities in the tone wheels. The wheels are cut so to give as nearly a sine wave as possible, but the generated voltage seldom reaches that ideal condition, since even a change in the air gap will change the waveform. The tip of each magnet, as well as the edge of each tone wheel, is coated with lacquer to prevent corrosion, for, should oxidation set in, the change in tooth shape would introduce irregular frequencies.

Locations of the various magnet and coil assemblies are shown in Figure 4. They are identified by their frequency numbers, and the broken line between any two numbers indicates that these two frequencies are supplies by one tone wheel assembly.

Each magnet is set at the factory with the set screw partially loosened, while observing an output meter. Experience has shown that the magnets seldom need adjustment and that setting them without proper equipment involves danger of damaging both magnet and wheel. Therefore it is not recommended that the serviceman attempt this adjustment.

As a means of eliminating any vagrant harmonics that may be present, there are filters consisting of small transformers and condensers associated with certain frequencies. The transformers have a single tapped winding, and this tap is grounded, so one side, which is connected to the corresponding magnet coil through a condenser, forms a resonant circuit for the fundamental frequency of that coil. This tends to emphasize the fundamental and suppress harmonics.

Figure 5 – Hammond Tone Generator

Locations of these transformers are shown in Figure 5 and 6. They are also shown in schematics in section 2.

Figure 6 – Hammond Tone Generator

These transformers and condensers are mounted on the top of the generator assembly. The transformers are mounted at an angle, thus minimizing the interference between them. The cores of the transformers are made of a special iron, and the number of laminations used is adjusted to secure the proper inductance. Wire from the magnet coils connect to the transformers, and wires from the transformers lead to the terminal strip on the generator.

This terminal strip carries the output frequencies of the generator, which are arbitrarily numbered from 1 to 91 in order of increasing frequency. This frequency numbering is continued throughout the instrument. In some models the frequencies are not in order on the terminal strip, and Figures 5 and 6 indicate the arrangement for different models. Several terminals at the right end are grounded to the generator frame and serve to ground the manuals and pedals.

Transformers and condensers are not used below frequency 44, but a length of resistance wire shunts each generator. Frequencies 44 to 48 have transformers only, while both transformers and condensers are used for frequencies 49 to 91 except in the case of the Model A consoles numbered below 2179, which do not have condensers for frequencies 49 to 54 inclusive.

Two condenser values are used – 0.255 mfd for frequencies 49 to 54, and 0.105 mfd for frequencies 55 to 91. The transformers are all different. Each transformer is matched to its condenser and any replacements are supplied as matched pairs by the factory.

There are several types of generators in use and the following information will aid the service technician in identifying the console on which work is being performed.

91 Frequency Generator

Model Aserial No. 1– 2676
Model Bserial No. 4000– 10,549
Model Cserial No. 1– 1247
Model Dserial No. 1– 3143
Model Eserial No. 8000– 8663
Player Consolesserial No. 9000– 9209

The number of tone wheels on the above models is 91, and 5 blank wheels are used to maintain the balance of the rotating units. There are twelve wheels with two teeth, one to operate at each of twelve speeds, and similarly twelve have four teeth, twelve have eight teeth, twelve have sixteen, twelve have thirty-two, twelve have sixty-four, twelve have one hundred and twenty-eight and seven have one hundred and ninety-two. An assembly with a two tooth wheel also has a thirty-two tooth wheel which generates a frequency 4 octaves above the other. The four and sixty-four tooth wheels go together, as do the eight and one hundred and twenty-eight tooth wheels. The twelve sixteen tooth wheels are mounted with seven one hundred and ninety-two tooth wheels and the five blank wheels. In this last group the high frequency is not four octaves above the lower.

This arrangement gives a total of 91 frequencies that are connected to the corresponding terminals on the generator, and then to the manuals and pedal switch. In all cases, as mentioned above, the generator must be used with the corresponding manuals and pedal switches and other types of generators cannot be substituted.

82 Frequency Generator

Model Aserial No. 2677– 2711
Model Bserial No. 10,550– 17,074
Model Cserial No. 1248– 17,074
Model Dserial No. 3144– 17,074
Model Eserial No. 8664– 8739
Model Gserial No. 4101– 7349
Player Consolesserial No. 9210– only

In the above consoles, frequency #1 to 9 have been omitted from the generator, and only 82 generator terminals are used. Similarly, there are only 82 tone wheels and magnets in the generator instead of 91. Blank wheels replace the nine two-tooth wheels formerly used to produce frequencies 1 to 9.

This generator change accompanies a wiring revision in the manual and pedal switches which makes the frequencies from 1 to 9 unnecessary. Generators having but 82 frequencies are easily identified by a blank space on the terminal strip at the left of the ground terminals. The first terminal at the left of this space is terminal #10.

91 Frequency Generator with Complex Tone Wheels

Model BVserial No. 17075– 29737
Model CVserial No. 17075– 30287
Model RTserial No. 1001– 1201
Model B-2serial No. 35000– 40303
Model C-2serial No. 35001– 40459
Model RT-2serial No. 1300– 2150

In the above consoles, the original two-tooth wheels in the generator have been replaced with twelve two-tooth complex tone wheels, which supply a fundamental tone that is enriched with the odd-number harmonics. Both manuals and pedal switch are wired differently and are therefore not interchangeable with earlier models.

91 Frequency Generator with complex tone wheels and narrow cover

Model B-2serial No. 40304– and above
Model C-2serial No. 40460– and above
Model RT-2serial No. 2151– and above.

This generator has twelve complex tone wheels and is idential to the one above except for the generator cover. Because the output terminals of this cover are not in order of frequency (see Figure 6) this type of generator is not interchangeable with the one above.

Model M Tone Generator

The generator used in Spinet Models M and M-2 has 86 tone wheels and differs from other models in several other respects. The twelve complex-tone wheels are different in shape from those used in other models, and the generator-to-manual cable connects directly to the filter transformer terminals. For details, refer to Model M or M-2 service book.

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4 thoughts on “Servicing the Hammond Tone Generator”

  1. Good afternoon.–When my Hammond A-105 is all started up, I press the keys and get a ‘hum’ sound. BUT, NO ACTUAL NOTES & TONES.

    All volumes are working. Just, NO TONE.

    Any idea what it could be? Speakers, maybe?

    Thanks…call me if at all possible.


    1. Hi, I just bought an A-105 and it has the exact same problem you described. Did any ever find a solution to fix the hum? On mine, the more notes I hold down simultaneously the louder the hum is. For example if I play one note I get a soft hum; if I play two notes it’s a louder hum, and so on… Any ideas?
      MY email address is GPHIGLV at gmail {dot}com

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