Hammond Organ Technical Information
Volume Control and Pre-Amplifier Assembly

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5-38. Typical Circuit Before Pre-amplifier.

5-39. Each voltage of predetermined frequency produced by the tone generator is connected to one or more key contacts. When the associated playing key is depressed, this voltage is impressed upon the bus bar and is carried through the pre-set key switch to the preset panel. The voltage is then fed to one of the several taps of the mixing transformer which is associated with the manual being played. From the high impedance secondary of the mixing transformer, this voltage (combined with others which may be fed through simultaneously) passes to one of the preamplifier input circuits. (Vibrato "ON" or "OFF" circuit). 540. Power to operate the preamplifier and power amplifier is supplied through the run switch circuit as shown in figure 3-3.

54 1. Preamplifier Circuit, Input.

5-42. The signal from each mixing transformer is sent to the Vibrato "ON-OFF" tablet associated with its particular manual, and is then carried to the "VIBRATO" or "NO VIBRATO" preamplifier input, depending on the position of the tablet.

5-43. The input circuits are similar, with one extra stage of amplification in the "VIBRATO" channel to compensate for the loss that occurs through the phase shift network and associated scanner. The input tube V4 receives the signals from "VIBRATO" and "NO VIBRATO" circuits and further amplifies them. The signal then is impressed on the "LOUD" stator of the volume control, and on the "SOFT" stator through a compensating network.

5-44. Volume or Swell Control.

5-45. The volume control is activated by the swell pedal connected by an appropriate linkage. (See figure l-4.) The volume control assembly consists of two sets of stator plates, similar to those used in the scanner assembly. (See figure 5-6.) A rotor assembly of similar size is moved by the swell pedal and is capable of meshing with either stator or a portion of each. The degree of mesh determines the strength of the entire signal.

5-46. The signal is further amplified by the second section of V4 and sent to driver tube V3 which in turn drives the 12BH7 output tube.

5-47. PERCUSSION SYSTEM. (See figure 3-3.)

5-48. The "Touch Response" percussion feature is controlled by four tilting tablets (figure l-l ). It is avaiIable only on the upper manual and only when the "B" adjust key is depressed. Percussion tones are produced by borrowing the second or third harmonic (depending on position of the "Percussion Harmonic Selector" tablet) from the corresponding drawbar of the upper manual "B" adjust key group, amplifying it, returning part of it to the same drawbar, and conducting the balance through push-pull control tubes which, when keyed, cause the signal to fade away at a pre-determined rate.

5-49. With the percussion tablet on, "B" adjust key pressed, and an upper manual playing key pressed, the second or third harmonic signal goes to percussion input terminal H on the pre-amplifier chassis and is amplified by T4 and T5. The percussion input transformer TS not only provides push-pull signal for the control tube V7 but also has a third winding which feeds signal back to the 2nd or 3rd harmonic drawbar through equivalent key circuit resistor R50 and terminal "J". 5-50. When a key is depressed, the note first sounds loudly, after passing through the control tube V7, transformer T6, a high pass filter, and terminal D to the grid of V4. Immediately, capacitor C3 1 in the control tube grid circuit begins to discharge, causing the signal to fade away.

5-65. Remove the four screws from the left and right-hand side panels of the music rack. Tilt the bottom of the music rack by lifting the side panels, and remove the rack by pulling outward.

5-66. Remove the 4 chassis bolts (underneath the console) and the 2 machine screws (under the front lower manual rail) that hold the entire manual chassis in place.

5-67. Disconnect the 79 manual leads, 68 pedal leads, 4 ground wires, and the pedal filter leads. The pedal filter is located on the rear surface of the upper manual assembly.

5-68. Pull out all drawbars to position 8, and then tilt the manual chassis from the front as far as the top of the console will permit. Place suitable wedges or blocks on both sides of the manual chassis to hold it in this position. The manual chassis must be tilted to provide adequate clearance for the bolts in the corners of the main generator assembly.

5-69. Unhook the four suspension springs on which the generator assembly rides.

5-70. Lift up the generator assembly and remove it at the rear of the console.

5-71. Install a replacement generator assembly by reversing the procedure given above for removing it.

5-72. MIXING TRANSFORMER ASSEMBLY. The Assembly of two mixing transformers is provided complete with all leads to the preset panels, and can be removed as follows:
  1. Remove the rear panel
  2. Remove the two screws which secure the transformer cover in place.
  3. Label and disconnect all leads from the mixing transformers where they connect to the preset panels.
  4. Unsolder green and yellow shielded wires at mixing transformers.
  5. Remove the two wood screws which secure the mixing transformer assembly to the manual chassis block.
  6. Secure the replacement mixing transformer assembly in place by reversing the procedures given above.

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