Hammond Model A, Manual, Page: 148 / 298

WIRING DIAGRAMS
In studying the operation of the pedal solo unit, refer first to the block diagram
(figure 1) and second to the more detailed schematic circuit (figure 2, 2B or 2C).
The schematic diagram of the console, apart from the pedal solo unit, is the same as
for the Model CV, C-2, or C-3 console, shown in Section 2. Actual connections
between the pedal solo unit and other parts of the console are shown in the section
containing the console wiring diagrams.
The Oscillator
The 32 coils which tune the audio frequency oscillator are shown in figure 2. When
the lowest C note is played (this pedal has no tuning contact), all 32 coils are
connected in series to form the tuning inductance of the oscillator. When any other
pedal is depressed, its tuning contact shorts out some of these coils (making less
total inductance) and thus tunes the oscillator to the higher pitch associated with
that note. If two pedals are depressed at the same time only the higher pitched of
the two will sound.
Frequency Dividers
Each divider includes three triodes. One acts as a driver and pulse rectifier,
supplying sharp and narrow negative pulses to actuate a symmetrical feed-back
tripping circuit comprising two triodes. Either one (but only one) of these two
triodes can be conducting at a time, for by drawing plate current it holds the other
in a cut-off condition.
Suppose, for example, that the first triode is conducting and the second is cut off.
Now a negative input pulse impressed on the grids of both triodes will not affect
the second one, which is already cut off, but will cut off the first. This produces
a positive pulse at the plate of the first triode, which is applied to the grid of
the second triode through its feedback connection. The second triode then suddenly
conducts current, producing a negative pulse at its plate. This negative pulse,
applied to the first triode grid through its feed-back connection, insures that the
first triode remains cut off. The situation is now exactly reversed, with the first
triode cut off and the second conducting.
The next input pulse will act on the second triode, cutting it off again and making
the first conductive; and thus two input cycles are required to produce one output
cycle. Each frequency divider circuit therefore divides its input frequency in half,
producing an output signal one octave lower than the preceding divider. One triode
plate of each divider stage furnishes a signal of rectangular wave shape to the
following driver tube, and output signals are taken from the driver and divider
plates as indicated in figures 2, 2-B and 2C.
This divider circuit is capable of operating satisfactorily with wide variations in
voltage, input frequency, and values of components, and therefore is remarkably
stable and requires no adjustments.
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