Kurzweil K2000 - K2000 Audio;Power Board Theory - Part 2; Front Panel, Wheels, and Pressure Sensor Theory

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SECTION 5 AUDPWRl. DOC 12/22/92
5.1.1. 7
PEDALS INTERFACES
J19 (A-1) receives variable inputs from a "control" pedal and can be used to control timbre,
pitch, etc. The circuit is optimized for a 10K linear taper potentiometer in the pedal, but will
work with higher resistances. Alternatively a control voltage of 0 to +5 volts can be input di-
rectly to J19 to produce the same effect. For use with a pot type pedal, R197 and R199 (B-1)
supply a current-limited reference voltage to the pedal. This voltage is equal to the +5ref when
the pot's resistance is 10K; thus there is no voltage drop across the limiting resistance.
Limiting resistance is necessary to prevent circuit damage or malfunction is a mono plug is
inserted into J19.
R195 pulls the pedal voltage input to zero when there is no plug in J19. R196 and 018 (C-1)
protect the AID converter from overvo1tage and C184 filters out noise.
J16 (I-1) and J17 (I-2) receive footswitch plugs and look for switch closures to ground. RP7-10
(G-2) and RP8-10 (F-2) are pull-up resistors and keep the M37450 inputs high when no pedals are
plugged in or the pedal contacts are open. RFI suppression is accomplished in the circuits of
R188, R189, C180 and C181 (H-1 to H-2). At power-up, the M37450 looks at the status of J16 and
J17 and assumes that state to be "OFF"; the opposite state being "ON". This allows for use with
NO or NC pedal switches.
5.1.1.8 LCD Vee VOLTAGE GENERATOR
The LCD on the front panel requires an adjustable negative voltage source to control the
contrast. This voltage must not be applied until after the LCD has been initialized. The LCD
Vee circuit (F-7 to H-7) performs these functions.
The pulse-width modulated output from the M37450 is used as a third D/A output to control the Vee
voltage. When filtered by R146 and C196, the duty cycle of the PWM waveform is converted into a
voltage between 0 and +5 volts. U59-14 converts this to a -12V to +12V swing. Note that the
center voltage from the Wheels Interface (+2.5V) is needed to do this properly. If the Wheels
Interface is not plugged in, the LCD contrast will be wrong. 020 and R154 prevent the Vee
voltage reaching the LCD from exceeding +5 volts. Vee will be affected by changes in the +5 volt
digital supply. If it varies by more than 0.1 volt high or low, the LCD contrast adjustment
range may not suffice to compensate.
Q20 and R152 are connected so that the Vee voltage is forced to the positive rail when RESET is
active, thus ensuring the display is inoperative when the processors are not running. Approxi-
mately one second after RESET goes inactive, software will set a default level for Vee, about -
8.5 volts. This voltage may be changed by a message from the Engine Board to set to a user
stored value, if so programmed.
5.1.1.9
AUDIO PATH CONTROL OUTPUTS
The audio path requires 6 digital signals to control switches and 2 analog signals to control
variable gain amplifiers. The digital signals are noise-isolated from the analog circuitry by
R79-80 and R136-l39 (F-6).
5.1.1.10
OPTION CONNECTOR
The 8-pin option connector (E-l) is included to utilize unused resources of the M37450 and to
provide for additional user control features in the future. Power (+12 and +5 volts), two analog
inputs, and one digital I/O line are provided.
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