Service manual

developed across current sense resistor R2 limits the
current.
The negative power supply voltage regulator
consists of IC2, Ql and associated components and
adjusts its output to have the same magnitude as the
regulated +9 volt output. No current limiting, other
than that supplied by R8, is provided.
2.3 KEYBOARD CIRCUIT
The keyboard circuit consists of IC3 thru IC7,
IC9, IC10 and related circuitry. The keyboard
contains a string of thirty-six 100-ohm resistors
connected between pins A5 and A6. The current
through the resistor string is regulated by IC7 so that
the drop across R79 and R80 is exactly 4.5 volts.
R79 is set so that the voltage at pin A6 is exactly
-4.5 volts.This sets a scale factor of 3 volts per
octave (250 mv per semitone).
2.3.1 TRIGGERING (SINGLE)
The voltage at the keyboard buss is applied to
voltage follower IC4. The keyboard buss voltage
rises to approximately 7 volts (detail A, Figure 2-1)
when no key is depressed because of R53. The output
of voltage follower IC4 is applied to comparator IC5
whose output swings from -16 to +7 volts whenever
the input goes below +4.8 volts (detail B). Q5 and Q6
comprise a monostable multivibrator producing a
pulse of approximately 20 milliseconds duration
(detail C). When the output of IC5 swings positive, a
positive spike is applied through C7 and D7 to the
base of Q6, initiating a 20 millisecond pulse. R63,
R73 and R72 are proportioned so that Q7 conducts
only when the output of IC5 is positive and the output
of the monostable multivibrator (the collector of Q5)
is negative. That is, Q7 begins to conduct approxi
mately 20 milliseconds after a key is depressed and
stops conducting as soon as all keys are released.
When Q7 conducts, Q8 is turned on and the voltage
at its collector rises from 0 to +9 volts. When this
happens, C13 discharges through R61 producing a
ramp voltage at the base of Q4 that decreases from
+9 volts to -0.6 volt in approximately 20 milliseconds
(detail E). Emitter follower Q4 supplies a current
through R62 and Q3 to turn on IC10. IC10 and Q51
with associated circuitry comprise a sample and hold
circuit. When the current ramp is applied to pin 5 of
IC10, the voltage at the source of Q51 rapidly
approaches that at the output of IC4. As soon as the
base of Q4 drops below 0.8 volt, the bias current
being applied to IC10 through Q3 drops to zero and
the voltage at the source of Q51 remains constant.
As long as the output of IC5 remains positive (that is,
as long as any key is depressed), a very small bias
current of approximately 50 nanoamperes flows
through R59 allowing IC10 to supply a small current
to C5 keeping its voltage constant. As soon as all
keys are released, the output of IC5 goes negative
and IC10 is virtually completely shut off. Thus,
when only one key at a time is depressed, the
voltage at the source of Q51 begins to approach the
new key voltage approximately 20 milliseconds after
the key is depressed and this voltage is equal to the new
key voltage well before the ramp current turning IC10
on goes to zero. As long as a key is depressed, the
correct voltage at the source of Q51 is maintained by
the small trickle current going through R59. When
the key is released, the trigger output at the
collector of Q8 drops to zero, and the sample and
hold circuit no longer samples the keyboard voltage.
The 20 millisecond delay supplied by Q5 and Q6 is
necessary to bypass the effect of contact bounce
during key depression.
2.3.2 TRIGGERING {MULTIPLE)
IC6 becomes important when two keys are de
pressed. Any abrupt change in voltage at the output
of IC4 is applied through R66 and C10 to the input
of IC6. Cll filters out spikes less than 1 millisecond
that are associated with contact bounce or spurious
interference. The resulting rounded pulse is amplified
by IC6 (detail G, Figure 2-1). Whenever the keyboard
buss voltage increases, the output of IC6 goes
positive, D9 conducts and fires Q6 producing a 20
millisecond positive going pulse at the collector of
Q5. While this 20 millisecond pulse is on, the trigger
voltage at the collector of Q8 drops to zero. Also,
the collector of Q50 drops to zero. This causes C13
to recharge to +8.8 volts at the leading edge of the 20
millisecond pulse and resets the contour generators
which are described later. Thus, when a key is held
down and a higher key is depressed, the keyboard
sample and hold circuit again samples and the trigger
is reset momentarily. The same happens when a
higher key is released while a lower key is being held
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