Moog MICROMOOG Technical & Service Manual page 10

Buss
833 A (I,)
HC
833A—— Ty
FIGURE 2-2 KEYBOARD CONTROL VOLTAGE GENERATION
When a key contact closes, holding capacitor
©212 charges (or discharges) at the rate set by R261
to the level established on the buss. R260 sets a
minimum charging time constant of 'TCH (min) = 0,22
HE x 1.5K = 0,83 msec, R250 decouples the 25 kHz
pilot oscillator signal (see below) from the DC control
voltage on C212. R250 also filters key contact scrape
and bounce that occurs when a lower contact closes
while an upper contact is being held closed, When all
keys are released, no further charging of C212 is
possible and it holds the last instantaneous value of
control voltage. Dual J-FET Q201 in conjunction with
R252, R263 and 1C204A comprise an extremely low
input bias current unity gain buffer. The buffer
provides substantial control voltage output drive
capability while draining C212 at less than a 200
pA rate.
1 _ 200 pA ~ T5cents
MAXIMUM DISCHARGE RATS = 5 = OBR = OTT
NOTE
'The output side of the buffer guards the
input side. This vastly reduces on-board
leakage since a potential equal to the value
being held surrounds the entire holding
circuit.
213 and C214 decouple RFI pickup by the
main PC Board, C214 also shunts capacitive audio
pickup by the keyboard control voltage output line.
CR203 and CR204 protect Q201 and 1C204A from
static discharge coming in via J8, R257 does not affect
the output level normally (it is inside the loop) but it
does allow control voltage to be forced into J3 for
testing purposes, or for introducing external control
voltages to override the keyboard control.
2.2.2 AC OPERATION (TRIGGER GENERATION)
Since the keyboard uses a single-buss contact
scheme, an AC pilot signal (generated by 10201)
must be injected onto the buss so that detection of
contact opening and closing can occur, When closure
occurs, 16203 and 1C202B amplify and detect the
signal and develop a clean DO trigger output.
Oscillator 10201 generates the 25 kHz + 5 kHz.
triangular AC signal. 25 kHz lies above the audio
range which reduces the chance that unwanted signal
pickup by other Micromoog circuitry will be audible,
The 3080 transconductance amplifier was chosen
8 10201 because it has very fast slew rate, does not
delay coming out of input or output saturation and
does not draw current slugs from the supply lines
while slewing or while saturated, The output of a
3080 is a bilateral current source the magnitude of
which is given by:
0.02 pA/mv (input)
pour nA (Bias)
5 TOUT (Max.) = [BIAS
R235 sets IBIAS (into pin 5) to 0.5 ma which
keeps the total package dissipation at 30 mW, well
below the absolute maximum of 125 mW.Iour will
tend to alternate between + 0.5 mA and - 0.5 mA,
2.3