5.3.2.8 Emergency Unload System
The Emergency Unload system, shown on Schematic No. 102810, sheet 1,
consists
of
an
Emergency Unload Relay (zone C9), a Relay Driver (zone E6), and an Emergency Unload
Charging
Network
(zone A9). The Emergency Unload Enable signal (LEUEG)
going
low at
J202 pin
32
turns
on 021 (zone E6), energizing
the
relay
coil.
The relay contacts are
connected in such a manner
that
when
the
relay
is
energized
the
power
amplifier
is
connected
to
the
positioner
coil. When relay is de-energized,
the
positioner
coil
is
connected
to
the
unload capacitor which in series
with
the parallel
combination
of
R129
and
R130.
J206
(zone A7) connects the unload capacitor
to
the
Servo PCBA. When
the
Emergency
Unload Relay is energized,
the
unload
capacitor
is
charged
to
-10v
with
R131
limiting
the
charging current. When
the
relay de-energizes,
the
positioner
coil
is connected
to-
the
unload capacitor and the
capacitor
discharges via
the
parallel
combination
of
R129 and
R130
and the
positioner
coil,
thus
retracting
the
carriage and heads
to
the
fully
retracted
position.
Capacitors C63, C64, C65, and C66 are provided across the contacts
of
the
Emergency
Unload Relay to suppress high frequency noise
(RFI) generated
while
switching
the
positioner
coil
to
either the Power
Amplifier
or
the
Unload Capacitor.
The Emergency Unload Relay Driver
consists
of
021 which is connected as a
transistor
switch
in series
with
the
relay
coil
and resistor R136. Diode CR30, across
the
relay coil, is
provided to protect
transistor
021 from large negative voltage surges when 021 is turned
on and
off.
In normal operation, LEUEG is
low,
emitter-base
junction
of
021 is
forward-biased,
turning
on
021.
If
LEUEG goes
high,
or
if
LEUEG is
low
and the +
5v
drops to approximately 3.8v, 021
turns
off,
de-energizing
the
relay, and retracting
the
carriage.
5.3.3
AC MOTOR CONTROL CIRCUITS
AC
Motor
Speed Control
circuits,
shown on Schematic No. 102810, sheet 1 (zone E3),
convert
logic
control
signals, namely Increase
Motor
Speed (NLlMS1), Brake Cycle Enable
(NLBCEG), and Drive
Motor
Enable (NLDMEG) at J202
pins
21,23,
and 20, respectively,
to
the appropriate
control
signal
at
TP22 which fires the triac
control
circuit
on the AC
Motor
Control PCBA. This
control
signal and + 5v
is
connected via J213 (zone
E2)
to
the
Motor
Control PCBA.
The
21
v ac (peak) 60 Hz
sine
wave (zone 05)
is
clamped by diodes CR34 and CR35 (zone
05), then converted
into
60 Hz square wave
digital
signal by comparator U21-A. The ac and
dc hysteresis in the comparator
circuit
is provided by
the
network formed by C15, R146,
R147, and R138.
Referring
to
the
21
v (peak) voltage at zone
05
of
the schematic, it can
be
seen
that
the
voltage is phase advanced by the network formed by C50,
C51
, C52,
R208
and R209, then
converted
into
a 60 Hz square wave
digital
signal at TP28
using
comparator U25-B (zone
C4).
The
digital
signal
outputs
of
comparators U21-A and U25-B
at
TP23 and TP28 are fed into
an
Inverse Exclusive
OR
circuit
formed by U20-C, U20-D, and U19-B, U19-C. The
output
signal at pin 8
of
U19-C is a 120 Hz
digital
signal which is used as the
switching
signal
for
transistor
switch
U26-A.
5-9
S30D
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