Similarly, if the +
20v
goes below approximately + 13v nominal,
034
turns
off
and the
cathode of
CR48
goes
to
ground potential. This, in turn, causes
037
to
turn
off
and the
sequence of events that occurs is identical
to
+
Sv
drop-off previously described. When
the
+
20v
is approximately +
16v
or
higher,
034
turns on and the cathode
of
CR48 is at
approximately 5.4v (if the
+
20v
line is at + 20v) which reverse-biases CR48, and the base
of
037
gets charged
to
2.7v again
to
turn on
037.
Similarly, if the
-20v
is maintained at its nominal potential,
040
is on and the base
of
039
is
at approximately
-2v
which turns
off
039.
The cathode
of
CR48 is at + 5.4v, which
reverse-biases CR48 and hence a normal charging sequence
of
C27
occurs
until
037
turns
on. When the
-20v
goes above
-13v,
040
turns
off
and
039
turns on. CR48 cathode goes
to
ground potential and the sequence of events that occur is identical
to
+
Sv
drop-off
previously described.
It should also
be
noted that resistors
R188
and
R190
provide hysteresis in the
circuit
so
that the voltage
(+
5v,
+
20v,
or
-20v)
at which SPCSA goes high is always
slightly
higher
than when it goes low.
5.4
READIWRITE
C
PCBA
The
following
paragraphs describe the Read/Write C PCBA installed in 03000 Series Disk
Drives. Refer
to
Schematic
No.1
03750 and Assembly
No.1
03751
.
The PCBA is approximately 247.7
mm
(9.75 inches) long by
16S.1
mm (6.5 inches) wide.
Figure 5-5
illustrates the placement
of
each connector,
test
pOint, and adjustable
component on the PCBA.
J30S
connects via a mating plug and 3M flat cable
to
the Logic
PCBA. J304 is a molex connector which connects via a mating plug and standard cabling
to
the Logic PCBA. J300, J301, J302, and J303 connect via shielded cabling
to
the
magnetic
Read
/Write
heads.
The Read/Write
PCBA description is divided
into
the
following
elements.
(1)
Head Selection Matrix and Select SWitches
(2) Write and Erase Driver
(3)
Read Switch
(4)
Read Preamplifier
(5)
Filter
(6)
Variable Gain
Amplifier
(7) Peak Detector, Squarer, and Pulse Former
(8)
Data Decoder
(9)
Emergency Condition Detection
A block diagram
of
the Read/Write electronics is shown in Figure 4-4 and should be
referred
to
in conjunction
with
Schematic No. 103750 and the electrical description
described in the
following
paragraphs.
5.4.1 HEAD
SELECTION MATRIX AND SELECT AMPLIFIER
Head selection
is
accomplished using a conventional diode matrix in conjunction with
center-tapped heads. Each head is comprised
of
a balanced Read/Write
(R/W)
center-tapped winding and a separate erase winding which has one end connected
to
the
R/W
center tap. Referring
to
Schematic No. 103750, zone E14, 13, and 12,
four
heads are
shown connected
to
J300, J301, J302, and J303. Three diodes are associated with each
head:
two
diodes (CR28 and CR30
for
the J300 head) connected
to
the balanced
R/W
bus
and the third diode
(CR29) is connected
to
the erase bus. .
...
5-13
630D
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