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In MFM generates a transition on the recording
medium.
3-100. Write Current Source.
The write current
source provides current for the read/write heads
and supplies a write control (WC) signal to the read
preamplifier/write driver IC's. The write current
source is enabled by the Write Gate line from the
microprocessor and has two outputs, selected by the
Inner Track line. For tracks 0 to 511, the write
current output is 27.5 milliamperes and for tracks
5 1 2 and above, the write current is 22.5 milliam-
peres.
3-101.
Writ4~
Fault Detector.
During a write
operation, the microprocessor monitors certain key
parameters of the operation via the write fault
detector. If an abnormal condition is detected, the
microprocessor activates, via the PLA, Write Fault
signal WFLT-L. Inputs to the write fault detector
include
Unsafe
signal
US
from
the
write
driver/read preamplifier IC, and lines from the
write current source and the head select logic. The
faults which can cause WFLT-L to become active
are listed in table 3-1 under the description for
signal WFL T .-L.
3-102. SELF TEST
Disc drive assembly A 1 performs a self test of cer-
tain key hardware functions at power-on. These
functions include head loading and seeking to
track O. Following successful completion of the
tests, Ready RDY-L and Seek Complete SKCMP-L
are set true, allowing A 1 to respond to DOC com-
mands via the ST- 506 Interface. During normal
operation of the disc drive, the microprocessor
monitors spindle speed and off -track conditions. If
either parameter exceeds predetermined limits, the
microprocessor shuts down operation via the RDY-
Land SKCM1P-L lines.
3-103. POWER SUPPLY A4
Note: Power supply assembly A4, part no.
07942-60024, used in the drive is a
higher wattage unit than power
supply
assembly
A4,
part
no.
07940-60094, described
in
the
Tape Drive Service Manual. Power
supply
assembly
A4,
part
no.
07942-60024, is described in the
following paragraphs.
Theory of Operation
7942 and 7946
Power supply assembly A4 develops dc operating
voltages from the ac line voltage and distributes
these voltages throughout the drive.
The power
supply assembly also generates a power-on reset
signal.
Power supply assembly A4 is a self -contained
switch -mode power supply mounted on a printed
circuit assembly. Located in the assembly are all
of the ac line voltage components including the
line cord connector, line fuse, line voltage selector
switch, line on/off switch, and line filter. Output
voltages are
+
5 V dc,
+
1 2 V dc, and - 1 2 V dc. The
power supply output voltages and power-on reset
signal can be measured attest poin ts a t the f ron t
of PCA-A4. See figure 4-7. The output voltages
are not adjustable. The power supply voltages are
connected via cable assembly WI to host dependent
controller PCA-A5, both device dependent con-
trollers PCA UO-A2 and PCA U 1-A2, disc drive
assembly UO-A1, and tape drive assembly U1-A!.
The following paragraphs provide a more detailed
description of the power supply A4 circuitry.
Refer to figure 4- 5 for detailed voltage and signal
distribution information.
3-104. AC INPUT CIRCUITS
The ac line voltage is connected to power supply
assembly A4 through a printed circuit assembly
(PCA) mounted - AC LINE connector.
A PCA-
mounted ac line on/off switch controls both sides
of the ac line into the power supply. The switch is
operated
by a
LINE- pushbutton
projecting
through an opening on the front panel of the drive.
There is a fuse in the line side of the ac input fol-
lowing the power switch. The fuse value is the
same (SA, 250V) for both 115-Vac and 230-Vac
inputs. A line filter following the fuse reduces the
level of line transients entering the power supply
and the amount of switching noise leaving the
power supply.
Also associated with the input circuitry is a
VOLT AGE SELECTOR switch which selects line
voltages of 115 Vac or 230 Vac. When the switch
is in the 115 Vac position, a surge voltage protec-
tion device protects the power supply from damage
if it is inadvertently connected to 230 Vac.
3-21
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