8300
The output
of
amplifier
U8
(TP2)
(zone
C6)
also drives a multiplexer digital-to-analog
converter. This multiplexer digital-to-analog converter consists
of
FETs
04,
OS,
06
(zone
OS)
and
amplifier
U7
and
U9
as
weU
as the associated computing resistors
R4,
RS,
R6,
R21, R22,
R23,
R24,
and
R25.
The switched current resistors
(R4,
R5,
R6)
receive their
commands from the current address counter
on
the logic card via
JS03
(zone 0, E-8).
The
three most significant bits are employed to provide off-track compensation and
are
comprised
of
LCAE1,
LCA71
,
LCAS1,
in order
of
declining bit weight. These signals are
buffered by
U1
and
U2
and applied to transistors 01,
02,
and
03.
These transistors
translate their input
levels to output levels which
are
compatible with the FET switches
04,
OS,
as. The outputs
of
the multiplexer digital-to-analog converters are applied to the
servo summing junction (pin 2 of U7), then through
U7
to
R26
and pin 1 of
JS04.
In addition to the temperature compensation electronics, the Temperature Compensation
PC8A contains a
time
delay circuit. This circuit consists
of
a monolithic timer,
US
(zone
8S), a 4-bit binary counter,
U3
(zone 85), and associated components. Transistor
07
resets the timer. When
07
is turned off, the timer will free run, generating one pulse per
24
seconds
as
determined by
R27,
R28, R29,
and
C3.
The binary counter is clocked
by
each
pulse and counts thirteen
of
these pulses before
an
output is delivered at
JS02-9
(zone A3).
This S.2-minute timer circuit allows the internal temperature of the disk drive to near
stabilization before a Temperature
GO
signal is generated at the interface. The timer
circuit operates after the completion of the start-up sequence, or
57
seconds after the
RUN/STOP switch is depressed.
(R38
is used for reducing testing time only.)
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