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Al7
Al7
Positive
Regulator,
Circuit
Description
Al?
Positive Regulator provides
+20
V,
+12
V, and
+5.2
V regulated voltages to the
instrument. It also provides for thermal shutdown at temperatures above 85°C or if the fan
fails, controls power to the oscillator in the A22 10 MHz Frequency Standard and ensures
power-up and power-down operation that will not invalidate data in Al5 Controller. The
relationship of Al7 to A8 Rectifier, A23 Motherboard, and Al8 Negative Regulator is shown
on the A8 Rectifier schematic.
Front-Panel
Shutdown
@
When the front-panel LINE switch is in STANDBY, the LSBY line is grounded, fan relay
Kl
on the A23 Motherboard is energized turning the fan off, and the STANDBY LED is turned
on.
Pin 9 of U3C is also at ground when LSBY is grounded so the output of U3C is at i-10 V,
turning on
Qll
through R41.
Qll
sinks all of the current supplied by the compound current
source
Ql
and Q2, which turns off the i-20 V supply series-pass transistor A23Q3, located on
the A23 Motherboard. All other supplies are referenced to the output of the $20 V Regulator,
so they are also disabled. (The
+22
V supply is an exception. It is always on as long as the
instrument has ac power from the line.)
When the front-panel LINE switch is switched to the ON position, the LSBY line is no longer
grounded, the fan relay
Kl
is no longer energized, and the fan turns on. LSBY rises to about
+22
V through the fan relay coil. This causes the output of U3C to go low, turning off
Qll.
The current source of
Ql
and
Q2
can now supply current to A23Q3 on the A23 Motherboard,
and the
-j-20
V supply will turn on.
Thermal
Shutdown
@
U3D senses the voltage across thermistor
RTl.
The resistance of
RTl
is nominally 1000 ohms
at 25°C and drops to 107 ohms at 85°C. When the resistance drops, the output of U3D rises,
turning on the thermal shutdown LED DS3 and also Q12, which disables the current source in
the
+20
V supply, shutting the supply off. CR5 and R52 provide hysteresis so the instrument
will not turn on again until the temperature drops to 55°C.
Al7 1