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oscillator. The power switch on the front panel acts only to enable the
unit's power supply regulators.
Power Supply Regulators (p. 12/14)
The 5 volt supply is regulated by pass transistor Q902. The base of
this transistor is controlled by U907 to maintain the output at 5 volts.
U907A compares the output (emitter) side of Q902 with the output of
U905, which serves as a 5 volt reference. Because the current gain of
Q902 remains high until the collector-emitter voltage drops to about .4
volts, the regulator exhibits a very low drop out voltage. The current
drawn by the +5 volt supply is sensed by the voltage drop across R917,
an 0.1 ohm resistor. When the current drawn exceeds about 1 amp,
comparator U909A turns on, pulling the reference input to U907A below
ground and turning off the regulator's output. The 5 V power required
by the unit is provided by three-terminal regulator U908.
Regulators U901 and U902 p15 and -15 volt regulated outputs.
The output of these regulators drops to about 1.25 volts when the front
panel power switch SW901 is opened. This also turns off the +5 V
supply, as the drive and reference for +5 V is obtained from the +15
volt supply. Regulators U903 and U904 are always on which provides
±15 volts to the timebase.
In addition to the power supply voltages, there are also two status bits
generated. - DROPOUT is asserted if the +5 V supply falls below 4.6
V, or if the +15 V supply falls below 11.5 volts. The - RESET line is
asserted if the +5 V supply is below 4.4 V, or has been in the previous 1
second.
Active Antenna (p. 14/14)
The active antenna electronics are located at the base of the antenna,
and provide filtering as well as drive for the antenna lead-in cable. L1,
L3, L4, and C1-C3 form a 3rd order Bessel bandpass filter for the 100
kHz LORAN-C signal. R1 and R2 provide a manually jumper-selectable
30 dB attenuator for use where strong signal levels might saturate the
FET (Q1). Neon bulb LP1 protects Q1 from high-voltage discharges.
Q1 drives the coaxial lead-in cable, and DC power for the FET stage is
provided on the same coax.
