Broadcast Electronics AM-2.5E, User Manual

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1-10. PWM Drive Fault Detector. A sample of the PWM drive signal from integrated circuit U2
is applied to comparator U3A. U3A compares the signal to a reference. When the PWM
drive signal is present, the output of U3A will go HIGH. The HIGH is inverted at U5F.
U5F will output a LOW to illuminate PWM drive indicator DS1. When the PWM drive
signal is not present, the output of U3A will go LOW. The LOW is inverted at U5F. U5F
will output a HIGH to extinguish PWM drive indicator DS1.
1-11. Modulator Fault Detector. A sample of the modulator circuit output is applied to
comparator U3D. U3D compares the signal to a reference generated by a divider consist
ing of resistors R43 and R44. When the output signal from the modulator circuit is not
present, the output of U3D will go LOW. The LOW is applied to a modulator status cir-
cuit (refer to the following text).
1-12. B+ Supply Fuse Fault Detector. The B+ supply is protected from over-current conditions
on the modulator circuit board by fuse F1. The status of F1 is monitored by a B+ supply
fuse fault detector circuit. The circuit consists of optical couplers U4/U12 and comparator
U3B. When the fuse has blown, the output of couplers U4/U12 will go HIGH. The HIGH
is applied to comparator U3B. U3B compares the signal to a reference. The output of
U3B will go HIGH. The HIGH is inverted at U5C. U5C will output a LOW: 1) to illumi
nate fuse indicator DS2 and 2) to a modulator status circuit.
1-13. B+ Power Supply Fault Detector. The status of the B+ supply is monitored by a B+ power
supply fault detector circuit. The circuit consists of B+ power supply fault detector
comparator U3C. U3C compares the power supply sample to a reference. When the B+
power supply sample is present, the output of U3C will be HIGH. The HIGH is applied to
inverter U5E and NAND gate U6B of the modulator status circuit. U5E will output a
LOW to enable power supply indicator DS3. When the B+ power supply sample is not
present, the output of U3C will be LOW. The LOW is applied to inverter U5E and NAND
gate U6B of the modulator status circuit. U5E will output a HIGH to disable power sup-
ply indicator DS3.
1-14. Modulator Status Circuit. The modulator status circuit consists of: 1) NAND gates U6A,
U6B, U6C, and U6D, 2) latches U7A, U7B, and U7C, 3) inverters U5B, U5D and U5G,
and 4) optical couplers U8 and U9. When a LOW from the modulator fault detector or the
B+ supply fuse fault detector is applied to NAND gate U6A, U6A will output a HIGH to
U6B. With a HIGH from power supply fault detector U3C, U6B will output a LOW to
latches U7B, U7C, and U7A. Latch U7C will output a HIGH to inverter U5G. U5G will
output a LOW to enable power supply mute optical coupler U9. U9 will output a LOW
power supply mute command to the power supply circuit board. When the supply is
muted, comparator U3C will respond by routing a LOW to U5E. U5E will output a HIGH
to: 1) disable power supply indicator DS3 and 2) NAND gate U6C.
1-15. Latch U7B will output a HIGH to inverter U5D. U5D will output a LOW to: 1) illumi-
nate modulator fault indicator DS4, 2) enable modulator fault detector optical coupler U8,
and 3) NAND gate U6D. U6D will output a HIGH to latch U7D. Latch U7A will output a
HIGH to NAND gate U6C. With a HIGH from U5E, U6C will output a LOW to latch
U7D. U7D will output a LOW to U5B. U5B will respond by routing a HIGH to bias relay
K1 on. Relay K1 is provided to immediately disconnect the forward converter transistors
from the power supply during a modulator fault or high B+ supply conditions.
1-16.
POWER SUPPLY.
A +20 volt operating potential for the modulator circuit board is pro-
vided by the applicable power supply circuit board. The +20 volt supply is applied
through fuse F2 to +15 volt regulator U10. Fuse F2 protects the +20 volt supply from
over-current conditions.
1-17. U10 is a three-terminal adjustable positive regulator containing internal thermal over-
load protection and short-circuit current limiting features. Further protection for U10 is
provided by diodes D8 and D9. D8 protects the regulator from a short circuit on the regu
lator input. D9 protects the regulator from a reverse polarity potential applied to the out
put. Capacitor C25 provides filtering for the +15 volt supply. A sample of the +15 supply
is regulated into a +5 volt supply by zener diode D1.