Special Shutdown Circuit

TM 11-5825-271-34

the reference voltage at the junction of R18 and R19 by

except that comparator U2-C responds only to the output

comparator U1-D. The time constant of C9/R17 is such

from modulation detector and threshold comparator.

that during normal operation the voltage at pin 11 of U1-

During normal operation Q11 is saturated, and +24V is

D never exceeds the reference voltage at pin 10 of U1-D

available through P1-1, for the rf oscillator in the exciter

(Q6 discharges C9 on each positive transition for U1-A,

assembly A1A2.

thus preventing C9 from being charged continuously),

NOTE

and the output from comparator U1-D (pin 13) is low.

The +24V for the rf oscillator in the exciter

Transistors Q7 and Q8 are therefore switched on and

assembly A1A2 is routed through the normal

provide indication of normal operation as follows

link or special link (fig. FO-1) as determined

(a) Shutdown relay K1 is energized by Q7 and

at the time of installation. When normal link

the +24V from power supply AlA1 is applied, through

is installed (special link removed), the + 24V

contacts 4-7 of K1 of the rf oscillator in the exciter

output from the special shutdown circuit (P1-

assembly A1A2 (fig. FO-1).

1, fig FO-10) is not used. When special link

(b) ALARM indicator DS2 is turned off.

is installed (normal link removed), the output

from the normal shutdown control circuit still

(3) If the output from U1-C is low (carrier too

controls the shutdown relay K1, but the +24V

low) or the output from U1-1 does not change state (loss

for the rf oscillator in the exciter assembly

of keying), Q6 assumes a steady off state. Thus, C9 is

A1A2 is taken from P1-1 (fig FO-10).

charged continuously through R17 and, after a delay of

e. Modulation Detector. This circuit is used to

20-35 seconds, pm 11 of U1-D becomes more positive

provide a reading of the modulation depth on the control

than the reference voltage at pin 10 of U1-D. Under this

panel TEST meter. The input at P1-13, obtained from

condition, the output of U1-D changes state from low to

the power probe A1A6, is applied to comparator U2-A

high, and transistors Q7/Q8 are both switched off,

which operates as a minimum voltage detector, i.e., the

resulting in the following alarm condition.

output of U2-A assumes a steady potential equal to the

(a) Shutdown relay K1 is deenergized and the

minimum positive excursion of the input signal. The time

+24V for the rf oscillator in the exciter assembly A1A2 is

constant of U2-A is chosen so that voltage at pin 2 of U2-

disconnected (fig.

FO-1); the NDB transmitter is

A follows the keying signal. When the NDB transmitter is

effectively switched off (no rf output).

operating without modulation, the output from U2-A is a

(b) ALARM indicator DS2 is turned on.

voltage proportional to the carrier level.

When

modulation is keyed on, the level falls abruptly, reaching

(4) NORM/BYP switch S1 applies a ground

zero for 100% modulation Comparator U2-B is used as a

(low) to either P1-2 or P1-3.

During testing or

voltage-follower, providing a high input impedance and a

troubleshooting, S1 is set to BYP position. Thus, Q7 is

low output impedance The output from U2-B is

held conducting regardless of the output level of U1-D,

connected to the voltage divider formed by set zero

ensuring that the system cannot shutdown. Transistor

potentiometer R1.

The wiper of ZERO SET

Q8, however, is still controlled by the output of U1-D. At

potentiometer R1 is connected, through the TEST meter

the same time, the BYP setting of S1 removes the

M1 and MOD % CAL potentiometer R4, to a fixed dc

ground from P1-3, effectively removing C9 from the

potential. The circuit forms a bridge, balanced by

collector of Q6 and eliminating the time delay formed by

potentiometers R4 and R1. When modulation is off,

R17/C9. Instantaneous indication of the transmitter

potentiometer R1 is adjusted for zero reading on the

status is, therefore, provided at P1-5, as Q8/DS1 are

TEST meter M1. When 100% modulation is present or

switched on and off at the keying rate.

is simulated by removing the rf input at P1-13,

d. Special Shutdown Circuit. This circuit is formed

potentiometer R4 is adjusted to read full scale deflection

by U2-C, Q9 through Q11 and associated components

of TEST meter M1

The operation of the special shutdown circuit is similar to

the normal shutdown control circuit described in c above,

2-10