NARROW BAND PRE-AMPLIFIER

TM 11-6625-524-14-4

c. The meter should indicate between 30% and 70% of full scale. If the reading

fails to reach 30% or exceeds 70% of full scale, the following procedures may be

used to check the applicable stages and components.

d. Readjust amplifier board control (3R53) for a meter reading of 50% of full

scale.

e . Visually check the meter coil for a sticky movement and/or continuity. If

d e f e c t i v e , replace.

f . Using the DC multimeter set to DC volts, measure the DC voltages at all

applicable transistor stages and compare values to those as shown on schematic

diagram, figure F0-1.

9. The synchronous demodulator's transistors, 3Q7 and 3Q8, may be checked by

substitution. Transformers 3T1 and 3T2, if suspected of being faulty, should be

c a r e f u l l y unsoldered and removed from the circuit board. Their winding continuity

should be checked by using the DC multimeter set to ohms.

h. Disconnect 3TP10 from 3TP8 and reconnect link wire 3W3 from 3TP8 and 3TP9.

Disconnect 3TP3 from 3TP13.

i . Disconnect multimeter.

5-8

NARROW BAND PRE-AMPLIFIER

a. With the voltmeter properly shielded (see paragraph 5-2) switch to the 0.001V

f u l l scale range position. Connect the equipment as shown in figure 5-1. Adjust the

oscillator frequency to 200KHz and the output level to 1mV as monitored by the AC

voltmeter.

b. Locate on the amplifier board, link 3W3. Unsolder one (1) end of 3W3 from

3TP8 .

c. Connect the oscilloscope to 3TP8. Disconnect oscillator from probe. Adjust

the SET REF control for minimum amplitude reading on the oscilloscope; the wave-

shape should agree with the value as shown on the schematic diagram, figure F0-1.

d. Reconnect the oscillator to the probe. With 1mV applied to the probe,

measure the waveshape. It should agree with the value as shown on the schematic.

I m p r o p e r shielding or zeroing may result in large errors.

e. I f this check reveals a faulty condition, the following procedure may be

used to check each pre-amplifier stage.

f . Using the DC multimeter set to DC volts, measure the DC voltages at each

stage's transistor (3Q10, 3Q11 and 3Q12) and compare to those as shown on the

schematic diagram, figure F0-1.

g. With the 1mV signal applied, measure with the oscilloscope the gain of each

stage. A typical gain figure is 10:1. Should the gain check reveal a defective

t r a n s i s t o r , verify its condition by substituting with a known good one.

h. Check the forward and reverse resistances of clipping diode (3DR5). Use the

DC multimeter set to ohms.

i . Reconnect 3W3 to 3TP8.

j . Disconnect oscilloscope and DC multimeter.

5-9

DC MODULATOR

a. With the voltmeter properly shielded (see Paragraph 5-2), verify the RANGE

s w i t c h is set to the 0.001V full scale range position.

b. Verify the DC modulator driver is functioning (see paragraph 5-6).

c. To check the modulator's chopping action, deliberately feed a 30mV, 200KHz

s i g n a l to the probe. With the oscilloscope set to a low level sensitivity, observe

the waveshape at 3Q9's gate. If a square-wave is not produced or fails to diminish

when the signal source is de-energized, t h e modulator is probably faulty. If

f a u l t y , replace.

d. D i s c o n n e c t oscilloscope.

5-4