Isolating Troubles

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under which the circuit failed. Paragraph c below is

ohmmeter. If the resistor is good, continue tracing

the signal path through the audio amplifier/detector

an example of how a trouble could be isolated in a

faulty audio amplifier/detector circuit.

to the TEST METER. If no 1 kHz voltage appears at

the sidetone output, another possible source of trou-

c. Assume that the trouble cited in paragraph

ble is the sidetone gate input. Table 2-4 shows that

3-6b was localized to the audio amplifier/detector

switch S4C connects a positive 10 volts dc through

circuit card. Using the extender card, extend audio

amplifier/detector circuit card HU7 above the card

S4D to the sidetone gate input of the module. Using

cage. Plug a transmit audio module known to be

a multimeter, check for +10 volts at J3-5. If the

voltage is not present, trace it back through S4C

g o o d into J3 and set the CIRCUIT SELECTOR

switches to 3673. Refer to the schematic diagram for

and S4D. The positive 10 volts originates in the

t h e audio amplifier/detector circuit (fig. 3-36).

power supply; refer to figure FO-2 to isolate

Using the oscilloscope, check for the presence of a 1

troubles in the power supply.

kHz signal at pin 17 (test point J2) on the circuit

card and note the amplitude of the signal. Next

NOTE

check for a 1-kHz signal at the emitter of Q1. Tran-

The audio amplifier/detector amplifies and

s i s t o r Q1 is an emitter follower; therefore, the

rectifies the 1 kHz signal. The output is,

amplitude of the signal at the emitter should be just

therefore, a dc voltage that is proportional

slightly less than the input. Transistor Q2 is an

in amplitude to the amplitude of the 1 kHz

amplifier. Check the waveform at the collector of Q2

input signal.

with the oscilloscope. The signal amplitude should

c. Use the same techniques as in b above to local-

be several times greater than that measured at the

ize any trouble that occurs during the performance

emitter of Q1. Transistor Q3 and Q4 are also ampli-

of the test procedures.

fiers. Check for a 1-kHz signal at the collectors of

each transistor and note that the signal amplitude is

3-5. Isolating Troubles

greater than at the output of the preceding stage.

a. Once a trouble has been localized to a func-

Diode CR1 is the detector diode. Measure the dc

tional circuit in the test set, further tests and meas-

level present at the cathode of CR1. The dc voltage

urements must be made to isolate the trouble to a

should be approximately equal to the peak value of

component part. The most useful tool that a techni-

the signal voltage at the collector of Q4. Resistor

cian has for troubleshooting any circuit is his com-

R24, R25, and R30 form an output voltage divider.

plete understanding of the circuit operation. When a

The dc output at pin 8 should be approximately one-

trouble has been localized to a functional circuit,

sixth of the value of the dc voltage at the cathode of

refer to chapter 2 and review the principles of oper-

CR1. When a signal level at the output of a stage

ation of the circuit. Chapter 2 contains simplified

appears to be low or if no signal is present, refer to

schematic diagrams of each functional circuit. These

table 3-2, voltage and resistance readings and check

schematic diagrams do not contain the components,

the dc voltages and resistances on the emitter, base,

switches, and circuits that are not directly involved

and collector of the suspected stage. If the tran-

with the circuit operation. These circuits are more

sistor appears to be faulty, check it with the tran-

easily understood than those on the overall sche-

sistor tester. Refer to paragraphs 3-9 and 3-10 for

m a t i c diagrams (fig. 3-32 through 3-44). It is

replacement of parts on circuit cards.

necessary, however, to refer to the overall sche-

d. Use the techniques in c above to isolate all

matic diagrams for pin numbers on switches and cir-

faults in the test set. For faults in the power supply

cuit cards. Usually a functional circuit, such as the

circuit, refer to the simplified power supply sche-

audio amplifier/detector, is completely contained on

matic diagram (fig. FO-2).

a single circuit card. Isolating a trouble on a single

3-6. Parts Location

circuit card is relatively easy, because all the com-

ponents are easily accessible and are close to each

When troubleshooting the test set, the exact loca-

other. A few circuits, such as the 500-kHz bandpass

tion of a component part may not be apparent. Fig-

filter and the +28-volt power supply, have com-

ures 3-2 through 3-24 show the location of each

ponents located on several circuit cards and on the

c o m p o n e n t part in the test set. Figures 3-11

chassis. The simplified schematic diagrams are par-

through 3-24 show the parts on each of the 13 circuit

ticularly useful for these circuits.

cards and RF power detector assembly. The remain-

b. The first step in isolating a fault in the test set

ing figures show the location of the chassis-mounted

is to extend the suspected circuit card above the

parts. Some parts require special procedures for

card cage with the board extender (fig. 3-9). Next,

removal and installation. Refer to paragraphs 3-11

energize the circuit, by setting up the conditions

through 3-16 before removing any parts.

3-4