Localizing Troubles

TM 11-6625-467-34

trouble would be an open terminating resistor. The

resistance introduced by dirty test probes may

test set contains many resistors that simulate ter-

make a good resistor measure out of tolerance. The

minating resistances for the AN/ARC-54 module

etched circuit boards in the equipment are covered

circuits. To localize a trouble of this type, first find

with an epoxy coating. The test probes used should

in which module test the trouble occurred. Obtain

be sharp enough to pierce the epoxy coating and

an AN/ARC-54 module of the same type; one which

make good contact to the circuit under test. The

is known to be in proper operating condition. Plug

c l e a r a n c e between socket terminals, wires, and

other components is usually very small. It is easy to

the module into the test set and perform the test

procedure for that module to the point where an

cause accidental short circuits with a test probe hav-

ing a long, exposed tip. Short circuits are very dam-

incorrect reading is obtained. At this point, note the

setting of the CIRCUIT SELECTOR switches on the

aging to transistor circuits; it is, therefore, a good

test set. Refer to the CIRCUIT SELECTOR switch

practice to cover all but about one-eighth inch of the

functional diagram (fig. FO-3) and trace the com-

exposed tips of test probes with plastic tape or some

plete circuit path of the test being performed. The

other insulation.

e. ohmmeter Checks of Transistors. If a transis-

CIRCUIT SELECTOR switches are composed of six

wafers each. Many of the wafers are not used on

tor tester is not available, a good ohmmeter may be

e v e r y test. T h e CIRCUIT SELECTOR switch,

used for testing. Be sure the ohmmeter meets the

requirements given in b above.

tables 2-1 through 2-7, show which wafers are used

and explain the function provided by each switch

(1) Checking PNP transistors. Connect the

wafer by every CIRCUIT SELECTOR switch set-

positive lead of the ohmmeter to the base and the

ting in the test procedures. Use these charts to aid

negative lead to the emitter. A resistance reading of

in tracing the signal path.

50,000 ohms or more should be obtained. Connect

the negative lead to the collector. The meter should

NOTE

again read 50,000 ohms or more. Reconnect the cir-

If none of the tests on the module can be

cuit with the negative lead of the ohmmeter to the

performed, there is a possibility that no

base. Connect the positive lead to the emitter and

p o w e r is being applied to the module.

r e a d the resistance. The meter should read 500

Refer to the power distribution diagram

ohms or less. Connect the positive lead to the collec-

(fig. 3-1) and make voltage checks at the

tor. The meter should read 500 ohms or less. Con-

appropriate pins on the module connectors.

nect the positive lead to the collector. The meter

b. The following is an example of a typical trouble

should read 500 ohms or less.

that might occur during the operating test. Assume

(2) Checking NPN transistors. Connect the

negative lead of the ohmmeter to the base and the

that step No. 7 of the transmit audio module test

positive lead to the emitter. A resistance reading of

( t a b l e 3-5, TM 11-6625-467-12) yields a TEST

50,000 ohms or more should be obtained. Connect

METER reading of O percent on a knov;n good

the positive lead to the collector. The meter should

transmit audio module. Referring to table 2-4 of this

manual, note that S6E connects the 1 kHz oscillator

again read 50,000 ohms or more. Reconnect the cir-

cuit with the positive lead of the ohmmeter to the

o u t p u t to the module audio input. Figure FO-3

base. Connect the negative lead to the emitter and

shows that the audio input to the transmit audio

r e a d the resistance. The meter should read 500

module is pin 1 of J3. With an oscilloscope, check for

ohms or less. Connect the negative lead to the col-

the presence of a 1 kHz voltage at J3-1. Again refer-

lector. The meter should read 500 ohms or less.

ring to table 2-4, note the S5B connects the module

sidetone output to S3C, and S3C connects the side-

3-4. Localizing Troubles

tone to the audio amplifier/detector input. Figure

a. There are two types of troubles that are likely

FO-3 shows that the sidetone output of the transmit

to occur in the test set: troubles that are detected

audio module is J3-11. With an oscilloscope, check

b y performing the self check procedures in the

for a 1 kHz voltage at this point. If no voltage is

operator/crew preventive maintenance checks and

present, there is no point in checking the input to

services (TM 11-6625-467-12), and troubles that

the audio amplifier/detector (XA5- 17). However, if

occur during the performance of the operating pro-

the voltage is present, notice that resistors A9R43

cedures and cannot be detected by the self check

and A9R44 form a voltage output (fig. FO-3). If

routines. The self check procedures localize the first

A 9 R 4 3 is open, the signal cannot reach S5B-8.

type of trouble to a circuit card or to the chassis of

Check for the presence of a 1 kHz signal at terminal

t h e test set. Aviation intermediate maintenance

8 of S5B. If 1.0 signal is present, the trouble is local-

must perform additional procedures to localize the

ized to circuit board A9 (load No. 1, HV5). Remove

the load No. 1 circuit card and check A9R43 with an

second type of trouble. An example of this type of

3-3