TROUBLESHOOTING TA-676/G.

TM 32-5895-227-14&P

a. Sectionalization. Listed below is a group of

CAUTION

tests arranged to reduce unnecessary work and to aid in

Before using any ohmmeter to test

tracing trouble. The AN/MSQ-71 consists of many units.

transistors or transistor circuits,

The first step is to locate the unit or units at fault. The

check the open circuit voltage across

following methods can be used:

the ohmmeter test leads. Do not use

(1) Visual Inspection. The purpose of visual

the ohmmeter if the open circuit

inspection is to locate faults without testing or measuring

voltage exceeds 1.5 volts.

Also,

the circuits. All meter readings, scope patterns, or other

since the RX1 range normally

visual signs should be observed and an attempt made to

connects the ohmmeter internal

sectionalize the fault to a particular unit.

battery directly across the test leads,

(2) Operational Tests. Operational tests

the comparatively high current (50

frequently indicate the general location of trouble. In

ma or more) may damage the

many instances, the tests will help in determining the

transistor under test. As a general

exact nature of the fault.

rule, it is not recommended that the

(3)

Substitution

Components.

RX1 range of any ohmmeter be used

Substitution of easily interchangeable components can

when testing low power transistors.

provide a quick method of localizing a fault. For

instance, if a microphone is suspected to be at fault,

(3) Intermittent Troubles. In all tests, the

another microphone can quickly be connected and the

possibility of intermittent troubles should not be

equipment tested for proper operation. If the system

overlooked. These can be noticed by tapping or jarring

then operates as normal, the first microphone or its

the equipment. Make a visual inspection of the wiring

cable can be assumed to be at fault.

and connections to the units. Minute cracks in printed

b. Localization. After the trouble has been

circuit boards can also cause intermittent operation. A

sectionalized, the methods listed below will aid in

magnifying glass is often helpful in locating defects in

localizing the trouble to a stage or module in the

printed circuit boards. Continuity measurements of

suspected unit.

printed conductors may be made using the same

(1) Troubleshooting. Meter indications, or

techniques ordinarily used on conventional wiring.

lack of meter indications, and operational checks

Observe the ohmmeter precaution set forth in paragraph

provide a systematic method of localizing trouble to a

(2) above.

stage or module.

(2) Signal Substitution. Signal substitution

7-6

TROUBLESHOOTING TA-676/G.

procedures enable the repairman to localize a trouble

General.

Perform the procedures for

quickly to a stage or module. A signal generator, an

sectionalizing, localizing, and isolating trouble to a

audio oscillator, and an oscilloscope are units of test

particular circuit in the Telephone Set TA-676/G.

equipment that may be used in signal substitution

b. Diagrams.

procedures. Observe the precautions in paragraph 74

(1) Wiring Diagram. See figure 7-1 for the

and follow the signal substitution procedures carefully so

Telephone Set TA-676/G wiring diagram. This diagram

that damage to transistors is avoided.

outlines the electrical means of performing maintenance

c. Isolation. After the trouble has been localized,

on this unit. The set, as shown, consists of connectors,

the methods listed below will aid in isolating the trouble

audio volume compression units (AMP 1 through AMP

to a defective circuit element.

4), switches, relays, and associated wiring.

(1) Voltage Measurements. The AN/MSQ-

(2) Circuit Diagram. The circuit diagram for

71 equipment is transistorized.

When measuring

power supplies PSI and PS2 is shown in figure 7-2.

voltages, use tape or sleeving (spaghetti) to insulate the

Resistor location wiring on AMP 1 through AMP 4

entire test probe, except for the extreme tip.

connectors is shown. Power supplies, components, and

momentary short circuit can ruin the transistor. Use an

output operating voltages are shown on the circuit

electronic multimeter (VTVM) if possible.

diagram.

(2) Resistance Measurements. Resistance

(3) Jacks. Location of jacks is shown in

measurements are an effective means of detecting open

figure 3-1 for the interconnecting cabling on the

leads, faulty resistors, or shorted capacitors. Since this

Telephone Set TA-676/G.

equipment contains transistors, the following caution

c. Troubleshooting Chart. Table 7-1 lists the

must be observed:

trouble symptoms, the probable troubles, and the

corrective actions to be taken.

7-2