Servicing Transistor Circuits

TM 11-6625-467-34

maintenance level. D u p l i c a t e the module test

(2) Multimeters that have sensitivities of less

wherein the fault occurred, then by the use of sche-

than 5,000 ohms per volt should not be used in tran-

sistor circuits. Multimeters with lower sensitivities

matic diagrams, functional diagrams, and the volt-

draw excessive current through many types of tran-

age and resistance table, the trouble can be localized

to a functional circuit. The CIRCUIT SELECTOR

sistors, thereby damaging them. Use 20,000 -ohms-

switches signal path tables 2-1 to 2-7 explain the

per-volt meters or vacuum tube volt-meters (vtvm)

function of each CIRCUIT SELECTOR switch for all

when checking all transistor circuits. Verify the

switch settings in the operating procedures. The

ohmmeter circuit (even those in vtvm) on all scales

s i g n a l paths for switch settings given in the

with an external, low-resistance milliammeter in

operator crew preventive maintenance checks and

series with the ohmmeter leads. If the ohmmeter

services are presented in table 2-8. The CIRCUIT

draws more than 1 milliampere on any range, this

SELECTOR switch functional diagram (fig. FO-3)

range cannot be used safely on small transistors.

shows the signal routing for any switch setting.

(3) Electric soldering irons may also damage

Both the table and the functional diagram will prove

transistors because of leakage current. To check a

very helpful in localizing troubles in the test set.

soldering iron for leakage current, connect an ac

b. After a trouble has been localized to a func-

voltmeter between the tip of the iron and a ground

tional circuit or circuit card, the trouble must be iso-

connection (water pipe or line ground). Allow the

lated. Isolation means the locating of defective part

iron to heat, and then check for voltage indications

or parts responsible for the abnormal indication.

on the meter. Reverse the plug in the receptacle and

Some defective parts such as burned resistors and

again check for voltage. If there is any indication on

arcing shorted transformers, can often be located by

the meter, isolate the iron from the line with a

s i g h t , smell, and hearing. Most defective parts,

transformer. If the iron is heated, it may be used

however, must be isolated by checking voltage,

without the isolation transformer if it is unplugged

resistance, waveforms, and so forth.

before touching the transistor. A ground wire be-

tween the tip of the soldering iron and the chassis of

3-3. Servicing Transistor Circuits

t h e equipment being repaired will also prevent

a. General. In general, the servicing procedures

damage from leakage current.

c. Soldering Techniques. Light duty soldering

and test equipment used for other types of elec-

tronic equipment may be used for transistor cir-

irons of 20 to 25 watts are adequate for transistor

cuits. Those cases which required special precau-

work and should be used. When installing or remov-

tions are outlined below. If the equipment under

ing a soldered-in transistor, grasp the transistor

lead between the solder joint and the transistors

test contains transistors, even though they may not

with a long-nosed pliers. The pliers absorb heat that

be in the circuit under test, the precautions should

would otherwise conduct into the transistor from

be observed because of the possibility of accidental-

ly connecting into a transistor circuit.

the soldering iron. Be sure that wires being soldered

b. Test Equipment.

t o transistor terminals are pretinned so that the

connection can be made quickly. Excessive heat may

(1) Transistor damage caused by test equip-

ment is usually the result of accidentally applying

damage the transistor.

d. Circuit Measurements. W h e n m e a s u r i n g

too much current or voltage to the transistor ele-

m e n t s . One source of such current is from the

r e s i s t a n c e s of circuits containing transistors or

powerline when test equipment with transformer-

semiconductor diodes, remember that these compo-

less power supplies are used. The test equipment

nents are polarity sensitive; therefore, follow the

r e c o m m e n d e d for testing the test set has trans-

directions in the notes given in the resistance tables

former-type power supplies; however, if substitute

to be sure that the correct polarity and range is

equipment should be used, check its power supply.

applied to the circuit from the ohmmeter. Capaci-

If line isolation is required, a suitable transformer is

tors used in transistor circuits usually have large

i d e n t i f i e d by National Stock Number 5950-00-

v a l u e s ; therefore, it takes time to charge these

356-1779. If the equipment is equipped with a line

capacitors when an ohmmeter is connected to a cir-

filter, it is also possible to damage transistors from

cuit containing them. Allow sufficient time for the

line current, even though the test equipment has a

capacitors to fully charge or the readings obtained

power transformer in the power supply. The line

will be subject to error. In some cases, it may be

filter may act as a voltage divider, applying 55 volts

b e s t to isolate the components in question and

ac to the transistors. To eliminate trouble of this

measure them individually. Test probes used for

type, connect a ground wire from the chassis of the

transistor circuit measurements should be clean and

e q u i p m e n t under test to the chassis of the test

sharp. Because many of the resistors used in tran-

equipment before any other connections are made.

sistorized circuits have low values, any additional

3-2