Isolating Trouble Within Stage

TM 11-6625-667-45

tual reading will be less than if taken with the

5.1-8. Isolating Trouble Within Stage

o h m m e t e r on the R x 1 0 0 r a n g e . W h e n i n

a. General. When trouble has been local-

d o u b t about the result of resistance measure-

ized to a transistor stage during troubleshoot-

ments, check a known good equipment of cor-

ing (para 5.1-7), isolate the defective part by

r e c t readings. The readings in b b e l o w w e r e

v o l t a g e measurements or resistance measure-

taken with each module removed from the

ments using the figure referenced in the note or

equipment. First measure between the base and

t h e Corrective measures column of the chart.

the emitter and between the base and the col-

For integrated circuit trouble isolation, refer to

l e c t o r with the positive ohmmeter lead con-

c below.

nected to the base; then measure between the

CAUTION

base and the emitter and between the base and

Carefully follow instructions and ob-

the collector with the negative ohmmeter lead

serve notes on voltage and resistance

connected to the base.

diagrams. Carelessness may cause

b. In-Circuit Transistor Resistance Charts.

m o r e troubles in the equipment and

Listed in the charts ((1) and (2) below) are

make the troubleshooting job more

r e s i s t a n c e measurements taken of the emitter

d i f f i c u l t . Do not remove or insert a

and collector with the transistors connected in

t r a n s i s t o r or integrated circuit ele-

t h e circuit. The measurements are made with

ment with voltage applied, to the cir-

M u l t i m e t e r TS-352B/U. These readings will

cuit. Do not perform resistance meas-

be valid only if the same type of ohmmeter is

u r e m e n t s of integrated circuit.

used and polarity and range scales are strictly

a d h e r e d to. Refer to figure 5.1-6 for the test

b. Transistor Testing. Since the transistors

setup.

are wired in the circuit, every effort should be

m a d e to troubleshoot the equipment without

physically unsoldering and removing the tran-

sistors.

c . Integrated Circuit Testing. Faulty inte-

grated circuits may be isolated by oscilloscope

s i g n a l tracing; figures 5.1-3 and 5.1-9 illus-

t r a t e the signal conditions at each applicable

pin.

d . W i r i n g D i a g r a m s . Use the wiring dia-

A. FORWARD

g r a m s (fig. 8-43, 8-44, and 8-45) to circuit

trace and isolate the fault part.

5.1-9. Analysis of Measurements

For an interpretation of readings when taking

t h e voltage and resistance measurements, re-

f e r to TB SIG 357.

a. In-Circuit Transistor Resistance Measure-

ments. When measuring resistance of circuit

elements connected across the junction of any

transistor (base-emitter or base-collector), con-

sider polarity of the ohmmeter and try meas-

u r e m e n t s with the ohmmeter connected one

way; then reverse the leads. Also, consider that

different values of resistance will be obtained

with the ohmmeter on different ranges; for ex-

B. REVERSE

ample, if the transistor junction, or a resistor

TM6625-667-35-89

plus the transistor junction, is measured in the

Figure 5.1-6. PNP transistor resistance

forward direction of the R X 10 range the ac-

measurements

Change 1

5.1-17