Tektronix 2213 - Page 93

M ainte nance-2213
Service
8 . C heck V oltages and W aveforms
Often t he d efective com p
o n ent
can
be located by
c heck ing t h e a pp ro pr iate voltage
o r waveform in t h e circuit .
Typical voltages a
r e liste d on t h e sc h ematic d iagrams .
W aveforms are s
h
own a
dj
ace
n t to t he sc h ematics, a nd wave-
form test
points a r e in d icate d on bot h t h e sch ematics and
circ u it b oard illustratio n s b y h exagonal-outlined
num b ers .
ΝΟΤΕ
V oltages and waveforms given on t
h
e sc
h ematic
d iagrams are n ot absol u te and may vary
slig h tly
betwee n inst r uments . To establis
h o
p
erating
con-
ditions similar to t h ose use d to
obtain t hese readi n gs,
see t h e " V oltage and Waveform Setup"
conditions in
t h e "Diagrams" sectio n fo
r
t
he p r eliminary equip-
me n t set
up . N
ote
t h e r ecommen d ed test eq u ipment,
initial front- p anel co n t rol settings, and cable-
connection instructions T
h e control-setting ch anges
(f r om initial
setu p ) req u ire d to obtai n t h e give n
wavefo r ms a
n
d voltages
a r e locate don t h e wavefo r m-
diagram page .
W A RN I NG
To avoi d elect r ic s h oc k , always d isconnect t
he instr u -
ment fr om t h e power input so
u rce befo r e removi ng
or replacing components .
9 . C h eck In d ivi d ual Com p
onents
T h e following procedu
r
es
d escribe met hod s of chec k ing
i nd ivi d ual components . Two-lead
com pone n ts t h at a r e
sol dered in place a re most acc
u rately c heck e d b y first
d isconnecting one end from t
he circuit boar d . T h is isolates
the measureme nt from t
he effects of surroun d i n g circuitry .
See F igure 9-1 for value
i dentification o r F igure 9-2for
ty p ical semicon d uctor lead
config u ration .
Wh en c h ec
k i n g semicond u ctor s, observe t he static-
sensitivity
precautions locate d at t h e beginni ng of
t h is section
.
TR A N SISTO RS . Α
goo d c heck of tra n sistor o p e r ation
is act u al p
erfor m ance und er op erating co nd itions . Α
t r an-
sistor can
most effectively be checked by s ub
stituting α
kn own goo d
com p onent . H owever, b e su r e t h at ci r cuit
con d itions
are not suc h t h at α re p lacement
transistor mig h t
also be
d amaged . If s ub stitute transistors
are n ot available,
use
α dynamic tester . Static-ty p e
teste rs are not recom-
mende d , si nce t h ey d o not chec
k
o p
eration un d er simulated
ope r ating con d itio n s .
Wh e n t r o ub les h ooting transistors
in t he ci r cu it wit h α
voltmeter, measure bot
h t he emitte r -to- b ase and emitte r -
to-collector voltages
to d etermine w h et h er t h ey are con-
sistent wit h normal
ci r cuit voltages . V oltages ac r oss α
tra n sistor may vary
wit h t h e type of device an d its circuit
function .
Some of t
hese voltages a r e p redicta b le . T he emitte r -
to- b ase voltage
fo r α cond u cti n g silico n tra n sisto r will
no r mally ra
nge fr om 0 .6 to 0 .8 V, an d t he emitter-to-
base voltage
for α
con d
ucting germani u m tra n sistor ra n ges
from 0 .2 to 0 .4 V . T
h
e
emitter-to-collector voltage for α
saturate d tra n sisto r is abo
u
t 0 .2
V .
B
ecause t h ese values
are small, t h e b est way
to check t h em is by connecting α
se n sitive voltmeter
across t he ju nction r at h er t h an com-
p aring two
voltages taken wit h r es p ect to grou nd . If t he
forme r meth od is u se d , bot h lea d s of t he voltmeter must
be
isolate d f r om ground .
If values less t han t h ese a re
o b taine d , eit he r t h e d evice
is s
h orte d o r no cu r r ent is flowing in t he external ci
r
c
u
it .
If
values
excee
d t h e emitter-to-base val u es given, eit h er t h e
ju nction is
reverse biase d or t he d evice is d efective . V oltages
excee d i ng t
h
ose
given for ty p ical emitte r -to-collector val ues
co
u l d in d icate eit her α n onsaturate d d evice o p e r ati n g
no r mally or α defective (open-ci r cuite d ) t r ansistor . If t h e
d evice is con d ucti n g, voltage will b e develo pe d across t h e
resistors i n se r ies with it ; if it is o p en, no voltage will b e
d eveloped across t
he
resistors
in series wit h it, un less
current is bei ng su p
plie d by α parallel p at h .
Wh e n c heck i ng emitter-to-base
jun
ctions, do
not use
an oh
mmeter range t h at h as α h ig h i n ternal cu rrent .
H ig h c ur rent can d amage t h e τ ransistor . R eve r se
biasing t h e emitter-to-base ju nction wit h α h ig h
c u rrent may deg r ade t h e t ra n sistor's cu r rent-transfe r
r atio ( B eta) .
Α transistor emitter-to-base j
unction
also can
b e check ed
for an o p en or s horted co nd itio n by m
easuring
t
h e
resistance b etwee n terminals wit
h
an o
h mmeter set to
α
ra n ge having α low i n ternal source curren t, such as t
h
e
R Χ 1 k Ω ra n ge . T h e j unction r esistance s
h
o u
l
d be
very
h ig h in one d irection a nd very low w
hen
t
he meter lea d
s
are reve r se d .
Wh en t r oubles h ooting α fiel d -effect t r an sistor, the
voltage across its elements can b e c hecked in t h e same
man
n
er as p
reviously d escribe d for ot h er transistors .
H owever, remem
b er t h at in t he n ormal d ep letio n mode of
operation, t h e gate-to-source j
unction
is
reverse b iase d ;
in t h e e nhance d m
od e, t he j unction is forwar d b iase d .