Tektronix 576 - Page 73

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p ower sup p ly is sh orte
d , th e d efective circuit ca
n be iso-
lated by d iscon n
ecting t h e p i n connectors at
the boards
until th e sh orting
co nd ition is remove d .
6 . C h ec k
V oltages a nd W aveforms . Ofte
n th e d efective
com p one n t can
be locate d by c h ec k ing for
t h e correct volt-
age or waveforms
as given on t he circuit
diagrams on fol d -
out pages
in t he bac k of t h is manual .
ΝΟΤΕ
V oltages an d
waveforms given on t he d iagrams
are
not absolute a
nd may vary slig h tly bet w een i n stru-
me n ts . To
obtain o p erating con d itions similar to
t h ose use d to
ta k e t h ese rea d ings, see t he first dia-
gram page .
7 . C hec k Semicon ductors
. M ost circ u it
failures result
from t h e failure of α
transistor, FE T, d io d e, or
integrate d
circuit due to normal aging
and use . T he followin g
ex p lains
vario u s met h o d s of chec k
ing semicon d uctor d
evices . Inser-
tion i n formation is p
rovi ded in F ig . 4-2 .
T R A NSISTOR S . Tra n sistor d
efects usually ta ke t h e
form of
t he transistor open ing, s h orting,
or
d
evelop ing
excessive lea
k
age
. T he best met h o d of c h ec k ing
tra n sistors
is by d irect
substitution . Be sure t he voltage
con d itions of
t he circuit are
not such t h at α re p lacement transistor
mig h t
also be damaged . If
substitute transistors are not
available,
use α d ynamic
tester (such as α Te k tronix Ty pe 576) .
Static-ty p
e testers a r e not recommen d e d si n
ce t h ey d o
not c
hec k t h e device unde r o p erating con d
itions . H owever,
if no ot
h er tester is imme d iately available, an o
h mmeter
will usually
indicate w he n α transistor is totally ba d .
As α
general rule,
use t he R Χ 1 k range w h ere t he current
is
usually limite d to
less t han 2 mA and t he internal voltage
is
usually 1 1/2 volts . C
heck t he cu r rent an d voltage of t
he
oh mmeterby inserting
α multimeter betwee n t h e o h
mmeter
lea d s and measuring t
h
e
current an d voltage of t h e various
ranges . After it h as been d etermined
w h ic h o h mmeter
ranges will
n
ot
h arm t he transistor, use t hose
ranges to
mcoourc th o trancictor' ε rocictanco
.
C
h oc k t ho roci ς tan
r_ σ
i n
both d irections t h roug h t h e j
unctions as liste d in Table 4-3 .
TA BLE 4-3
Transistor R esistance C hec
k
s
M aintenance-Ty p
e 576
F I EL D
EFFE CT T R A N SISTO R S .
T h e voltage a nd
resistance of
fiel d effect transistors can be
c
h
ec ke
d
in th e
same manner as
transistors . 1 1/2 V a nd
less th an 2 mA
sh oul d be use d
for o h mmeter c h ec ks . R
esistance rea d ings
sh oul d be :
d rain-to-source

L
ess th an 500 Ω
gate-to-source an d
400 Ω to 10 Ω (a pp r oximately) i
n
gate-to- d rain

one d irection ; mo r e t h an 200 k Ω
wit h
lea d s reverse d .
I N T E GR AT E D CI RCU
ITS
.
Integrate d circuits are best
c h ec k e d with α voltmeter,
oscillosco p e, or by d irect substi-
tution .
DIOD E S . Dio d es (excep t for tunnel d io d
es) can be
ch
ec
k ed
for
an
open or sh ort-circ u ite d con d
ition by meas-
uri ng t
he resistance between t he termi n als
after u n sol d ering
one en d of t he com p
onent . U se α resistance scale with an
internal voltage
between 800 m V an d 3 volts . T h e resist-
ance s h oul d measure
very h ig h (in mego h m r ange) in one
d irection an d low in t
he oth er .
8 . Ch ec k Ot
he r Compo n ents . If t he semicon d ucto
r s in
t he circ u it
h ave been foun d to be goo d , t h e r est of
t he
com p onents s
h oul d be chec k e d . Com p onents w h ic h
are
sol d ere d in p lace
are best c hecked b y disconnecting
one
end . T h is isolates
t he measurementfrom t he effects of sur-
roun d ing circuitry
.
9 .
R e p air and R ead j ust t h e Circuit
. If any defective
p arts are locate
d , follow t h e re p laceme
n t p roce d ures give n
in t h is
section . Be sure to c heck t
he p erformance of any
circuit t
h at h as been re p aire d or
t h at h as h a d any electrical
com p
one n ts re p lace d . If α com
p onent h as been re p laced,
recalibration is usually
necessary .
A dd itional
Troublesh ooting Information
T r oubles h ooti
n g t he R ea d o ut . M alfunction of
t he read-
OU L U is μ
lay υαιι
Du
ιαυαα U by ιΙιιτυ
th i n gs : α burne d out
rea d out
lam p , im prop er op eratio n of
t he rea d out logic or
im prop er op
e r atio n of α cam switc
h . T he best met hod of
locati ng
t he malfunction is by ch ec k
ing t he i n p uts an d t he
out p uts of
t he d eco d ers for various p
ositions of t he f r ont
panel
switch es . Tables 4-4 t hr oug h
4-7
s
h
ow
to w h ic h
decoders t h e p ins on
t h e J 950 are i n p uts . T h e state of t h
ese
p ins (h ig h or low)
for vario u s front- p an el control
settings
can be obtaine d
from t he Rea d out Switc h ing and
Intercon-
nection s sc h ematic in t he Diagrams
section . T he out puts of
t h e deco d ers a r e ch ecked
by fi r st d eter m ining w h at t h
e
read o u t o u g h t to be for t he given
settings of t he front- pan el
controls (be sure to
note t he effects of t h e M OD E switc
h ,
DIS
PL AY O FFS E T Selector
switch an d STEP MUL T .1 Χ
button) . Wh en
t he pro p er readout h as been
determine d ,
locate t he p ins on t h e Rea d out
L ogic circuit boa r d w h ic h
must
be low to cause t h at rea d out
(see Tables 4-4 t h roug h
4-7) .
Wh en t h e pro p er states of t
h
e
i nputs and out p uts of
t he d eco d ers h ave
bee n determine d , c heck t h ese
levels wit h
α voltmeter .
4- 7
O h mmeter
Resista nce Rea
d
ings
T h at Can Be
Conn ections
) Ex p ected U sing
t h
e
R Χ 1 k R ange
E mitter-Collector
H ig h readings
bot h ways (about 60
k Ω to aroun d
500 k Ω ) .
E mitter- Base
H ig h reading one way
(about 200
k Ω or more) . L ow rea d
ing t h e
ot h er
way (about 400 Ω to 2 .5 k Ω
Base-Collector
H ig h reading o n
e
way (about
500
k
Ω or more) . L ow rea d ing t
h e
ot h er way
(about 400 Ω to 2 .5
k Ω ) .