Tektronix 7704A User Manual

clamps t he collector of Q3361 at abo u t -0 .6 volt if α
negative voltage is ap plied to t
h
e ou tput con nector .
R
ea
doutControl
Q3367
an
d Q3373, along with S3368, control t he
op
erating mo de of t he Reado u t System . Wh en Rea d out
M ode switch S3368 is in t he F ree- R un position, t he
Rea d out System runs contin uously in α free-running
manner . T h e emitter of
Q3373 h
as no ground
return, so
Q3373 can not con d uct and its
collector rises positive
(t h roug h circuitry in t
h
e
Rea
d out
System) to enable t he
Rea do u t System .
In t he Gate Trig' d p osition, t h e emitter of
Q3373 is
connecte d to groun d t h roug
h R
3373 an d
S3368
to pro
duce
α L O loc kou t level to t
h
e
Rea
d out System
. At t he end of
t he selecte d gate, α negative
level
is ap p
lie d to t h e base of
Q3367 . Th is negative
level
is
differentiated by
C3368-133368 ; t h e resulting
negative-going p ulse reverse
b iases Q3373 to momentarily allow its
collector to go HI .
T h is enables t he Rea d out System so t
h
at
it can p rod uce one
complete frame (eig h t wor d s)
eac h time t h e selecte d gate
goes n egative .
L ine In p ut
Ο
I NVER T ER / RE CTI F I ER S
T he Inverter/ R ectifiers circuit provi d es t
h
e op erating
p ower for t h is instrument from an AC line-voltage source
.
T h is circuit inclu d es α L ine Selector assembly to permit
selection of t h e
nominal
op
erating
voltage
for t he instru-
ment
.
F igure 2-29 s h ows α d etailed bloc k diagram of t h e
Inverter/ R ectifiers circuit .
P ower is applied t h roug h t he L ine
F
ilter,
li n e fuse
F 3001, P O WER switc h S5020, an d
L i n e Selector switc h
S5022 . T h e L
ine F ilter is designed
to
k eep p ower-line
interference from entering t h e instr u ment and to k eep t h e
25- k ilo h ertz Inverter signal from entering t
h
e p ower
line .
L 5020- L 5022 provide
common-mo d e EMI (electro-
magnetic
interference) filtering with C5020-C5022, and
d ifferential
filtering wit h C3005-C3006 . R5020- R5022
provi de common-mo d e resonance d amping .
L ine Selector switch S5022 allows t he instr u ment to
operate from eit her α 115-volt nominal or α 230-volt
nominal line-voltage source . In t he 115-volt p osition,
rectifier CR3015 op erates as α full-wave do u bler wit h
cap acitors C3016-C3017 so t he voltage across t h e two
capacitors in series will be t h e peak -to- p ea k value of t h e line
voltage . F or 230-volt op eration, C R 3015 is connecte d as α
bri d ge rectifier an d C3016-C3017 op erate as energy-storage
filter cap acitors . As α result, t h e outpu t voltage applie d to
t h e Inverter stage is ab out t h e same for eit h er 115-volt or
230-volt operation .
Ci r cuit
Description- 7704A Service
T he r mistor RT3009 limits t h e surge curre
n
t d ema
n
ded
by t he power su pp ly w
h
en it is
first turne d
on .
After the
instrument is in operation, t
he
t
h ermistor resistance
drops,
so it h as little effect on t
h e
op
eration of t h is stage . T he
stored c
h
arge on
C3016 an d C3017 limits t he surge cu rrent
if t h e
P O WER switch is turned off an d back on after the
resistance
of
RT3009 h as d ro p pe d . If t h is occurs, t he
Inverter Control L ine Sto p stage will s hut t he Inverter off
so t h at C3016 an d C3017 will slowly d isc
h
arge t
h
ro
ugh
R
3021 .
Th e disc h arge time constant of
C3016-C3017- R 3021 is a b out eq ual b ut opposite to t
h
e
t h ermistor t h ermal recovery time
. Th
is
ensures sufficient
t h ermistor resistance to limit t h
e
turn-on
surge
c
u rrent
to α
safe level . Since C3016 an d C3017 disc
h arge slowly,
dangero u s p otentials exist wit
h in
t
he power supply for
several minutes after t
h
e
P O WER switch is turned off . T he
presence of voltage
in the circuit is in d icate d by t he
relaxation
oscillator R3019-C3019-DS3019 . N eon bul b
DS3019 will
blin
k
until t h e potential across C3016-C3017
dro p s to about
80 volts .
DS3008-DS3013 are
surge
voltage
p rotectors . Wh en t h e
L ine Selector switc
h is in the 115-volt position, only
DS3008 is
connecte d across t he line input . If α pea k -voltage
s u rge in excess of about 230 volts is present on t he line (or
if t he instr u me n t is acci d entally connecte d to α 230-volt
sou rce), DS3008 will b rea k d
own
an
d d eman d h ig h c u rre n t .
T h
is excess
current will q uic k ly op en line fuse F 3001 to
interrupt t h e input power before t he instrument can be
damage
d . In t h e 230-volt p osition, DS3008 and DS3013 are
connecte
d in
series across t h e line in p ut to provi de
protection
for p
ea
k voltages in excess of about 460 volts .
Transformer Τ 3001 provi d es α sample of t h e li n e voltage
to t he p l u g-in connectors in t he M ai n Interface circuit for
internal triggering at line frequencies . T h is line-fre q uency
signal is also connecte d to t h e Inverter Control L ine Stop
stage to
in d icate w hen line voltage is applie d and t h e
P O WER switch is on .
T h e rectifie d out put voltage from t he L ine In pu t stage is
connecte d to t h e Start N etwor k t h rough I n verter fuse
F 3003 . F 3003 protects t h e Inverter stage
if
it d
emand s
excessive current d ue to α malfunction .
Start N etwor k
T h e input line voltage is connected to d ivi d er
R3010- R3048 . T h is voltage c h arges C3048 on eac h h alf
cycle . Wh en t he charge on C3048 reac h es about 32 volts,
trigger d io d e CR 3040 con ducts to
provide
α turn-on
trigger
current to Q3047 t
h roug h C3042 . T h is current allows t h e
I n verter stage to start operating . After t h e I n verter stage is
o
p
erating, t he recurrent waveform at t h e collector of
Q3047 keeps C3048 d isc harged t h ro ug h C R 3046 .
T h
is
d isables t h e Start N etwor k w h ile t
h
e instr
u
ment is on .
2- 33

Tektronix 7704A Specifications

General

Brand	Tektronix
Model	7704A
Category	Test Equipment
Language	English