Tektronix 2213 - Page 34

wit h t he exceptio n of an additional
p
ai
r
of
tra n sistors
t h
at
p erforms t h e inverting f u nction
.
In
t he No r mal mode
of op eratio n , Q257 and Q267
are biased on and Q258 and
Q268 biase d off b y I NVER T
switc h S264 groun d ing one
end of R263 . In t
h
e Invert
mo de (I NVER T switch p resse d
in), c
r oss-wired transistors Q258 and Q268 are biased on
and Q257 an d Q267 b iase d off by g
r oun d ing t h e ju nction
of R256 and R266 . Invert ΒαΙ
p oten tiometer R264 is
ad j uste d to correct
for
do
offsets between the two
switc h i n g-transistor
pairs
.
Wh en R264 is correctly a d j usted,
α baseline t
r ace will maintain t he same vertical p ositio
n
as
t
h
e am
p lifier is switched b etween Invert an d N ormal
.
Internal Trigger P ic k off Am p lifier
T he Internal Trigger P ic k
off
Am
p lifier su pp lies trigge r
sig n als to t h e Internal
Trigge r
Am
p lifier in the Trigge r
circuitry (Diagram 4) . Inter n al t r
igger signals are provided
by t he vertical p ream p lifiers an d
are ap plie d to t h e b ases
of U 170 B an d U 170C (for C
h annel 1) and U270B and
U 270C (for C ha nn el 2) . T
h ese transistor p airs are biase d
on, eit h er in d ivi d ually or
togeth er, f r om t he Internal
Trigger Switc h ing L ogic ci
r
cuit
(Diagram 3) .
Wh e n C hannel 1 is t
h e selecte d internal trigger sou rce,
Q173 and U 170A (C H
1) will b e biase d on an d Q273
(C H 2) biase
d off . Curre n t flowing t hrough R 173, R183,
an d R197
will b ias on U 197A to keep U 197 E c u t off .
E mitter current
is su
p
plie d to U 170A by U 197D . In turn,
U 170A t
h en sup p lies emitter curren t to U17013 and U 170C
to
enable t h e C hannel 1 internal trigger signals to pass to
t h e Inte r nal Trigger Am p lifier .
Wh en C hannel 2 is
selecte d as t h e internal trigger source,
Q273 a
nd U 270A will be biased on a nd Q173 biase d off .
Transistor U 197A will remain on, and current su pp lie d by
U197D will su p p ly emitte r c u
rrent
to
U
270A .
T h en U 270A
in turn sup p lies t he emitter
current
to U270B
and U 270C
and en a b les t
h
e C
hannel
2
internal trigge r signals to pass to
t
h e Internal Trigger Am p lifier .
Wh en t h e T RIGG ER ΙΝΤ s
w itc h is set to VER T M OD E ,
t h e actual signal source selecte d d ependson
t he setting of
t h e VER TICA L M OD E switc
h
es . If
eit h er CH 1 or C H 2
VER TICA L M OD E is selecte d , t
h e p recedi ng discussion on
C
h annel 1 or C h annel 2
internal trigger signals a pp lies .
Wh en t he VER TICA L
M
OD
E switch is set to BOTH, the
VER TICA L
M OD E ADD-A L T-C H O P switch setting d
eter-
mines t h
e switch ing action for selecting t he internal trigger
source
.
Selecting ADD VER TICA L
M
OD
E causes bot h i n ternal
trigger-select signals (C H 1 Trig and C
H
2
Trig) to be L O,
an d bot h Q173 a nd Q273 are biase
d off . Transistor U 1 97A
t h en b ecomes biase
d off causing U 197 E to saturate . Wit h
U 197 E saturated,
emitter current is s upp lie d to bot h
T h eory of O p
e r atio n -2213 Service
C h a nn el 1 an d C
h
an
n
el 2
Trigger P ic k off Amplifie rs
( U170C an dU 170 B
for
C
h a n nel 1 an dU 270 B an d U 270C
for C hannel 2)
via
R196-C
R 196 a nd R 296-C R 296 res p ec-
tively . Wh en b ot h pic koff amplifiers are ena b le d , t
h e
resulti n g trigge r signal is t h e sum of t h e C h annel 1 an
d
C han nel 2 i nte rn al trigger sig
n
als . T
he
s
u m
of
the current
sup p lie d by U 197
E to both pic k off am p lifiers is t h e same
magnitude as
t h e curren t from U 197D w hen eit he r CH 1
or C H 2 is
selected i n dividually . T h erefore, t he do ou t p ut
to t
he Internal Trigger Am p lifier will b e t he same for C H 1,
C
H 2, and ADD VER TICA L M OD E trigger signals .
Wh
en A L TVER TICA L M OD E is selected
wit h t he
previously
esta b lis h e d settings ( VER TICA L M OD E
to
BOTH,
ΙΝΤ to VER T M OD E , an d SO UR CE to ΙΝΤ ), t
he
inte
r
nal
trigger-select signals alter n ate between c h annels . On
one swee p t
he
C
h a nn el 1 inter n al trigge r will b e selecte d as
previously
descri bed . On t he alternate swee p , C h annel 2
internal trigger will be selecte d , agai n as p reviously
d escri b e d .
U
n
d
er t h e same switc h -setting cond itions, selecting
CH O
P VER TICA L M OD E p roduces t he same
trigger-
selection
con d itions as d escri b ed for ADDVER TICA L
M OD
E . T he sum of t h e C h a nn el 1 and C
h
an
n
el 2
i n ter n al
trigger sig n als will be p asse
d to t he Internal Trigger
Am p lifie r . See t h e "Internal
Trigger Switc h i n g Logic"
d iscussion for α d escri p tio
n
of
h ow the internal trigger
selectio n signals a
r e generated .
C H ANN EL S
W ITC H
A NDVER TICA
L
O U
T
PU
T
T
h
e C
h annel Switc h circ u itry, s h own on Diagram 3,
selects t h e in
p
ut
signal or combination of in put sig n als to
be co n nected to t
h e V ertical
O
u t pu t Am p lifier . By setting
t h e logic in
p
ut
into t h e C h ann el Switc h ing L ogic circuit,
VER TICA L M OD
E switch es S315 an d S317 select t he
i np ut signal
com b inations to be displayed . T he i n ter n al
trigger-select signals a r e also generate d in t h e C hannel
Switc
h circuitry .
Diod e Gates
T
h
e Diode
Gates, consisting of eig h t d iodes, act as
switch es t h at are controlle d by t he C h ann el S w itc h ing
Logic circuitry . T h e Q- an d Q-out p uts of U 317A ( p ins 5
and 6 res p ectively) control forwar
d biasing of t he d io d es
to t u r n t he gates onan d
off .
C HANNEL 1
DISPL AYO NL Y . To dis p lay only the
C h a nn el 1
sig n al, t h e CH 1 Enable signal ( U 317A p in 5)
is
ΗΙ
an
d t h e CH 2 E nable signal ( U 317A p in 6) is L O .
3-7