Α Trigger Generator; Α Trigger Level Circuit; Α Trigger L Evel Comparator; V Trigge R Circuit - Tektronix 2235 Instruction Manual

Α
TR
IGG
ER
G
ENER
ATO
R
biased on an d U 460A and
U
460D are
b
iase d off.
F
or the
negative slope
p
osition, the transistors reverse states to in-
Th e Α Trigger Generator, shown on Diag ram 3, supplies
vert the comparator outpu t
p
olarity.
trigger signals to the Α Sweep Generato r. Included in the
Α Trigger Generator circ uit are the Ρ-Ρ Auto Trigger, Auto
Baseline, and TV Triggering circ uitry .
Α Sc hmitt Trigger
a
nd
T
V Trigge r Circuit
This circuitry ge nerates α signal that drives the Α Trigger
L
ogic as α fu nctio n of the Trigger
L
evel Comparator output
signal and the Α TRIGGER Mode switches .
Th e Α Trigger
L
evel Circ uit establishes voltages at the
ends of the Α TR IGGER LEVEL potentiometer as α function
of the Α TRIGGER
p
ush bu tton selection and trigge r signals The output signals from the Α Trigger
L
evel Comparator
selected by the Α SO UR CE switch. drive Q460 and Q463 . These transistors are configured as α
current mi rror that converts the
d
ifferential output to α sin-
gle-ended current to drive am plifier U480C . Slope
B
alance
In the Ρ- Ρ Auto a n d TV F ield modes, 0413 is off and
p
otentiometer
R
471 D corrects for
d
o offsets
b etwee n p osi-
C R414 a n d C R415 a r e reverse biased . Trigger signals se-
tive and negative slope. S h u n t-feed bac k amplifier U480C
lected
b
y the Α SO UR CE switch
a
re applied to
p
ea k detec-
co nverts α curre n t in put to α voltage output to drive th e
tors consisting of Q420-Q422 and 0421-Q423. These peak input of the Sc hmitt Trigger,
U 480D, through R 469 . Positive
detectors trac k do levels a n d
h
ave α
h
ig h voltage transfer feedback for the Schmitt Trigger is
p
rovided by
p
otentiome-
efficie n cy . Th e positive- and negative-pea k sign al levels ter
R
479, and C479 reduces trigger
j
itter by increasing
p
osi-
stored by C414 a n d C415 are near t h e pea k levels of th e tive feedback at
h igher frequ encies . T he setting of
R 479
trigger signal . Am p lifiers U426A a n d U 426B are configured determines the circuit
h
ysteresis.
as voltage followers with transistors 0428 an d Q429 in the
feedbac k loops. Th ese transistors thermally compensate for
Q420 and 0421 and level shift the am plifier outpu ts
b ac k to Wh en TV Field is not selected, the TV Trig Ena ble line
the original do levels of t h e input trigger signals . Th e out p ut connected to
R
402 and
R
473 is
L
O . Transistors 0402 and
of U426A will b e th e positive p ea k voltage of the input trig- Q403 are biased off an d α
L
O is
p lace d on one inp ut of
ger sign al and th e out p ut of U 426B will
b
e the
n
egative
p
ea k U480A by
R
474. This
L
O in put will cause U 480A to invert
voltage. Potentiometers
R
434 and
R
435 adjust for do off- the outpu t from
U 480D . Wit h Q403 off, α
L
O will b
e placed
sets in the trigger ci rcuit ry. on one input of U480B by
R
405 and U480B will also act as
an inve rter. The Α Trigger signal at the outpu t of
U 480 B is
therefore the same as the input signal to
U 480A .
In t h e Norm mode, +8 .6 V is app lied to the
j
unction of
R
411 and
R
414. Diod e C R414 is forward
b
iased, turning on
Q413 which forward
b
iases CR415. Input transistors Q420
Wh en TV Field is selected, t h e TV Trig E nable line is HI .
an d Q421 are then biased off and no trigger signals will The outputs of U480D will
d
etermi ne the co nd uction states
reac h
t
h e Α Trigger
L
evel ci r cuit . Th e inputs and outputs of of 0402 and Q403, an d
t
he i np ut of U 480A con n ected to
U 426A and
U
426 B will then be fixed voltages and indepen-
R
473 will
b
e HI . The output of
U 480A will be
L
O and
U 480 B
dent of trigger-sig nal am plitude .
will invert t h e signal at its ot h er input. Signals at the collec-
tor of Q403 are filtered b y C408,
R
405, and C405 to
r
eject
TV video information and average the TV horizontal-sync
Α Trigge r Level Comparatoι
p ulses. Setti ng t h e trigger-level th res h old near the center of
t h e
h
orizontal-sync-pulse swing establis h es th e untriggered
The Α Trigger
L
evel Comparator compares signals se-
level. Wh en t h e TV vertical-sync bloc k occurs, the o utpu t of
lected by t h e Α TRIGGER SO UR CE switch to α voltage set
t h e filter app lied to U 480B pin 7 rises to α level that will
b y th e Α TR IGG ER LEVEL co ntrol . Positive or negative
cause t h e Sc hmitt Trigger circuit to switch . Precise TV fiel d
slope trigge ring is selected
b
y the Α TRIGGER S LO PE
sy n c h ro n ization is obtained as α result of this filtering action.
switc h.
The Α Trigger signal outpu t will b e t h e in ve r se of t h e filtered
Α Trigge r L evel Ci rcuit
signal appearing at U480B p in 7.
Theo ry of Operation-2235 Service
Tran sistors U 460B and U 460E compare the wi per volt-
age on the Α TRIGGER LEVEL control to the input trigger
signal, and the transistor with
t
he
h
ig her
b
ase voltage will Α
S
WEEP
G
ENER ATOR
A
ND
L
OGIC
conduct more of the available emitter current. The output
collector currents su pply emitter current to two transistor
The Α Swee p Ge n erato r and
L
ogic circuitry, shown on
p
airs whic h serve as cross-wired switches that are
b
iased Diagram 4,
p roduces α li nea r voltage
r
amp th at is amplified
on or off by the Α TRIGGER S LOPE switch. Wh en S464 is by the
H orizontal Am plifie r to provide
h
orizontal deflectio n
set to the
p
ositive slope
p
osition,
U
460C and U460F are
of the crt beam . The Sweep Gene r ato r circuits also produce