Tektronix 2213 - Page 47

T heo r y of O pe r ation-2213 Service
in p ut current i n α manne r t h at com p lements t h e nonlinear
trio d e ch aracte r istics of t he crt . T h is ou t p ut voltage is
a p plie d to t he Ζ -Axis Am p lifier via the Ζ -Axis Gati
ng
ci r cuit . T h e Intens L evel sig n al is also applie d to t
h
e
F
ocu s
circuit (Diagram 9) for use i n focus trac k ing of t he inte n sity
level c han ges .
T he intensity of t he d isplay is allowe d to re duce to zero
t h ro ug h t h e action of CR 809, VR 809, and R 809 . W it hou t
t h is circ u it, t he Auto Intensity circuit woul d not
allow the
i n tensity to go to zero w h en th e A U TO I N T EN SITY
co
n t r ol is set to min im u m i n tensity .
Ζ -Axis Am p lifier
T he Ζ -Axis Am p lifier controls t he crt intensity level
via several
in
pu t-signal
sources . T
he effect
of t
h ese in p
ut
sig n als is eit h er to increase o r d ecrease trace intensity o r to
completely blan k portions of t h e dis p lay . T h e Ζ -Drive
signal current an d t he inp u t current fr om t h e Z-AXIS
I NPU T connector (if in u se) are summed at t he emitter of
common- b ase am p lifier transistor Q841 . T
h
e algebraic sum
of th ese signals determines t
h
e
collector current of
Q841 .
In pu t transistor Q841 p rovides α low-impedance
termina-
tion
fo
r t h e i n p ut signals
an
d isolates t h e signal sources from
followi n g
stages
of
t
he Ζ -Axis
Am
p lifie r .
Signal cur r ent fro m Q841 flows t h roug h CR 844 an d
d evelo p s α signal voltage d ro p across R 844 . I n creasing
cu rrent t h roug h Q841 re d uces t h e forwar d bias of Q844,
t h ereby reduci ng t he current t hr oug h Q844 . T h is action
causes t h e collector voltage of Q844 to go more n egative
(towar d t he -8 .6 V su pp ly) and increases the forwar d b ias
on emitter-follower Q845 . As emitte r cur r ent of Q845
increases, negative-going voltage develo p e d across R847 is
a p plied to t h e b ases of com p lementary- p air ou t pu t tran-
sistors Q847 an d Q850 . P ositive transistions of t he Ζ -Axis
signal are couple d to t he b ase of Q850 via C852 . T he fast-
rise transitio n s are am p lified by Q850 to s p ee d up t he
res p onse time . F or negative transitions of t h e Ζ -Axis signal,
as well as for do an d low-frequency signal com p o n ents,
Q847 acts as t h e amplifier, with
Q850
s upp lying t
he
cu
rrent .
Dio d e C R 856 p revents t he Ζ -Axis outp ut signal from
goi ng negative, and n eon lamps DS854 and DS856
p
rovide
protection to t h e Ζ -Axis Am p lifier i n t h e event of hig h -
voltage arcing in t h e crt .
T
h
e am
p
lifier gai n with respect to t
h
e
Ζ
-Drive current is
set to about 10 b y t h
e
negative feed b ac k
supp
lied from t h e
collectors of Q847 an d Q850 to t he b ase of Q845 via
fee d b ac k resistor R 846 . T he gain with res p ect to t h e
external Ζ -Axis I nput signal is h eld to a bout t hree by R 801,
R 802, and R 803 in se r ies with t
h
e exte r nal i nput signal .
Dio d es C R 801 an d C R 802 p rovi de p r otectio n for t he
3-
2
0
Ζ -Axis Amplifier in case of an acci d ental a pp licatio n of
excessive signal amplitu d e to t he Z-AXIS I NPU T connector .
Wh en C H O P VER TICA L M OD E is selecte d , t
h e
C
h
op
B la nk signal is ap p lied to t h e collector of 0841 d uring t h e
d isplay switc h ing time . Signal current is s h unted away from
CR 844, and t he forward bias of Q844 inc r eases to t he
b lan k ing level . Wh en b lan k ed, t h e ou t p ut of t h e Ζ -Axis
Amplifier drop s to about +10 V, an d t he crt b eam curre n t
is r e duced to below viewing i n tensity to eliminate chop
switc h ing transients from t he dis p lay .
F or
an
Χ - Υ d isplay, t he Swee p Ζ -D r ive signal cu r rent is
switc h ed off . Wh en t h e ΧΥ signal is L
O,
C R 837 is forwar d
b iased and Intens L evel current flows t h roug h R 837 to
Ζ -Axis Amplifier tra n sistor Q841 to establis h t h e d is p lay
intensity .
T h e last inp u t to the Ζ -Axis Am p lifier is t h e B eam
F i nd
curre n t . N ormally, BEAM F I ND switc h S390 is close d , an d
-8 .6 V is supp lie d to t he base bias network of Q841 an d
Q844
.
Wh
en
t
he
BEAM
F I N
D
switc h is
ope
n ed , t he
-8 .6
V
is
removed ,
an
d t he bias voltage becomes more p ositive .
Transistor
Q841 b ecomes mo r e forwar d b iase d w h ile Q844
b ecomes muc h less forwa r d b iase d . T h e current t h roug h
Q844 is re duced , an d t h e base b ias voltage of Q845 is
t h ere by increased . T he ou tput of Q845 t h en goes to α level
t h at p ro d uces α fixe d , pre d etermi ned Ζ -Axis o u t p ut signal
level . T h us neit h er t he A U TO I N T EN SITY control nor t h e
Ζ -Drive signal have any con trol over t h e intensity level
of t he crt display w h enever the BEAM F I ND pus h button
is p resse d in, a
n
d α b
r ig h t trace (or
d ot if no sweep is
prese
n
t) will be d is
p
laye d
.
H orizontal P ream p lifier
H O R IZO N TA L
T he H orizo n tal Amplifier ci r cuit, s h ow n on Diagram 7,
provides t he output signals t h at drive t he h orizontal crt
d eflection p lates . Signals a pp lied to t h e H orizontal P re-
amplifier can come f r om eit h er t h e M iller Swee p Ge n erato r
(for sweep d eflection) or
from
t
h
e ΧΥ
Amplifier
(wh en
Χ - Υ d is p lay mo d e is selecte d ) . See
F ig
u re 3-7 for α detaile d
b loc k diagram of t h e H orizontal Amplifier circ u it .
T he H orizontal P OSITIO N control, Χ 10 magnifier
circuitry, and t he h orizontal portion of t he beam finder
circuitry are also containe d i n t h e H orizo n tal Amplifier
circuit .
T h e sum of t he sweep an d positioning current is applie d
to t he in pu t of one side of α
differe n
tial am p
lifier
compose d of Q730 an d Q731 . F or all conditions ot
h
er t h a n