Tektronix 434, Instruction Manual

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C irc u it D e s c rip tio n —434
signals present at the a m p lifie r inputs in to c u rre n t varia­
tio n s to drive the o u tp u t p o rtio n o f the U nblanking
A m p lifie r. C R T in te n s ity variations are accomplished in the
fo llo w in g manner.
When a sweep starts, the DC level present at pin 2 o f
U 9 4 0 A steps negative, resulting in a reduced c u rre n t flo w
fro m pin 9 o f U 9 4 0 A . T his re duction in c u rre n t increases
the fo rw a rd bias on Q 954 and reduces the fo rw a rd bias on
Q 955. Q 954 conducts harder, resulting in a positive m ove­
m e n t at the e m itte r o f Q 954. T his positive m ovem ent is
coupled th ro u g h C942 to the e m itte r o f Q 940 and starts a
positive m ovem ent a t the c o lle c to r o f Q 940. A t the same
tim e th a t this is occurring, Q 955 is decreasing its co n d u c­
tio n through R 955 and Q 950, w h ich causes a positive
m ovem ent to occur at the c o lle c to r o f Q 950, w hich
reinforces and sustains the positive m ovem ent started by
Q 940. A t the end o f the sweep, co n d itio n s are reversed.
The DC level at pin 2 o f U 9 4 0 A steps positive, causing
increased c u rre n t flo w fro m pin 9 o f U 9 4 0 A and the c o lle c­
tors o f Q 940 and Q 950 move negative. The variations in DC
level at th e collectors o f Q 940 and Q 950 are connected to
the DC Restorer c irc u it.
T o reduce the p o s s ib ility o f burn in g the CRT phosphor,
the e m itte r voltage o f Q 940 is changed by the T IM E /D IV
sw itch , S 700A. In the 50 m s /D IV p o sitio n and up, the
T IM E /D IV sw itch applies +115 vo lts to the ju n c tio n of
R 943 and V R 9 4 4 . In all sw itch positions below 50 m s /D IV ,
+ 115 volts is no longer connected to the ju n c tio n o f R 943
and V R 9 4 4 . Instead, the e m itte r o f Q 940 is returned to
+ 115 volts th ro u g h
R 942, R 943, and V R 9 4 4 . This
produces a less positive level a t the e m itte r o f Q 940 to lim it
th e c o lle c to r level o f Q 940 and Q 950 w h ich protects the
C RT phosphor in these switch positions.
DC Restorer C irc u it
Q 924 and Q 925 fo rm a com plem entary a m p lifie r, each
side o f w h ich conducts on a h a lf cycle o f the w aveform
coupled through T 9 2 0 fro m the secondary o f the power
supply tra n sfo rm e r. The voltage on the c o lle c to r o f Q 924 is
determ ined by the o u tp u t o f the U nblanking A m p lifie r. On
the positive h a lf cycle, Q 924 saturates and the com m on
em itte rs seek the Q924 c o lle c to r voltage. On the negative
h a lf cycle, Q 925 saturates and the com m on em itters go to
ground p o te n tia l.
C 924, C 925, C R 926, C R 928, and R 928 fo rm a DC
restorer n e tw o rk . A ll DC voltage levels in th is c irc u it are
referenced to the negative high-voltage p o te n tia l. The
voltage diffe re n ce across R 928 a p p ro x im a te ly equals the
voltage swing present on the com m on em itters o f Q924 and
Q 925. The upper end o f R 928 is more positive than the
lo w e r end, b u t negative w ith respect to th e C R T cathode.
V a ryin g the drive signal to the U nblanking A m p lifie r
changes the voltage level at the a m p lifie r o u tp u t, w h ich in
tu rn varies the am p litu d e o f the voltage swing on the
com m on em itters o f Q924 and 0 9 2 5 , thereby resulting in
changes to the CRT bias. CR924 and CR925 lim it the
positive excursion th a t can occur at the c o lle c to r o f Q 924
and th e com m on em itters o f Q 924 and Q 925 respectively.
C R T G rid Bias adjustm ent R 980 determ ines the level o f the
range o f adjustm ent o f the IN T E N S IT Y c o n tro l. C R935
provides p ro te c tio n to the C R T if the grid tries to rise more
positive than the cathode.
High Voltage Regulator
Feedback fro m the high-voltage c irc u it is connected to
pin 20 o f integrated c irc u it U 940B. A n y change in this
sample o f the high-voltage level produces an in-phase error
signal at pin 17 o f U940B w hich is a m p lifie d and inverted
th ro u g h Q 989, Q 990, and G 995. R egulation occurs as
fo llo w s :
If the negative high-voltage level attem pts to go positive
(less negative), a sample o f th is positive-going voltage is
applied to p in 20 o f U940B. This results in a positive-going
e rro r signal at pin 17 o f U940B w hich is applied to the base
o f Q 989 through C R 982. Q 989 am plifies and inverts this
e rro r signal, w h ich receives fu rth e r a m p lific a tio n through
Q 990 and Q 995. The resulting negative m ovem ent a t the
c o lle c to r o f Q 995 is applied to the positive pole o f the
m inus high-voltage supply, and is o f s u ffic ie n t a m plitude to
m aintain the o u tp u t level o f the supply.
V R 9 8 5 through V R 9 9 0 prevent more than 900 volts
fro m appearing across Q 989, Q 990, and Q 995 to prevent
exceeding th e m axim um voltage rating o f these devices in
the event o f c irc u it fa ilu re . C R983 prevents the base o f
Q 989 fro m going more negative than a b o u t —0.5 v o lt.
R963 adjust the negative high-voltage pow er supply fo r the
proper o u tp u t voltage.
CRT C o n trol C ircuits
Focus of the C R T display is determ ined by the Focus
c o n tro l R82B. T his c o n tro l is pa rt o f voltage divider
R 962C, R 968, R 82B, R83, R 962D , R962E between the
negative high-voltage and ground. T h e re fo re , the voltage
applied to the focus grid is m ore positive (less negative)
than the voltage on e ither the c o n tro l grid o r cathode.
R 968 sets the range of adjustm ent o f the Focus c o n tro l.
The Focus co n tro l is ganged w ith the front-panel IN T E N ­
S IT Y c o n tro l, and is a u to m a tica lly adjusted when changes
are made in the IN T E N S IT Y setting. The A S T IG adjust­
m e n t R85, w h ich is also adjusted to o b ta in a w ell-defined
display, varies the positive level on the astigmatism grid.
G eom etry a d justm ent R 967 varies the positive level on the
h o riz o n ta l d e fle c tio n plate shield to c o n tro l the overall
geom etry o f the display. The T R A C E R O T A T IO N adjust­
m e n t R80 c o n tro ls the c u rre n t through L100, w h ich varies
the m agnetic fie ld around the C R T to h o riz o n ta lly align the
C R T trace.
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