Tektronix 2213 - Page 52

voltage on C957 . This voltage will bias
0940 and Q942 to α
level that will maintain the peak-to-peak input
voltage of
Τ 940 . T he amplitude of the voltage across the transformer
primary winding and thus, that of the
secondary voltages of
Τ 940, is set by adjusting -8 .6 V A dj potentiometer
R946 .
At turn on, Q948 is b iased off and Q954 is b iased
on . All
t he current of the error amplifier will therefore go through
Q954 to bias on Q956 . Diode CR956 allows the
b ase of
Q956 to go p ositive enough to initially turn
on
Q940
or
Q942 . The current through Q956 controls the b ase drive for
0940 and Q942 . B ase current provided by b ase-drive trans-
former Τ
942 will charge C957 n egative with respect to the
Inverter circuit floating ground (common) level .
NOTE
Th
e following paragrap h applies only to inst r uments
having t he C urr e n t L imit
b oar d .
V oltage from CR940
and CR942 also p rovides α
measurement of the minimum
collector voltage of Q940 and
Q942 with respect to the
Inverter circuit floating ground .
This voltage is fed b ack to the Preregulator through
o ptical
isolator U 931 to control the
out
p
ut voltage fro m t he
P reregulator circuit . As the n egative p eak
voltage at the
collectors of the switching transistors is regulated
by
the
error amplifier with respect to the output of the Preregulator,
control of the do level from the p reregulator will control the
minimum voltage with respect to the floating ground .
P otentiometer R 952 (Head R oom V oltage Adjust) is use d to
set this minimum voltage level to α point that prevents
saturation and excessive power d issipation of the Inverter
switching
transistors .
C RT Supply
H igh-voltage multiplier U 990 utilizes the 2-kV wind ing of
Τ 940 to generate 8 kV at one output to drive the crt anode .
It also uses an internal h alf-wave rectifier diode to produce
-2 kV
for
the crt cathode .
The
-2
kV
supply is
filtered
by α
t hree-stage low-pass filter compose d of C990, R 992, R 990,
C992, R 994, and R 995 . N eon lamp DS870 p rotects against
excessive voltage between the crt h eater and ε rt cathode b y
con d ucting if the voltage exceeds approximately 75 V .
Auto F ocus Circuit
Focus voltage is also develope d from the
-2
kV
supply
via α voltage divider composed of R 884, R 882,
AUTO
F OCUS potentiometer R
883,
R 881, R 880,
R 879, R 878,
R 872, Auto F ocus Adjust p otentiometer R
875, and 0877 .
The focus voltage tracks the intensity level through t
he
action of Q877 . The Intens L evel signal from the
Auto
Intensity circuit (Diagram
6) is applied to
the
emitter of Q877
through R877 . When the Intens L evel signal
changes due to
α changing display intensity, the current through t he divider
resistors changes proportionally . Auto F ocus
Adjust
potentiometer R 875 is adjusted to p roduce the best focus
tracking .
REVNOV 1982
L ow-Voltage Sup p lies
DC R estorer
Theory of Operation-2213 Service
The low-voltage supplies u tilize the secon d
ary
windings
of Τ 940 and are all full-wave, center-tapped b ri d ges . The
+100 V s upply u ses CR961 and C R963 for rectification and
u ses C961 for filtering . Diodes CR965 and C R967 rectify ac
from taps on the 100-V winding, an d C965 filters the
output
to p roduce +30 V dc . The diode bridge consisting of CR971
through C R974 produces the
+8
.6 V a nd -8 .6 V supplies .
F iltering
of
the
+8 .6 V is accomplished by C971, C975, and
L971 ; while filtering of the
-8 .6
V
is done b y C972, C976,
and L 972 . V
oltage
r
egulator U 985 uses the rectified
+8
.6-V
s upply
to produce the +5 V out p ut . Diode CR985 protects
the regulator by not allowing the
output voltage to gomore
p ositive than the
+8 .6
V input
voltage .
The DC R estorer
circuit produces the crt control-grid
bias andcou p les both do an d
low-frequency components of
the Ζ -Axis Amplifier o utput to the crt control gri
d . Direct
coupling of the Ζ -Axis Amplifier out p ut to the crt
control grid
is n ot employed d ue to the h igh potential
d ifferences
involved . R efer to F igure 3-8 during the following d iscussion .
The ac drive to the DC R estorer circuit is obtained f rom
p in
16 of Τ 940 . The drive voltage h as α peak amplitude of
about
150
V
and α frequency of about 20 kHz . The
sinusoidal
drive
voltage is coupled through C863 and R863
into the
DC
R
estorer circuit at the j unction of CR860,
CR863, and R864 . The
cathode end of CR860 is h eld at
about +85 V by the voltage
applied from the wiper of Grid
B ias p otentiometer R 860 . When the
p ositive p eaks of the
ac-drive voltage reach α level that
forward b iases CR860,
the voltage is clamped
at that level .
The Ζ -Axis Amplifier output-signal voltage
is applie d to
the DC R estorer at the anode end of
CR863 . The Ζ -Axis
signal voltage level varies b etween +10V and
+75V,
depending on the setting of the AUTO INTENSITY
control
.
The ac-drive voltage will h old C R863 reverse b
iased
until
the
voltage falls below the Ζ -Axis Amplifier output voltage level .
At that p oint, CR863 b ecomes forward biased and clamps
the j unction of CR860, CR863, and R864 to the Ζ -Axis
output level . Thus, the ac-drive voltage is clampe d at two
levels on the positive swing of the cycle to produce
an
approximate square-wave signal with α positive dc-offset
level .
The DC R estorer
is referenced to the
-2 kV crt cathode
voltage through R867
and C R867 . Initially,
both C865 and
C864 will charge
up to α level determined
b y the difference
b etween the Ζ -Axis
output voltage and
cathode voltage .
Capacitor C865 charges
from the
crt cathode through
R867,
CR867, CR868, and R
865 to the Ζ -Axis
output .
Capacitor C864 charges
through R867,
CR867, R 864,
and
CR863
to the Ζ -Axis output .
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