Heathkit IO-4550 - Page 26

Page 24 [egg =rmarercerrs]
HIGH VOLTAGE
Diodes 0404 and D405, and capacitors C408, C409, C410,
and C411 comprise a voltage doubler that produces
approximately —2000-volts DC at nominal line voltage. The
Positive output of the doubler is connected to the collector
of pass transistor 0404/0405/Q0406. (The three transistors
are wired in series to form a single high voltage pass
transistor.) At nominal line voltage, the voltage at the
collector of 0404 is approximately +300 volts. The +300
volts and —2000 volts add together to produce the —1700
volts supplied to the cathode of the CRT. Control R409 is
adjusted for optimum voltage regulation, while resistor
R427 provides cathode current limiting. Divider resistor
string R423, R424, R3, R425, and R426 supply a reduced
voltage for the Focus control (R3). A current summing
junction at the end of the divider string adds the high
voltage current (approximately 340uA) to a reference
current supplied by the +15 volt supply through control
R419 and couples it to operational amplifier 1C401. Control
R419 is adjusted so that the reference current is equal to,
but opposite the high voltage (HV) current, when the high
voltage is at the correct level (—1700 volts).
If the high voltage falls below —1700 volts, the current
difference at 1C401 will cause 1C401 to increase the base
drive to 0404/0405/0406 and thus decrease the collector
voltage until the high voltage again equals —1700 volts. This
will again make the HV current equal to the reference
current. The reverse will occur when the high voltage
exceeds —1700 volts.
The frequency response of this circuit is high enough so that
it also operates as a filter to remove 120 Hertz ripple from
the high voltage. Upper frequency response is limited by
capacitor C414 to suppress any possible high frequency
oscillation in 1C401. Diode D406 protects the pass transistor
from a reverse output from 1C401. Resistors R411, R412,
and R413 insure equal voltage distribution between Q404,
2405, and Q406.
CRT BLANKING
The CRT blanking circuit is used to control the electron
beam in the CRT. This includes blanking the CRT during
“retrace” and “hold-off’, and trace intensity. To fully
understand this circuit, you must keep three ideas in mind:
1. The CRT is blanked when the control grid is 68 volts
more negative than the cathode.
2. As the 68-volt difference between the grid and
cathode is reduced the CRT is unblanked, and the
beam intensity is increased.
3. Since the cathode of the CRT is at —1700 volts, the
grid must vary between —1700 volts and —1768 volts.
Therefore, the blanking circuit must be completely
isolated from the other oscilloscope circuits.
CAUTION: When measuring voltages in the blanking
circuit keep in mind that “circuit common” is 1700
volts below oscilloscope ground.
Diodes
D402
and
ZD403,
capacitors
C401
and
C402,
and
resistor
R401
comprise
a
—68
volt
regulated
power
supply.
The
positive
end
of
the
supply
is
tied
to
the
—1700
volt
supply.
The
—68
volt
supply
powers
a
simple
flip-flop
(Q402,
Q403).
This
flip-flop
is
toggled
through
high
voltage
capacitors
(C403,
C406)
by
blanking
control
1C203A.
Because
of
the
capacitor
coupling
normal
ground-referenced
logic
levels
can
be
used
for
control.
Assume the flip-flop is toggled so that Q402 is off, and
Q403 is on (unblanked CRT condition). The cathode is near
—1700
volts
and
the
other
end
of
control
R2
is
at
—1768
volts
[—68
(+)
—1700].
As
the
Intensity
contro!
(R2)
is
turned
clockwise,
the
beam
intensity
will
increase
(grid
voltage approaches cathode voltage).
A blanking signal from IC203A (low-to-high logic transition
at C406 and high-to-low logic transition at C403) will toggle
the flip-flop and turn Q402 on and Q403 off. Now both
ends of contro! R2 are at —1768 volts, and the CRT will
blank. The reverse will occur when an unblanked signal from
1C203A is sensed. Resistors R404 and R405 hold the
flip-flop in a stable state after each toggle; while capacitors
C404 and C405 speed up the switching cycle. The RC
network, R407, R408, and C407, shape the blanking signal.
Resistor R408 also isolates the circuit.
CALIBRATOR
n oscillator circuit comprised of 1C208A and IC208D, and
their
associated
circuitry
components
generate
an
output
of
approximately
1000
Hz.
This
is
coupled
to
the
base
of
transistor
switch
Q209.
When
Q209
is
turned
off,
a
precise
1-volt level is connected to the output. Divider resistors
R236, R237, and R238 set the output level. When Q209 is
turned on, it grounds the output.
Q