voltage will be returned to its established value
of + 100 volts. C671 improves the response of
the circuit to sudden changes in the output volt
age.
A small sample of the unregulated bus ripple
appears at the screen of V666B through R662.
The ripple signal which appears at the screen
(which acts as an injector grid) will produce a
ripple component at the grid of V677 which will
be opposite in polarity to the ripple appearing
at the plate of V677. This tends to cancel the
ripple at the cathode of the tube, and hence re
duces the ripple on the + 1 00-volt bus. This
same circuit also improves the regulation of the
supply in the presence of line voltage variations.
The +350-volt supply functions in the same
manner as the + 100-volt supply. Rectified volt
age from the cathode of V722 is added to the
voltage supplying the + 100-volt regulator to
supply voltage for the -f-350-volt regulator. As
mentioned previously, the -|-350-volt supply also
furnishes an unregulated output of about +485
volts for the crt high-voltage supply.
The —6.2-volt supply works in essentially the
same manner as the vacuum-tube supplies. The
divider R648-R649 establishes a reference voltage
at the base of V644. If we now assume that the
output tends to go more negative, the emitter
of V644 will also go more negative since it is
strapped directly to the output. The collector of
V644 will then go more positive, carrying with it
the base of V634. The collector of V634 will
then go up, carrying with it the base of V647.
The series-regulator transistor V647 is essentially
an emitter-follower, so the emitter will follow
the base. Hence, the emitter of V647 also goes
up. This increase in the voltage at the emitter of
V647 will be coupled through the rectifier SR642
back to the output and will thus pull the output
back up to its specified value.
CRT CIRCUIT
A single 60-Kc Hartley oscillator circuit fur
nishes energy for the two power supplies that
provide accelerating voltages for the crt. The
main components of the Oscillator circuit are
V800 and a portion of the primary of T801 tuned
by C807.
The two half-wave rectifier circuits employ
capacitor-input filters. Separate supplies are re
quired for the grid and cathode circuits of the
crt in order to provide dc-coupled unblanking to
the crt grids.
V822 supplies about —2900 volts for the cath
odes of the crt. V862 supplies about —3100 volts
for the grids (the actual voltage depends on the
setting of the INTENSITY control.)
In order to maintain a constant deflection
sensitivity in the crt, and thereby maintain the
calibration
of the instrumentf it is necessary that
the accelerating potentials in the crt remain con
stant. This accomplished by regulating the two
supplies by comparing a sample of the cathode
voltage to the regulated —150-volt supply. This
sample voltage, obtained from the arm of the
H.V. ADJ. control R826, is applied to the grid of
V814A; the cathode of this tube is connected to
the —150-volt regulated supply. The error volt
age is amplified by V814A and V814B; the out
put of V814B varies the screen voltage of the
Oscillator tube and thus controls its output.
Unblanking
As mentioned previously, dc-coupled unblank
ing is accomplished by employing separate high-
voltage supplies for the grids and cathodes. The
cathode supply is tied to the +350-volt supply.
The grid supply, on the other hand, is not tied
to any other supply and is therefore floating.
The unblanking pulses from the Time-Base Gener
ator are transmitted to the grids of the crt via
the floating grid supply.
The stray capacitance in the circuit makes it
difficult to move the floating supply fast enough
to unblank the crt in the required time. To over
come this, an
isolation network composed of
R864, C868 and R868 for one grid circuit, and
R864, C866 and R866 for the other, is employed.
By this arrangement, the fast leading edge of
the unblanking pulse is coupled directly to the
grids of the crt via C866 and C868. For short-
duration unblanking pulses (at the faster sweep
rates) the power supply itself is not appreciably
moved. For longer unblanking pulses, at the
4-10
CIRCUIT DESCRIPTION — TYPE 502
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