HP 120A - Page 40

Sect, in Page 6
pulse is also ac-coupled into this supply to unblank the
cathode-ray tube quickly as is explained under the
Gate Inverter part of the section on the Sweep Genera 
tor.
The other secondary is connected to the cathode of the
Cathode-ray Tube. It is also connected to the regula 
tor resistor string. Voltage is taken from this string
and fed into the input of a two tube de-coupled regu 
lator (both sections of a 12AU7). The output of this
regulator is then fed back to the screen of the Hart 
ley oscillator in the proper phase to oppose any
change in the de-output of the cathode supply.
The negative unblanking pulse is direct-coupled to
this supply to keep the Cathode-ray Tube unblanked
for the duration of the sweep.
The resistance-capacitance values of this circuit are
fast enough so that in combination with the ac-coupled
positive gate the pattern will unblank quickly and re 
main on. The Intensity Modulation terminals are also
ac-coupled to the cathode of the Cathode-ray Tube.
A negative voltage input will brighten the trace while
a positive voltage of approximately twenty volts will
blank the Cathode-ray Tube from normal intensity.
3-8 CALIBRATOR
The Calibrator consists of two neon bulbs connected
to put out a square-wave with an amplitude of 60 mil 
livolts. When the VERTICAL SENSITIVITY control is
switched into the CALIBRATOR position this signal
is applied to the vertical amplifier input. The pattern
on the face of the Cathode-ray Tube should then be
adjusted to give a height of six centimeters (VER 
NIER in CALIBRATED). The oscilloscope will then
be calibrated within ±5% to the sensitivities engraved
on tlie front panel.
Operation of the calibrator is explained, with refer 
ence to the schematic of the Calibrator (on the Low
Voltage and Filament Supply schematic), as follows:
When the switch, S 2, is in the off position both
sides of the neons are at the same potential, so there
is no action.
When the switch is turned to the on position, the volt 
age at the top of the neons will try to go to + 380 volts
since no current is being drawn. I 301 will fire first
as the voltage across it will increase more rapidly
because it is returned to the negative supply. When
I 301 fires it will draw current through R 358. How
ever, the voltage across R 358 will build up slowly
because C 334 must be charged at the same time.
When this condenser allows the voltage drop across
R 358 to rise, the voltage at the top of the neons will
also rise. When this voltage rises to 70 volts (ap 
prox.) above ground, I 302 will fire and stay lit un 
til the voltage across C 334 discharges through R 358
to a voltage approximately 70 volts below the voltage
at the top of the neons. I 301 will now fire and the
action will repeat itself.
I 302 is thus alternately turned off and on at a rate
of about 400 cps. The output of the calibrator is taken
from the current passing through this neon. The out 
put is approximately a square wave which can be set
with R 365 to be of exactly 60 millivolts in amplitude.
When the output of tins calibrator is fed into the ver 
tical amplifier it will show a pattern six centimeters
in height when the amplifier is calibrated.