b. High-Voltage Power Supply. Accelerating voltages for the cathode-ray tube are obtained by rectifying a 30-KHz high
3-5.1. Low Voltage Power Supply (Newer 765 MH, 765MH/F and 765MA)(Figs. 3-1.2, 3-1.3, and 6-1.1)
voltage produced by transistor oscillator Q2002. This transistor is connected as a class C oscillator, with the primary of
transformer T2001 tuned by the strayed capacitance and the reflected secondary impedance. The output of the oscillator is
a. Power Transformer. Low voltage and heater power for this instrument is provided by power transformer T1001. The
stepped up by T2001 and applied via a conventional solid-state tripler network consisting of CR2050 through CR2052, to the
primary is wound with two equal 115-volt windings that can be switched either in parallel for 115-volt operation, or in series for
post-deflection accelerator of the cathode-ray tube. The negative high-voltage supply is derived from a tap on the secondary
230-volt operation via the 115V/230V selector switch S101. The secondaries contain four separate windings which provide
winding of transformer T2001, which is connected to half-wave rectifier CR2001. The rectified output of -1440 volts is
power to the regulated supplies and two separate heater windings.
capacitively filtered by C2008, C2009, and C2011 and provides the necessary operating potentials for the proper intensity and
b. Regulated Supplies. Four regulated and two unregulated supplies are provided. All of the regulated supplies are of
focusing of the cathode-ray tube. Regulation of the high-voltage negative supply is accomplished by sampling a portion of the
the constant output voltage series-passing type.
rectified output and applying a correction signal back through HV regulator Q2000 to Q2002 to the base of HV oscillator
The basic reference for all of the regulated supplies is established by the fixed drop across zener diode CR1304. This
Q2003. This action changes the amplitude of oscillation of Q2003 and T2001 in such a manner as to restore the high-voltage
constant drop or reference voltage is applied to the base of Q1302, which is one of the inputs to differential amplifier Q1301
output to its present level. Variable resistor R2009R, HV ADJ, is provided to preset the output of the high-voltage supply to its
and Q1302. The other input to the amplifier is connected across a divider consisting of R1302 through R1305 which samples
a portion of the -50V regulated output.
c. Normal Sweep Unblanking.
The cathode-ray tube used
in this oscilloscope has two additional beam deflection plates,
A portion of this sample voltage is tapped by the -50V ADJ variable resistor R1303, and is applied to the base of Q1301.
which cut off the electron beam independently of the control grid. In beam gate blanking, the electron beam is always turned
R1303 is adjusted so that the output is also the voltage reference source for the other low-voltage regulated supplies. Thus,
on and passes through an aperture in the center of a blanking shield. Behind this shield is a set of deflection plates, one of
voltage adjustments are always made in the sequence starting with the -50-volt supply and ending with the +200-volt supply.
which is connected to a fixed +50 volt source, while the second deflection plate is connected to the unblanking gate from the X
Since the operation of the low-voltage regulated supplies is similar, only the +50-volt supply is described. CR1201 and
plug-in. When the time base is sweeping, both beam gate deflection plates are at the same potential, and the electron beam
CR1202 are connected in a conventional full-wave rectifier circuit. The rectified output is capacitively filtered by C1201,
passes through the aperture in the blanking shield. When the sweep ends and the unblanking gate voltage increases, the
shunted by resistor R1202, and applied to the voltage regulator.
beam is deflected behind the blanking plate and the screen is dark. Beam control pulses (trace brightening and retrace
Operation of the regulator is as follows. Assume the output voltage tends to decrease.
blanking pulses) can easily be dc coupled since these beam deflection plates are electrically close to ground. Hence, the-
This will lower the base voltage of error detector dc amplifier Q1201, and raise the base voltage of emitter follower driver
display is visible only during the forward sweep interval and is blanked during the retrace and lockout interval.
d. Fast Single Sweep Unblanking.
A back of-panel slide switch
on the high-voltage and high-frequency main
The increase in voltage at the emitter of Q1202 is applied to the base of series-passing transistor Q1203. The resultant
frame models so that both NORMAL (beam gate blanking) and SINGLE SHOT (grid-cathode blanking) are available.
increase in voltage at the emitter of the pass transistor is such as to return the output voltage to its proper value.
Cathode drive for beam brightening is desirable when high writing rates are required to assure maximum beam current with
c. Voltage Calibrator.
The power-line frequency voltage from T1001
to the base
of voltage calibrator Q1051
minimum spot size. When it is desired to record high-speed, single-shot transients, the back-of-panel beam brightening
through limiting resistor R1051. Negative and positive voltage excursions will drive the transistor into and out of conduction at
switch is set to FAST SINGLE SWEEP ONLY. This technique completely turns off the electron beam, eliminating the
the line-frequency rate. The resulting square wave is applied to a voltage divider consisting of R1053 and R1054. The
possibility of film fogging and loss of information when the lens is open for an extended period.
attenuated calibrator output of one volt peak-to-peak is coupled to the amplifier plug-in when the VOLTS/DIV switch is set to
CAL. The plus 100-volt supply and the saturation voltage of the transistor will determine the peak-to-peak voltage excursion
3-5.3. CRT Circuits (765MA)(Fig. 6-1.3)
a. CRT Beam Control Circuits. FOCUS and ASTIGMATISM controls are provided for optimum spot shape and size
3-5.2. CRT Circuits (Newer 765MH and 765MH/F)(Fig. 6-1.2)
adjustment. The Pattern control adjusts the shape of a full raster display for minimum distortion. The BEAM ROTATE front
panel screwdriver adjust permits the trace to be perfectly aligned with the CRT internal graticule.
a. CRT Beam Control Circuits. INTENSITY control R2016, part of the negative high-voltage divider, is used to vary the
b. High-Voltage Power Supply. Q2005
is a power oscillator operating
at a frequency
40 kHz and
cathode-ray tube grid voltage to adjust the beam current. FOCUS control R2014 is provided to vary the voltage at the
controlled in amplitude by the error amplifier consisting of Q3001, Q3002 and Q3003. The error amplifier compares the
focusing anode to set the second cross-over point right at the screen of the cathode-ray tube. ASTIGmatism control R2022F
rectified and filtered high voltage with the +100 volt power supply, through appropriate divider resistors, to obtain a regulated
is provided to vary the voltage at the astigmatism anodes to focus the spot in both axes simultaneously. The PATTERN
-1150 volt power supply. This voltage is set exactly using R3026, H.V. ADJ. Identical bifilar windings on T2001 provide
CORRection service adjustment is set to vary the field the cathode-ray beam encounters as it emerges from the deflection
independent grid and cathode supplies for the CRT to allow a DC coupled Z-Axis.
system to control the linearity at the extremes of deflection. BEAM ROTATE front-panel screwdriver control R2022 is
provided to vary the current through coil L2001 around the neck of the CRT. The current varies the field strength which the
CRT beam intercepts, causing it to rotate. This compensates for the earth's magnetic effect on trace alignment.