Theory of Operation—2465B/2467B Service
The ac waveform, now carrying both the grid-bias infor-
mation and the Z-Axis drive information, is applied to a DC
Restorer circuit in the High Voltage Module where it is
raised to the high-voltage levels of the crt control
grid.
DC RESTORATION. The DC Restorer circuit in the
High Voltage Module is referenced to the crt cathode
voltage via a connection within U1830. Capacitor C (in
Figure 3-9), connected to pin 15 of U1830, initially charges
to a level determined by the difference between the Z-Axis
signal level and the crt cathode potential. The Z-Axis
signal sets the level on the positive plate of capacitor C
through R1920, CR1930, and
R1941;
the level on the
negative plate is set by the crt cathode voltage through
resistor E and diode A. Capacitor D is charged to a similar
dc level through resistors F, R1922, and R1913.
When the ac waveform applied to pin 15 begins its
transition from the lower clamped level (set by the Z-Axis
signal) towards the upper clamped level (set by the Grid
Bias potentiometer), the charge on capacitor C increases.
The additional charge is proportional to the voltage
difference between the two clamped voltage levels.
When the ac waveform begins its transition from the
upper clamped level back to the lower clamped level, diode
A becomes reverse biased. Diode B becomes forward
biased,
and an additional charge proportional to the nega-
tive excursion of the ac waveform (difference between the
upper clamped level and the lower clamped level) is added
to capacitor D through diode B and resistor G. The
amount of change added to capacitor D depends on the
setting of the front-panel INTENSITY control, as it sets the
lower clamping level of the ac waveform. This added
charge determines the potential of the control grid with
respect to the crt cathode.
The potential difference between the control grid and
the cathode controls the beam current and thus the
display intensity. With no Z-Axis signal applied
(INTENSITY control off), capacitor D will be charged to its
maximum negative value, since the difference between the
two clamped voltage levels is at its maximum value. This
is the minimum intensity condition and reflects the setting
of the Grid Bias potentiometer. During calibration, the Grid
Bias pot is adjusted so that the difference between the
upper clamping level (set by the Grid Bias pot) and the "no
signal"
level of the Z-Axis drive signal (VZ OUT) produces
a control grid bias that barely shuts off the crt electron
beam.
As the INTENSITY control is advanced, the amplitude
of the square-wave Z-Axis signal increases accordingly.
This increased signal amplitude decreases the difference
between the upper and lower clamped levels of the ac
waveform, and less charge is added to capacitor D. The
decreased voltage across capacitor D decreases the
potential difference between the control grid and the
cathode, and more crt beam current is allowed to flow.
Increased beam current increases the crt display intensity.
During the periods that capacitor C is charging and
discharging, the control-grid voltage is held stable by the
long-time-constant discharge path of capacitor D through
resistor F. Any charge removed from capacitor D during
the positive transitions of the ac waveform will be replaced
on the negative transitions.
The fast-rise and fast-fall transitions of the Z-Axis
sig-
nal are coupled to the crt control grid through capacitor D.
This ac-coupled fast-path signal quickly sends the crt elec-
tron beam to the new intensity level, then the slower DC
Restorer path "catches up" to handle the dc and low-
frequency components of the Z-Axis drive signal.
Neon lamps DS90 and DS91 prevent arcing inside the
crt should the control grid potential or cathode potential be
lost for any reason.
CRT Control Circuits
The CRT Control circuits provide the various potentials
and signal attenuation factors that set up the electrical ele-
ments of the crt. The control circuitry is divided into two
separate categories: (1) level setting and (2) signal
han-
dling.
The level setting circuitry produces voltages and
current level necessary for the crt to operate, while the
signal-handling portion is associated with changing crt
sig-
nal levels.
LEVEL-SETTING CIRCUITRY. Operational amplifier
U1890B, transistor Q1980, and associated components
form an edge-focus circuit that sets the voltages on the
elements of the third quadrapole lens. The positive lens
element is set to its operating potential by Edge Focus
adjustment pot R1864 (via R1897). This voltage is also
divided by R1893 and R1982 and applied to the non-
inverting input of U1890B to control the voltage on the
other element of the lens.
The operational amplifier and transistor are configured
as a feedback amplifier, with R1891 and R1990 setting the
stage
gain.
Gain of the amplifier is equal to the attenuation
factor of divider network R1893 and R1892, so total
overall gain of the stage from the wiper of R1864 to the
collector of Q1890 is unity. The offset voltage between
lens elements is set by the ratio of R1891 and R1990 and
the +10 V reference applied to R1990. This configuration
causes the two voltages applied to the third quadrapole
lens to track each other over the entire range of Edge
Focus adjustment pot R1864.
3a-39
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