Theory of Operation—2465B/2467B Service
Other adjustable level-setting circuits include Y-Axis
Alignment pot R1848, used to rotate the beam alignment
after vertical deflection. This adjustment controls the
amount of current through the Y-Axis Alignment coil
around the neck of the crt and is set to produce precise
perpendicular alignment between x- and y-axis deflections.
The TRACE ROTATION adjustment R975 is a front-panel
screwdriver-adjustable control. The effect of the adjust-
ment is similar to the Y-Axis Alignment pot, but when
adjusted,
it rotates both the x-axis and the y-axis
deflections of the trace on the face of the crt. A final ad-
justable level-setting control is the Geometry pot R1870,
adjusted to optimize display geometry. The potential at pin
8 for the vertical shield internal to the crt is produced by
zener diode VR1891 and associated components.
SIGNAL-HANDLING CIRCUITRY. The crt termination
adjustment R1501 is set to match the loading
characteristics of the crt's vertical deflection structure to
the Vertical Output Amplifier.
HIGH VOLTAGE POWER SUPPLY AND
MCP-CRT FOR 2467B ONLY
The High-Voltage Supply and CRT circuit, diagram
<8> 2467B, provides to the MCP-CRT (Micro-Channel
Plate Cathode-Ray-Tube) the high voltage levels and
necessary control circuitry for proper operation. The
MCP-CRT produces high brightness on low rep-rate
tran-
sient waveforms while limiting the brightness of high-rep
rate waveforms.
The circuitry consists of the 2467B MCP-Cathode Ray
Tube,
MCP Bias Supply, High Voltage Oscillator, the
Cathode Supply, the High Voltage Regulator, the DC Re-
storer, the Anode Current Limiter and Multiplier, the Focus
Circuitry, and the various CRT Control circuits.
2467B MCP-CRT
The MCP-CRT has a Micro-Channel Plate element
added between the PDD Lens and CRT Screen to multiply
electrons, therefore boosting CRT performance. A low bias
voltage across this element causes the electron multiplica-
tion to be low. Raising the bias voltage across the Micro-
Channel Plate increases the multiplication of electrons
going through the MCP. This higher bias voltage increases
the MCP-CRT viewable writing rate a thousand times over
a conventional crt. Full intensity drive to the MCP-CRT
increases both the cathode current and the bias voltage
across the MCP electron multiplier.
MCP-Bias Supply
The MCP-Bias Supply provides a variable bias voltage
across the MCP (Micro-Channel Plate) element of the
CRT. The MCP Bias Supply voltage is set by Intensity
control information (DIR input voltage) and MCP Bias
con-
trol R4365. As the Intensity control voltage is increased
from minimum to maximum the MCP Bias Supply also
increases from minimum to maximum. When the DIR input
is between 0 to +2.5 V the MCP Bias stays at its
minimum voltage. When the DIR input is varied between
+ 2.5 V to +5 V maximum the MCP Bias voltage linearly
follows the DIR input voltage and increases by about 400
V.
MCP-BIAS-SUPPLY VOLTAGE REGULATOR. The
MCP-Bias-Supply Voltage Regulator consists of non-
inverting operational amplifier U4367B and associated
components. The regulator monitors the MCP-Bias-Supply
output voltage at Test Point 4301 and varies the bias point
of switching transistor Q4460 to hold the MCP-Bias-
Supply DC voltage in regulation.
When the MCP-Bias-Supply output voltage is at the
proper level, the sum of the currents through R4377 (MCP
Bias),
R4378 (intensity control, DIR), and R4380 (feedback
resistor) hold the voltage developed across C4377 at zero
volts.
This balance condition sets base drive to Q4460 via
regulator U4367B. Varying the base drive to Q4460 holds
the rectified and filtered secondary voltage in regulation.
If the MCP-Bias-Supply output voltage level (T4480 pin
14) is too negative, a slightly negative voltage will develop
across C4377. This voltage causes the output of regulator
U4367B to move negative. The negative shift charges
capacitor C4470 to a different level, around which the
induced feedback voltage at the base-drive winding will
swing.
The added negative bias causes Q4460 to turn on
earlier in the oscillation cycle, causing a stronger induced
current pulse in the secondary winding. The increased
current in the secondary winding increases (makes less
negative) the secondary voitage (T4480 pin 14) until the
MCP-Bias-Supply output voltage returns to the balanced
condition (zero volts across C4377). Opposite action
occurs if the MCP-Bias-Supply output voltage is too
posi-
tive.
Intensity of the MCP Bias Supply is controlled by
U4367A and associated components. Operational amplifier
integrator U4367A has a DC gain of -4. The input is
offset through R4461 to cause the Output voltage to be
Zero volts when the DIR input is at +2.5 Volts (output
range is ±10 V). Only the negative voltage out of U4367A,
through CR4374 and R4378, changes the input current to
regulator U4367B. This negative voltage is amplified and
inverted by regulator U4367B, oscillator Q4460, and
transformer T4460, increasing the MCP-Bias supply output
voltage up to 400 Volts.
3a-40
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