Tektronix 2445, Инструкция по эксплуатации

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Theory of Operation—2445 Service
The ac waveform, now carrying both the grid-bias
information 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 w ithin U1830. Capacitor C (in
Figure 3-10), 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 o f 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 resistor F and R1922.
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 d iffer­
ence 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
negative excusion 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 o f charge 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 o f the control grid w ith
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 w ill 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 o f 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 o f the Z-Axis drive signal (VZOUT) produces a
control grid bias that barely shuts o ff 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 w ill be replaced
on the negative transitions.
The fast-rise and fast-fall transitions o f the Z-Axis signal
are coupled to the crt control grid through capacitor D.
This ac-coupled fast-path signal quickly sends the crt
electron beam to the new intensity level, then the slower
DC Restorer path "cathes u p" 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
elements o f the crt. The control circuitry is divided into
tw o separate catagories: (1) level setting and (2) signal
handling. The level-setting circuitry produces voltages and
current levels necessary fo r the crt to operate, while the
signal-handling portion is associated w ith changing crt
signal 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, w ith 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 over­
all 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 o f R1891 and R1990 and the
+10-V reference applied to R1990. This configuration
causes the tw o voltages applied to the third quadrapole lens
to track each other over the entire range of Edge Focus
adjustment pot R1864.
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