Detailed Operating Information; Intensity Controls; Crt Phosphor Protection; Display Focus - Tektronix 7904 Instruction Manual

Operating Instructions--7904 (SN 8260000-UP)
8. Set the deflection factor of both amplifier units for
the desired display and center the display with the position
controls . The amplifier unit in the A HORIZ compartment
controls the horizontal deflection, and the unit in the LEFT
VERT compartment controls the vertical deflection .
Astigmatism Adjustment
If a well-defined display cannot be obtained with the
FOCUS control, adjust the ASTIG adjustment as follows:
®ETAILECI OPERATING INFORMATION
Intensity Controls
General. T'he 7904 has three separate intensity con-
trols. The A INTENSITY control determines the brightness
of the display produced by the plug-in unit in the A HORIZ
compartment. Likewise, the B INTENSITY control deter-
mines the brightness of the display produced by the plug-
in unit in the B HORIZ compartment. The READOUT
intensity control determines the brightness of only the
readout portion of the crt display.
NOTE
To check for proper setting of the ASTIG ad~usi-
ment, slowly turn the FOCUS control through the
optimum setting. If the ASTIG adjustmentis correct-
ly set, the vertical and horizontal portions of the
display will come into sharpest focus at the same
position of the FOCUS control. This seating of the
ASTIG adjustmentshould be correct forany display.
1 . Connect the CAL VOLTS connector to the input of
the vertical unit with a bnc-to-bnc jumper lead .
Crt Phosphor Protection . To protect the crt phosphor,
this instrument contains protection circuitry which limits
the display intensity by limiting the crt beam current to a
safe level. If the intensity controls) is advanced to a point
where the crt beam current exceeds a potentially damag-
ing level for more than about ten milliseconds, the circuit
action atuomatically limits the beam cu rrent to a safe level .
This circuit action also de-focuses the trace to indicate
that the setting of the intensity controls) should be
reduced . The crt beam current is limited to an even lower
level when operating in an X-Y mode,orifeitheroneofthe
time-base units is set to a slow sweep rate (even if the time-
base unit with slow sweep rate is not selected for display
by the HORIZONTAL MODE switch) . 1 his reduces the
danger of damaging the crt phosphor with a stationary or
slowly moving spot . Since beam-current limiting does not
take effect until after about ten milliseconds, the full
display-intensity capability of this instrument is available
for most single-shot and photography uses .
Light Filters. Light filters reduce the observed light
output from the crt. When the highest intensity display is
desired, remove the filters and use only the clearfaceplate
protector (permanently installed behind bezel) . Apparent
trace intensity can also be improved in such cases by
reducing the ambient light or using a viewing hood .
2. Set the CALIBRATOR switch to 4 V and the RATE
switch to 1 kHz . Adjust the vertical deflection factor to
produce a two- or three-division display.
3 . Set the time-base unit for a sweep rate of 0.2
millisecond/division .
4. Set the A INTENSITY control so the display is at
normal intensity (about midrange) .
5. Turn the FOCUScontrolfullycounterclockwiseand
set the ASTIG adjustment to midrange .
6. Adjust the FOCUS control so the top and bottom of
the displayed square wave are as thin as possible, but not
elongated .
7 . Set the ASTIG adjustment so the top and bottom of
the displayed square wave are as thin as possible .
8 . Repeat steps 6 and 7 for the best overall focus.
Display Focus
This instrument contains an automatic-focusing circuit
which maintains optimum focus after correct setting of the
FOCUS control is established . This eliminates the need to
re-adjust the FOCUS control for various intensity settings
within the range of the crt phosphor-protection circuitry
(see Crt Phosphor Protection) . Set the FOCUS control for
best definition of a low-intensity display.
Trace Alignment Adjustment
If a free-running trace is not parallel with the horizontal
graticule lines, set the TRACE ROTATION adjustment as
follows: Position the trace to the center horizontal line and
adjust the TRACE ROTATION adjustment so the trace is
parallel with the horizontal graticule lines.
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