Tektronix 2246 1Y, Service Manual

Page: 78 / 433

T h e o ry of O p eratio n —2 246 1Y and 2246 M od A Service
then follows the Z-Axis output-voltage level to set 11) where it is digitized and sent to the Measure-
the new bias voltage for the crt control grid. ment Processor.
Neon lamps DS2702 and DS2701 protect the crt
from excessive g rid-to-cathode voltage if the
potential on either the control grid or the cathode is
lost for any reason.
Table 3-25
Front-Panel Multiplexer
Channel Select Bits
AUTO FOCUS DC RESTORER. The action of the
Auto Focus circuit has to be in reverse of the action
of the Z-Axis circuit. The differential transistor pair
of Q2708 and Q2709 provides drive to the Auto
Focus Amplifier that is inverted in polarity to the
Z-Axis signal. As the intensity increases (more
beam current), the focus grid bias must become
more positive to maintain the focus of the beam.
Also, since the focus grid operates at a less nega­
tive level than the control grid, the Auto Focus DC
Restorer is referenced to the -2750 V supply via a
voltage divider chain.
The FOCUS potentiometer (R2758) voltage is taken
across the middle resistor of the divider string to
provide an adjustable focus voltage that sets the
nominal focus level. Capacitor C2758 filters the
reference supply voltage for the focus circuit.
CONTROL LINE
Analog
POT7 POT6 POT5
Signal
Selected
0 0 0 A INTEN
0 0 1 RO INTEN
0
1
0 CH 1 PROBE
0 1 1 CH 2 PROBE
1 0
0
CH 3 PROBE
1 0 1 CH 4 PROBE
1 1 0 B INTEN
1 1 1
ANALOG GND
Volts Cal Signal Source
This circuit provides the precision voltages required
for setting the voltage measurement constants
during the SELF CAL routine. Ground is connected
to the vertical input when GND Input Coupling is
selected.
Five voltages are selected from a precision voltage
divider, R921, and multiplexed through U931 to the
vertical inputs at the appropriate time during the
SELF CAL routine. Selection is controlled by three
binary coded lines (VOLT CAL 0, 1 , 2 ) from U303.
Those control bits and the selected output voltage
may be checked one at a time by running the VOLT
REF exerciser from the Service Menu.
Control Multiplexer
Multiplexer U506, controlled by Data Latch U2313 on
Diagram 11, selects the A INTEN, B INTEN, and
READOUT control levels and probe code voltages to
be sent on the PROBE MUX signal line to multiplexer
U2309 on Diagram 11. The bit coding is shown in
Table 3-25. The selected output from U2309 is
applied to the A -to -D Converter (U2306, Diagram
Scale Illumination
Front-panel SCALE ILLUM control R905 varies the
base current of Q905, Q907, and Q908 to set the
intensity levels of the scale illumination bulbs
(DS901, DS902, DS903).
NOTE
Bulb life Is extended by keeping SCALE
ILLUM control set low or off except when full
intensity is required.
Probe Adjust Circuit
The Probe Adjust circuit generates a 0.5 V square
wave signal at about 1 kHz. Operational amplifier
U930A has a gain of about 4. The +2.5 V reference
on its noninverting input produces a little over 10 V
at the output pin. That voltage is divided by the
voltage divider formed by R936, CR936, and R937
for a peak amplitude of the signal of 0.5 V during the
time CR936 is forward biased. When CR936 is
reverse biased by the output of U930B, the Probe
Adjust output voltage is pulled down to 0 V through
R937 to ground.
3-41