Tektronix 2246 1Y, Service Manual

Page: 43 / 433

Theory of Operation—2246 1Y and 2246 Mod A Service
SWITCH BOARD AND INTERFACE
(Diagram 10)
Most of the front panel switches that can be read by
the Measurement Processor are “ s o ft” switches;
they are not connected directly into the circuit to be
controlled. The front-panel control physical parame­
ters of capacitance, leakage resistance, and induc­
tance, therefore, cannot affect the operation of the
controlled circuit. The wiper voltage of some of the
potentiometers is digitized, and that digitized data is
used by the Measurement Processor to set up the
circuitry under its control as dictated by the control
change.
The momentary push-button switches are rapidly
scanned at short intervals by the Measurement
Processor to check if one is being pressed. When a
switch closure is detected, the Measurement
Processor makes the necessary circuit or display
changes as directed by its firmware instructions for
that button and the existing operating states.
Functions are shown to be on by turning on the LED
(light-em itting diode) that back lights the push but­
ton or panel label. The Measurement Processor con­
trols the lighting via control registers (U2523 and
U2524) that it reloads with control data to enable the
correct LED with each button or mode change.
ADC AND DAC SYSTEM (Diagram 11)
The ADC and DAC system is the Measurement
Processor’s control link to the analog circuitry.
When the Measurement Processor does a scan to
determine the front panel control settings, the DAC
system drives the input to the A -to -D converter
comparator (U2306) in a binary search pattern to
determine the voltage level applied to the other input
of the comparator. The smallest incremental change
in the DAC input data that produces a switch in the
com parator’s output identifies the digital value of the
unknown voltage. The output of the comparator (AD
COMP) is applied to the Data Buffer U2515 on
Diagram 8.
POWER SUPPLY (Diagram 12)
The low and high voltages required to power the
2246 1Y and 2246 Mod A are produced by a high-
efficiency, switching power supply. Input ac voltage
from 90 to 250 volts and from 48 to 445 Hz is con­
verted to a dc voltage that powers a preregulator
circuit. The preregulator supplies regulated power to
an inverter switching circuit in the primary of the
power transformer (T2204). The secondary voltages
produced at the secondary windings of the trans­
form er are rectified and filtered to provide the low
voltage power requirements of the instrument.
High voltage to drive the crt is generated by a multi­
plier circuit (U2203) that provides the +14 kV post­
deflection anode voltage and the -2 .7 kV to the
cathode. The 6.2 Vac heater voltage is supplied by a
isolated secondary winding from the power trans­
form er that is referenced to the -2 .7 kV cathode
voltage.
DETAILED CIRCUIT DESCRIPTION
VERTICAL INPUTS (Diagram 1)
Channel 1 and Channel 2 input circuits on this sche­
matic diagram
are arranged identically. Only
Channel 1 circuit numbers are referred to in the
discussion. CH 3 and CH 4 are also arranged
identically to each other and described separately
from CH 1 and CH 2.
Attenuator and Vertical Mode Control Registers
The switching relays of Channel 1 are driven by tran­
sistor array U174. Drive to each of the transistors in
the array to switch the relay states is supplied by the
Measurement Processor (U2501) via U171. That
device is a portion of a shift register formed by
U171, U172 (for channel 2 relays), and U173 (for
Preamplifier gains). The devices are connected in
series to form one long shift register (designated
Shift Register 0). Serial data bits for the entire
register string are loaded at pin 2 of U171
from the
SR DATA line by the SRO CLOCK applied to pin 3 of
all three devices. See Table 3-1 for data bit assign­
ments. Tables 3 -2 , 3-3, 3-4, and 3 -5 define the bit
states for controlling the switching.
3-6