Tektronix 2465B Service Manual

Comparator U2510 compares the analog voltage of
each pot to the output voltage from the DAC (pin 18). To
determine the potentiometer output voltage, the processor
performs a binary search routine that changes the output
voltage from the DAC in an orderly fashion until it most
closely approximates the voltage from the pot.
The conversion algorithm is similar to successive
approximation and generates an eight-bit representation of
the analog level. When the pot's value is determined, the
Microprocessor stores that value in memory. Once all of
the pots have been read and the initial value of each has
been stored, the processor uses a shorter routine to deter-
mine if any pot setting changes. To do this the DAC out-
put is set to the last known value of the pot (plus and
minus a small drift value), and the status bit is read to see
that a HI and LO occurs. If within the limits, the processor
assumes that the pot setting has not changed and scans
the next pot. When the processor detects that a pot set-
ting has changed, it does another binary search routine to
find the new value of that pot.
Analog Control
The operating mode and status of the instrument
requires that various analog voltages (for controlling instru-
ment functions) be set and updated. The digital values of
the controlling voltages are generated by the Microproces-
sor and converted by the DAC. Analog multiplexers U2521
and U2530 (on diagram 2) and U170 (on diagram 4) route
the DAC voltages to sample-and-hold circuits that maintain
the control voltages between updates.
The Microprocessor writes three selection bits to regis-
ter U2301 that directs the DAC output to the appropriate
sample-and-hold circuit and charges a capacitor (or capac-
itors) to the level of the DAC. When the processor discon-
nects the DAC voltage from the sample-and-hold circuit
(by disabling the multiplexer) the capacitor(s) remains
charged and holds the control voltage near the level set by
the DAC. Due to the extremely high input impedance of
the associated operational amplifiers, the charge on the
capacitor(s) remains nearly constant between updates.
FRONT-PANEL CONTROLS
The Front Panel is the operator's interface for control-
ling the user-selectable oscilloscope functions. Along with
the crt, it provides visual feedback to the user about the
present operating state of the instrument.
Theory of Operation—2465B/2467B Service
Most of the Front-Panel controls (diagram 3) are "cold"
controls; i.e., they are not connected directly into the sig-
nal path. Therefore, associated circuits are not influenced
by the physical parameters (such as capacitance, resis-
tance, and inductance) of the controls. In addition,
translating the analog output levels of most of the poten-
tiometers to digital equivalents allows the processor to
handle the data in ways that result in a variety of
enhanced control features.
To maintain the front-panel operating setup between
uses of the instrument, the digitized values of the poten-
tiometers and front-panel switch settings are stored in bat-
tery backed up RAM so that when the instrument power is
turned off, these control settings are not lost. Then, when
power is next applied, the instrument will power up to the
same configuration as when the power was last removed
(assuming the settings of the non-digitized pots and
switches remain the same).
The Front-Panel Controls also allow the user to initiate
and direct the diagnostic routines (and when enabled, the
calibration routines) programmed into the read-only
memory (ROM). These routines are explained in the
Maintenance section of this manual.
Front-Panel Switches
The Front Panel Switches are arranged in a ten-row-
by-five-column matrix, with each switch assigned a unique
location within the matrix (see Figure 3-3). A closed switch
connects a row and a column together through an isolat-
ing diode. To detect a switch closure, the switch matrix is
scanned once every 32 ms (every tenth Microprocessor
interrupt cycle). When scanning, the Microprocessor
sequentially sets each individual row line LO. A closed
switch enables the LO to be passed through the associ-
ated diode to a column line. When the processor checks
each of the five column lines associated with the selected
row, the LO column is detected. The intersection of the
selected row and the detected column uniquely identifies
the switch that is closed. Further information about switch
scanning is found in the "Front-Panel Scanning" descrip-
tion located in the "Analog Control" discussion.
As each switch is read, the processor compares the
present state of the switch to its last-known state (stored
in memory) and, if the same, advances to check the next
switch. When a switch is detected as having changed, the
processor immediately reconfigures the setup conditions to
reflect the mode change and stores the new state of the
switch in memory. The detected status of the switch on
each of the following scan cycles is then compared against
the new stored data to determine if the switch changes
3a-11

Tektronix 2465B Specifications

General

Bandwidth	400 MHz
Channels	4
Vertical Sensitivity	2 mV/div to 5 V/div
Maximum Input Voltage	400 V (DC + peak AC)
Display	CRT
Trigger Modes	Auto, Normal, Single
Time Base Range	500 ps/div to 0.5 s/div
Rise Time	875 ps
Power Supply	48 to 440 Hz
Dimensions	495 mm

Summary

Service Manual

WARRANTY

Tektronix warranty for product defects in materials and workmanship for a period of three years.

OPERATORS SAFETY SUMMARY

SERVICING SAFETY SUMMARY

Do Not Service Alone

Prohibits servicing or adjustment unless another person capable of first aid is present.

Use Care When Servicing With Power On

Warns about dangerous voltages and touching exposed components.

Section 1—2465B/2467B Service

PERFORMANCE CONDITIONS

Defines the electrical characteristics and environmental requirements for the instrument.

Specification—2465B/2467B Service

Table 1-8

Details environmental requirements, including temperature, altitude, humidity, vibration, and shock.

Section 2—2465B/2467B Service

SAFETY

Provides crucial safety information for operating and connecting the oscilloscope.

START-UP

Describes the automatic diagnostic tests performed when the instrument is turned on.

Section 3—2465B/2467B Service

INTRODUCTION

Provides detailed information on the electrical operation and circuit relationships of the instrument.

BLOCK DIAGRAM

Provides a simplified block diagram of the instrument showing basic interconnections.

BLOCK DESCRIPTION

Discusses the Low Voltage and High Voltage Power Supply circuits and DC Restorer function.

DETAILED CIRCUIT DESCRIPTION

PROCESSOR AND DIGITAL CONTROL

Explains how the microprocessor directs oscilloscope functions and interacts with controls.

Theory of Operation—2465B/2467B Service

Reset Control

Ensures Microprocessor starts execution from a known memory point and handles power-down states.

Address Decode

Generates enabling signals and strobes for Microprocessor control of various devices.

Random-Access Memory (RAM)

Provides temporary storage for data and calibration constants, retaining settings after power-off.

FRONT-PANEL SCANNING and ANALOG CONTROLS

Explains how the Analog Control circuitry reads front-panel controls and sets analog voltages.

Front-Panel Switches

Describes the front-panel switch matrix arrangement and scanning process.

Pot Scanning

Describes how the Pot Scanning circuitry derives digital values for front-panel potentiometers.

Analog Control

Explains setting and updating various analog voltages for instrument functions.

Front-Panel Pots

Details the front-panel potentiometers that control linear functions and are digitized indirectly.

ATTENUATORS AND PREAMPS

Explains how the Attenuators and Preamps circuitry allows selection of vertical deflection factors.

Auxiliary Control Register

Allows Microprocessor to control mode and range dependent functions, including attenuation factors.

Channel 1 Preamplifier

Converts single-ended input signal to differential output for Vertical Channel Switch.

DISPLAY SEQUENCER, TRIGGERS, AND SWEEPS

Controls and sequences analog-type oscilloscope functions in real time.

Display Sequencer

Holdoff Circuitry

Delays sweep start until circuits recover from previous sweep, setting charging current for integrating capacitor.

HOLDOFF BOARD (2467B ONLY)

Triggers

Selects triggering signal source for horizontal sweep and produces triggering gate signal.

A Sweep

Calibrator

VERTICAL CHANNEL SWITCH AND OUTPUT AMPLIFIERS

Selects vertical signal source and provides amplification necessary to drive CRT deflection plates.

Vertical Output Amplifier

Z-Axis Amplifier

READOUT

Responsible for displaying alphanumeric readout characters in the CRT.

Section 4—2465B/2467B Service

INTRODUCTION

Explains the procedure for verifying instrument controls and performance.

PREPARATION

VERTICAL

Details performance checks for vertical controls, input coupling, and signal display amplitude.

COUNTER/TIMER/TRIGGER CHECKS

Covers performance checks for options affecting horizontal timing modes.

Check Delay Time

Check Delta Time Accuracy

Verifies delta time accuracy by aligning cursors and checking readout.

Verify Delay-By-Events

Check Logic Trigger

Verify Trigger Delta Delay

Checks trigger delta delay by aligning reference and delta cursors.

HORIZONTAL (cont)

ADDITIONAL FUNCTIONAL VERIFICATION

Verify ALT, CHOP, and ADD Modes and TRACE SEP

Verify BEAM FIND Operation

Check Probe Encoding

WORD RECOGNIZER CHECKS

Initial Setup

Check Data Setup Time

Verifies data setup time by varying pulse duration and checking signal display.

Check Data Hold Time

Verifies data hold time by varying pulse duration and checking signal stability.

Check Minimum Clock Pulse Width

Check Delay From Selected Edge to WORD RECOG OUT

Verifies delay from selected edge to WORD RECOG OUT by aligning cursors.

ADJUSTMENT PROCEDURE

IMPORTANT-PLEASE READ BEFORE USING THIS PROCEDURE

Highlights critical information and general rules before performing adjustments.

PARTIAL PROCEDURES

PREPARATION FOR ADJUSTMENT

POWER SUPPLIES AND DAC REF ADJUSTMENT

Procedure for checking and adjusting power supply DC levels, regulation, and ripple.

CRT ADJUSTMENTS

Covers adjustments for Z-AXIS DRIVE, GRID BIAS, TRACE ROTATION, FOCUS, ASTIGMATISM, and GEOMETRY.

AUTOMATIC CALIBRATION CONSTANTS, HORIZONTAL AND VERTICAL GAIN, CENTERING, AND TRANSIENT RESPONSE ADJUSTMENTS

Procedures for generating calibration constants for timing, gain, trigger level, and transient response.

CAL 01—HORIZONTAL

Checks and adjusts horizontal timing, X1 gain, X10 gain, Hrz Ctr, and Trans Resp.

CAL 02—VERTICAL

Checks and adjusts vertical preamp gain, gain (R638), and vertical centering (R639).

CAL 03—TRIGGERING

Checks and adjusts triggering parameters, including source selection and coupling.

CAL 04—CH 2 DELAY ENABLE/DISABLE

CAL 06—VERTICAL TRANSIENT RESPONSE

Checks and adjusts vertical transient response for flattest corner.

CAL 07—READOUT CENTERING AND GAIN

Checks and adjusts readout centering and gain for optimal display.

CAL 09—PARAMETRIC MEASUREMENTS

Adjusts parametric measurements, including time markers and bandwidth limit.

Section 6—2465B/2467B Service

STATIC-SENSITIVE COMPONENTS

Provides precautions for handling components susceptible to static discharge damage.

PREVENTIVE MAINTENANCE

Outlines procedures for cleaning, visual inspection, and checking instrument performance.

INSPECTION AND CLEANING

Details visual inspection and cleaning procedures for both exterior and interior of the instrument.

CLEANING

PERIODIC READJUSTMENT

TROUBLESHOOTING

Provides information for locating faults and aiding in troubleshooting the instrument.

TROUBLESHOOTING AIDS

Lists aids for troubleshooting, including diagnostic firmware, schematic diagrams, and circuit board illustrations.

Diagnostic Firmware

Explains the built-in diagnostic routines that aid in locating malfunctions via CRT readout or LED indicators.

Troubleshooting Charts

TRANSISTORS, INTEGRATED CIRCUITS, AND HYBRID CIRCUITS

Guidance on replacing semiconductor devices, emphasizing checking for actual defects.

SOLDERING TECHNIQUES

Provides general soldering techniques applicable to maintenance of precision electronic equipment.

REMOVAL AND REPLACEMENT INSTRUCTIONS

Describes special techniques and procedures required for replacing components.

Cabinet Removal

Section 8—2465B/2467B Service

Section 9—2465B/2467B Service

REPLACEABLE ELECTRICAL PARTS

Lists electrical parts available for replacement, including Tektronix and manufacturer part numbers.

Section 10—2465B/2467B Service

DIAGRAMS AND CIRCUIT BOARD ILLUSTRATIONS

Explains symbols, component values, and numbering systems used in diagrams.

Assembly Numbers and Grid Coordinates

Figure 10-3. Locating components on schematic diagrams and circuit board illustrations.

Provides a flowchart and explanation for locating components on diagrams and boards.

Figure 10-4. Instrument block diagram.

A simplified block diagram illustrating the instrument's functional blocks and interconnections.

ANALOG CONTROL (SN B050000 & ABOVE)

Section 11—2465B/2467B Service

Section 12—2465B/2467B Service

REPLACEABLE MECHANICAL PARTS

Lists mechanical parts available for replacement, including Tektronix part numbers.

PARTS ORDERING INFORMATION

Provides information on ordering replacement parts, including required details.