Tektronix 485 User manual

TABLE OF CONTENTS
485/R485 Service
SECTION 1SPECIFICATION
Pogo SECTION 3CIRCUIT DESCRIPTION (conti
p,,.
Introduc1ion
,.,
Focus DC Restorer 3-13
V.tical Deflection SyRem
... Beam Current Limit 3-14
Probe Perlormanu
...,
Po_r Supply Shutdown Ceused by
A tlOd B Triggerin;
,~
Excessi.... Beam Current 314
Horizontal Deflection
'4
CRT Control Circuits 3-14
Calibrated Sweep Delay
'4
Auto Focu' Circuit 3-14
X·Y
,~
Calibrator 3-15
CRT
H
Calibrator
,~
Other CharacteriRics
,~
SECTION 4 MAINTENANCE
Ell'lironmental CharactMiRics
,~
IntroduC1ion 4-,
Cover Removal and Reinstallation 4-,
SECTION 2OPERATING INSTRUCTIONS Cleaning
,.,
Lubrication
'.'
Gen .... 1
,.,
Visuallnlf)aC1ion
'·3
Praliminary Information
,.,
Semiconductor Checks 4-3
Controls and Connection'
'.,
Recalibration
'·3
Front Pane!
,.,
Troublll$hootin;
,~
Rear Panel
'4
Introduction
'·3
Bottom Side
'4
Troublashootin; Aids
'·3
Circuit Board.
,~
Troublashoo1in; Equipment
,~
Troubllt'ShOOlin; Techniques
,~
SECTION 3CIRCUIT DESCRIPTION Special
Troubleshooting Information
4-,
Correcti ... Mainter'anca 4-11
Introduction
3·'
Dblainillll Replacament Paru
4·11
50 n A ttenuttor
3·'
Solderillll Tachniques
4-11
1Megohm
Buffer Amplifier
3·'
Component Repl_ment
4-12
50 nOverload Protection
3·'
Circuit Board Repl_ment 4·12
V.tical Preamplif • .,
3·'
Timin; Board Removal 4-12
Transform" Board Remonl
4·13
Signal Channel Switch
3·'
InYartar Board Removal 4_14
Internal Trigger Amplifiar
3·'
Semiconductor Replacement
4-14
Main Vertical Amplifier
3·'
Circuit Board Pin'
4·14
Scale Faetor Readout
3~
End Lead Pin Connectors
4·15
Vertical Mode Control
3~
Cathode Ray Tuba Repl_ment
4-15
ZAxis 3·3 Rapachllillll
for Shipmant
4·15
ATrig9er Amplifier
3~
Inlarboard C9mb Connection Loutions
4-"
AEXT Trigger Identify 3~
A Triggoer Gen .... ,or 34
SECTION 5 CALIBRATION
B Tr.r Generator J.4
Introduction
5·'
A and B Sweep,
J.4
TEKTRON IX Field Sllt'Yiea
5.'
Horizontal Amplifier
3~
Usin; Thi, Procedure
5·'
Horizontal Control 3~ Test Equipment Required
5·'
Power Inverter/Regulator
~
Special Calibration Filltures
5·'
Line Filt.,
3~
Calibrttion Equipm""t AII.,nalil/ft
54
Line Input Circuil
3~
Perlormance Check
54
Slart Natwork
'.'
Calibration Procedure 5~
Inverter 3~
I nvart.,/R'lIulltor 3~
lnvarter Currant Limitillll Circuit
,~
SECTION 6RACKMQUNTING
OvlltVoltage Stop Circuit
3~
Introduction
,.,
L1na Stop Circuit and Surge Limiting 3~
Rack Dimension,
,.,
Procedure for Converting the 485 to
Slida Out TtlICks
6·'
DC Operation 3~
Mounlin; Procedure
6·'
low Volt. Supplies
,.,
Alter"'te RNr Mounting Methods
5·'
Power Supply Protaetion Circui1 3·10
Removing Or ln$larung thaln$lrumant
,.,
Stop Mono$lable 3·10
Slida Out Track Lubrication
,.,
Balenea Nod. 3·10
Overcurrant Protection 3-12 SECTION 7ELECTRICAL PARTS LIST
Low Line Voltage Protaction 3·12
Samplin; Period Ti..- and Dver_ SECTION 8DIAGRAMS
AND CIRCUIT BOARD
Voltage Protection 3·12
CRT Clfcuil 3-12 ILLUSTRATIONS
Filament Voltage
3·12
His;, Volt. Supplies 3-12
Anode Supply 312 SECTION 9 MECHANICAL PARTS LIST
C,thode SupplV 3-12
Cathode Regulator
3·12
Grid DC RlI$Iorar 3-13 CHANGE
INFORMATION
REV. B. MAR. 1976

Section 1--485/R485 Service
SPECIFICATION
Introduction
The 485/R485 is ageneral-purpose, environmentalized,
high·performance, portable, wide-band oscilloscope which
has adual-channel vertical amplifier with selectable input
impedance (DC to 350 MHz bandwidth with 50 il input
impedance; DC to 250 MHz bandwidth with 1 Mil input
impedance). The 485 has a 1 ns sweep rate, stable triggering
to bandwidth limits and calibrated X-V capabilities.
Delayed sweep has calibrated delay time, can be triggered
aher delay and can be displayed with the intensified main
sweep
in an alternate sweep switching display. Additional
features are XtO, X100
probe scale factor readout, 8div
Xl0 div graticule area, small spot size and high writing rate.
A
20 MHz bandwidth limiter, 1 MHz and 1kHz last·rise
calibrator
and autofocus are also included. The 50 n input
is automatically disconnected from excessive Voltages. An
external trigger view feature
is also provided. The 485·1 and
485-2 have no external trigger view. The
485-2 has Only 50
nvenical input impedance.
Input Coupling Selection
AC; DC; GND (provides zero reference, precharges
coupling capacitor, disconnects
50 nload in 50 n
model.
Lower -3 dB Point (AC coupling from 50 n
source)
50 ninput, 1kHz or less.
1
Mn input lX, 10 Hz or less.
Deflection Factor
5mV/div to 5V/div in 10 calibrated steps (1-2·5
sequence), accurate within
2%. Uncalibraled, continuo
ously variable between steps
to al least 12.5 V/div.
Lights at edge of knob skirts indicate correct defleClion
faclor for 1
X, lOX and l00X probes.
ELECTRICAL CHARACTERISTICS
Gain can be recalibfated at fronl panel. 1 Mil SAL is
available at botlom panel to eliminate step allenuator
shift above to mV/div, in the 1 Mn mode.
VERTICAL DEFLECTION SYSTEM
(2 identical channels)
Display Modes
Channell; Alternate; Chopped (approximatelv 1 MHz
rate); Added; X·Y (CH 1 -y and CH 2 -XI; Channel 2
(+Up
or Inverted).
(VARIABLE gain
Terminated Source
,
,
-
c..---
--
TYPICAL BANOWIOTH. 50 n
VS.
VARIABLE GAIN SETTING
300
BW.
MH.
200
Internal Trigger Source
Normal (displaved signal). Channel 1 or Channel 2
signal.
500
400
BandwidthIand Risetime2
CALIBRATED3) From son
_15°C to +35°C
From 50 nterminated source -1SoC to +35°C.
1
Mn within 1% paralleled by approximately 20 pF.
50 nDC to at least 350 MHz, 1ns
Selectable Input Impedance
50 nwithin 0.5%. VSWR t;;;;1.25: 1on 5mV/div and
10 mV/div, 1.15:1 Irom 20 mV to 5V!div to 350 MHz.
1
Mn DC to at least 250 MHz, 1.4 ns
Fill. 1·1. Typical Bandwidth, 50 n vs. Vari.bl. G;oinSIII;ng tvaries
considerablv depending on instrumant SNI
,.,
1 Bandwidlh tBWI mellsure<! 81 -3 dB down.
lRiSBlime c;otcul;oled trom O.35/BW. From +35°C 10 +55°C, BW is
300 MH~ for 50 0 and 200 MH~ for 1 MO.
)Sea Fi9. 1·1 for effect of VARIABLE gain conlrol.
REV C. MAR 1979
so
SO%
Per<;ent of CAL Gain
'00%

Speeification-485/R485 Service
200 V
100 V
60V
<DV
20V
10V
'V
2V
":::::
~~
~
""'-
150 V
~~E~A~ !,~CING-..~
~
h.
~
~
~
1 m,
50V
"'-,- TYPICAL
'"
50 n PROTECTION_
~o,."' ....
DISCONNECT
Oi$l;onnlll;:1
CHARACTERISTIC
Threshold
l
::::
Ilatd!if'llJl
~o,
.......
~
-"
' ...
NONRECURRING SIGNALS
..............
IN THIS AREA 00 NOT
TRIP THE DISCONNECT
............ ~1
-"
, ....
5V
1 V 0.1 ms
50 nProtection
10 rnl
Tima from 5urt of Applied Voltage
Fig. 1-2. Typical 50 n Protection Disconnect Charae:teristic.
Bandwidth limiter
0.1 s1 •
Internal detection circuitry automatically disconnects
excessive signals of up to 50 V. The disconnected
condition is indicated, and has manual reset.
Maximum Input Voltage
50 nDisconnect occurs for voltages that exceed approx,
imately 5 V
RMS continuous or 0.1 watt-second for
instantaneous voltages of 5 V to 50 V.
Repeated application of voltages in excess of 50 volts
and greater than 0.1 watt-second will cause deterioration
of K1S1 relay contacts. due to arcing. Signals in excess
of 150 volts will damage the instrument.
1
Mil·DC coupled 250 V (DC +peak ACI. 500 V pop to
1kHz. AC coupled 500 V(DC +peak ACI. 500 V pop to
1kHz.
Common Mode Rejection
Added Mode with Channel 2invened. At least 10:1 at
50 MHz for a·division
signal after adjusting CH 2gain
for
best CMR at 50 kHz and 20 mV/div.
1-2
Limits to approximatelv 20 MHz the Vertical Amplifier
and Internal Trigger signals.
Channel Isolation
At least 30:1 at 350 MHz.
A Ext Trigger Display (R485 and 485 only)
Momentary pushbutton in VERT MODE area enables
display
of EXT ATrigger when Asource is in EXT
position. Deflection factor is approximatelv 50 mV/div.
Risetime
is approximately 1.5 ns. Delav match to CH 1
or
CH 2 input is within 0.5 ns at 50% amplitude.
Triggering level
is within 0.5 div of center-screen.
Probe Power
Two 4-pin connectors at the rear of the instrument
provide power suitable for optional active probes
such as
the P6201.
®i

Specification-485/R485 Service
PROBE PERFORMANCE
(Probes are Optional Accessories)
Temperature Range
Lower Bandwidth for this
Input
-3 dB point with
485
Bandwidth
Type Atten. Impedance (maximum) (minimum) Specification
For Use WIth
50 nInput
010-6056·03
~ 6ft.
P6056
lOX 500!"! 1 pF
200 Hz 350 MHz
010-6056·05 -9ft.
010·6057·03 - 6 ft.
_15°C to +35°C
P6057 100X 5k!"! 1
pF 20 Hz
350 MHz
010·6057-{)5 - 9ft.
1X 100 kn 3pF 10 Hz
P6201 010·6201·01
-6ft.
Plug·
lOX 1 MS1 <1.5 pF
1 H,
330 MHz
DoC to +35°C
(FET Probe)
00
100X 1 MS1 <1.5 pF 10 Hz
For Use With 1 Mil Input
010·6053·11 -
3112lt.
1M!"! 9.5 pF
250 MHz
P6053A
010·6053·13 -6ft. lOX 1M!"! 12
pF 1 H,
250 MHz
-lS0Cto+35°C
010·6053·15 -9ft. 1M!"! 13.5 pF 200 MHz
Aand B TRIGGERING Time Base Aand BTrigger Sensitivity
Trigger Mode To 50 MHz At 350 MHz
DC lot 0.3 div 1.5
div
DC Ext 20 mV 100 mV
AC
Signals below 16 Hz are attenuated.
LF Reject Signals below 16 kHz are attenuated.
HF Reject Signals below 16 Hz and above 50 kHz
are attenuated.
Sweep free·runs
in the absence of atriggering signal
and lor signals below 20 Hz.
Sweep runs only when triggered.
Sweep runs one time on the first triggering event
alter
pressing the reset selector.
AUTOMATIC.
SINGLE SWEEP.
ATrigger Mode
NORMAL.
level and Slope
BTrigger Mode
BRUNS AFTER DELAY TIME.
Starts automatically
at the end of the delay time.
Internal, permits selection of triggering point at any
level
on the positive and negative slope of the displayed
waveform. External,
level is adjustable through at least
to.5 Vfor either polarity It5 Vfor EXT +10l.
BTRIGGERABLE AFTER DELAY TIME.
Runs when triggered. The B(delayed) sweep runs
once, in each of these modes, following the Asweep
delay time.
ASource
Internal, Line, External, External +10.
REV. B, AUG. 1977 1-3

SpeCification--485IR48S Service
BSource
Bruns after Delay Time, Internal, External, External
+10.
Minimum Sweep Holdoff (A Trigger Holdoff in
Norm
detent)
ATime Setting
External Inputs
1Mil paralleled by approximately 20 pF. Maximum
input voltage 500 V
(DC +peak AC), 500 V p.p to
1kHz.
Jitter
less than 0.1 ns at 350 MHz and 1nsldiv.
1ns/div
to 0.1 ~s/div
0.2 ~s/div
0.5 ~s/div to 0.5 s/d;v
ATrigger Holdoff
less than 0.4 ~s.
Less than 1.0 IJ.s.
Less than 2times the
ATIMEIDIV setling.
HORIZONTAL OEFLECTION
Time Base Aand B
1nsldiv to 0.5 sldiv in 27 calibrated steps /1-2·5
sequence). Uncalibrated, A is continuously variable
between steps and to at least 1.25 sldi ....
Adjustable control permits astable presentation of
repetitive complex waveforms. The control covers at
least the time of one full sweep for sweeps faster than
0.2 s/di ....
BEnds A
The Asweep is reset at the end of the Bsweep to allow
the fastest possible sweep repetition rate for increased
trace intensity
in the delayed sweep mode.
CALIBRATED SWEEP DELAY
Delay Time Range
o 10 9.9 times the Delay TimelDiv settings of 10 ns/div
to
0.5 s/di ....
Exclude the first (above 0.0) 1.0 turn or the first 40 os
of
the DELAY dial.
Differential Delay Time Accuracy (+1S'"C to
+35"Cl
'Full sca'e i110X Ihe OeillY Tima/Oiv Sening.
Time Base Aand BSweep Accuracy (Center 8
screen divisions)
Sweep Rate +lS'"C
to +3S'"C
-lS'"C to +SS'"C
1nsldiv to 20 nsldiv '3% '5%
50 nsldiv to 0.1 sldiv '2% "'I'
0,2 sldi ... to 0.5 sldi ...
'3% '5%
Display Modes
A, A INTEi~sified during Bdelayed, ALTernate display
of AINTEN and B(delayed sweep), Only Asweep
is
displayed for Asweep rates of 1,2 and 5ns/div.
1-4
DELAY TIME Setting
10 ns/div and 20 ns/div
50 ns/div to 1ms/div
2ms/div to
0.5 s/div
± (1% of mt:8surement
+0.2% of full
scale')
±(0.5% of measurement
+0.1 %of full scale)
± (l%of measurement
+0.1 %of full scale)

•
,.,
,.,
SPECIFIED OIFFERENTIAL
OELAY ACCURACY
V,.
SIZE OF MEASUREMENT
10 n .. div 80 20 ntldiv
2mild;" 10 0.5 sid;"
Specification-485/A485 Service
CRT
TEKTRONIX CRT
4-inch rectangular tube; ax 10 divisions (0_8 cm/div)
display area.
P31 phosphor normally supplied; Pl1
optional without extra charge. 21 kV accelerating
potential.
50 nsld;" to 1 mildi ..
~IO.5'" of me.", •• mlnt
+0.'% of lull lUll)
DIFFERENCE Batween Two Dial
Raad;n!p,1Y.3je" D,T~. Oivision1
Fie. '-3. Specified DiH .... ntial Oelav Accuracy tIS. Size of Meawr.
manto
Jitter
less than 0.005% (one part in 20,000) of full scale plus
0.2 os. (Wrap-around cover must be in place.)
X-y
Full Sensitivity
(CH 1Vert, CH 2Horizontal.) 5mV/div to 5V/div in
10 calibrated steps (1·2·5 sequence) accurate within 2%.
Gain can be recalibrated at front pane1.
YPerformance
Identical to CH 1. See Vertical Deflection.
X-Axis Bandwidth
DC to 4MHz (with 10 div reference signal).
x-v Phase Match (Full BWand BW Limit)
Within 3° to 4MHz.
X·Gain Match to CH 2
+15°C to +35°C: 1%
Photographic Writing Speed
At least 3div/ns for standard P31 phosphor; at least
6div/ns for optional
Pl1 phosphor (with the
TEKTRONIX C-31·R Camera and POLAROID 10,000
ASA film).
Auto Focus
Automatically maintains beam focus for all intensitY
settings. (Intensified zone and EXT Zaxis are not
auto-focused).
Graticule
Internal, no parallax. Variable edge-lighting. Markings
for measurement
of risetime. Graticule is dark with
illumination off.
Beam Finder
limits display wilhin graticule area.
External Z-Axis
Risetime 15 ns. Input R 500 n. +0.2 V {DC 10 20 MHzl
blanks trace of average intensitY.
+2 V (DC to 2MHz)
blanks maximum intensity trace.
Beam Current limit
Automatically limits the average beam current to protect
the CRT phosphor.
(limits average current to 20 fJA for
sweep rates faster than
50 ms/div; 5 fJA for 50 ms/div
and slower sweep rates and for X·Y. Backup system
shuts down power supply
if average current reaches
approximately
30 fJA.)
'-5

Specification~85/R485 Service
CALIBRATOR
Two-Frequency Fast-Rise Calibrator
ACCtJracies +15D C to +3SD C
-lSOC to +SSD
C
SVamplitude 10
0.5%
'"
1 Mn load
0.5 Vamplitude to
'"
1.5%
50 n to.5% load
50 mA amplitude to
optional
BNC acces- ,% 1.5%
sorv current loop
Selectable Frequency 0.25% 0.5%
of 1 MHz and 1kHz
Output Resistance
450 n
Risetime
Positive edge into 50 n. less than 1ns. O'JershOOl and
ringing less than
2% into 50 nat 1GHz bandwidth.
1kHz Duty Cycle
49.8% 10 50.2%
OTHER CHARACTERISTICS
Signal Outputs
Posilive gates from both time bases lapprollimately 4V).
and apositive-going Sawtooth from Time Base A
(approximately 10 V). Gates and Sawtooth are approxi-
mately
0.5 Vamplilude into 50 n.
Power Requirement
Two·position line voltage selector 115 V(90 V to
136 VI and 230 V(180 Vto 272 V) 48 to 440 Hz. 60
watts mallimum at 115 Vand 60 Hz. 0.9 Amaximum at
115 Vand
60 Hz. line fuse: 3 A fast.
Cooling
Ouiet, filtered. forced-air ventilalion. Fan speed
increases smoothly with increasing ambient temperature.
ENVIRONMENTAL CHARACTERISTICS
Ambient Temperature
Operating: _15
D
C
to +S5D C
Storage:
_3S
D
C
to +75D C
1·6
Altitude
Operating: 1015,000 feet. Maximum allowable ambient
lemperature decreased
by lD Cper 1000 feet from 5,000
10 15,000 feet.
Nonoperating: 1050,000 feet.
VibrationS
Operating: 15 minutes along each of lhe three axis,
0.025 inch peak-to·peak displacement (4 g's at 55 Hzl
1010551010 Hz in l-minute cycles.
ShockS
Operating and Nonoperaling: 30 g's, one half sine,
11·ms duralion, 2shocks per axis in each direclion for a
10tal
of 12shocks.
Electromagnetic Interference
(With 485 EMI modificalion option 41. Meets inter-
ference requirements of
Mll-1·618D: power line
conducted, 150 kHz
to 25 MHz; radiated, 150 kHz to
GHz.
Humidity
Operating and Storage: 5cycles (120 hours) to 95"
relalive humidity referenced 10 Mll-E-l6400F (par.
4.5.91hrough 4.5.9.5.1, class 41.
Dimensions and Weight
Heighl; 6and 9/16 inches.
Width: (with handleI12 inches.
Depth (including panel
coved: 18 and 1/2 inches.
Depth (handle extended): 20 and
5/8 inches.
Net weight (wilhoul accessory case
and panel coved: 21
pounds.
Net weight (with panel cover, accessory case
and
accessories): 24 pounds.
Domestic shipping weight:
31 pounds.
Export·packed weight: 42 pounds.
SR48S strapped 10 tab~, nol mounted in rloC:k.

Section 2~85/R485 Service
OPERATING INFORMATION
PRELIMINARY INFORMATION
The 485 is cooled by air entering through the air filter
on the rear panel and exiting through the holes on the sides.
Adequate clearance must be provided at these locations.
Allow at least one inch clearance behind the air filter and at
least one inch on the sides.
Athermal
cutoot in this instrument provides thermal
protection and disconnects the power
to the instrument if
the internal temperature exceeds asafe operating level.
Power is automatically restored ....... en the temperature
returns to asafe level.
Oean the air filter periodically: a
dirtY filter prevents adequate air flow into the instrument.
Operating Temperature
The 485 can be operated where the ambient air
temperature
is between -ls'"C and +5s'"C (+5°F and
+131°F). This instrument can be stored in ambient
temperatures between
-5s'"C and +75°C (-67°F and
+167"F). After storage at temperatures beyond the opera·
ting limits, allow the chassis temperature
to come within
the operating limits before power
is applied.
WARNING I
This instrument is designed for operation from a
power source with irs nautra' at or near earth
(ground) potential
with aseparate safety-earth
conductor.
It is not intended for operarion from two
phases of amulti-phase sysrem, or across the legs of a
single-phase, three-wire system.
Operating Voltage
-----,
The 485 can be operated from either a115 Vor 230 V
nominal line voltage source,
or a220 Vellternal battery
pack. The Line Voltage Selector switch on the rear panel
converts this instrument from one operating voltage to the
other.
General
To effectively use the 485, the operation and capabilities
of the instrument must be known. This section describes
the operation of the controls and connections and gives
first-time and general operating information.
CONTROLS ANO CONNECTORS
Power Cord Conductor Identification General
Conductor Color Alternate Color
Ungrounded (Line) Brown Black
Grounded (Neutral) Blue
lftfhite
Grounding (Earthlngl Green·Yellow Green·Yellow
~
This instrument may be damaged if operated with the
Line Vo/rage $elector switch
set to incorrect posi-
tions for the line volrage applied.
The major controls and connectors for operation of lhe
485 are located on the front panel of the instrument. Some
aU;l{iliary functions are provided on the side, top, bonom
and rear panel. To make full use of the capabilities of this
instrument,
the operator should be familiar with the
function and use of each of these controls and connectors.
Abrief description of each control and connector
is given
here.
Front Panel
POWER Pushbutton Turns instrument on or off.
115 V
230 V9(J.136 V
180-272 V
CAL 5 V Connector BNC connector for square·wave
voltage calibrator output signal.
The 485 is designed to be used with athree-wire AC
power system. If the three to two·wire adapter is used to
connect this instrument to atwo-wire AC power system, be
sure to connect the groond lead of the adapter to earth
(ground). Failure
to complete the groond sV1tem may allow
;he chassis of this instrument to be elevated above ground
potential and pose ashock hazard.
FREO Pushbutton
INTENSITY Control
BINTENSITY
Control
Selects one of two frequencies 11
kHz or 1 MHz) for the last-fise
calibrator signal.
Controls intensitY of writing beam.
Provides additional intensitY coo-
trol for the BSWEEP.
AEV.C,MAA.1977
2·'

Operating Information-485/R485 Service
SCALE
ILLUM
Control
BEAM FINDER
Pushbutton
BW llMIT
Pushbutton
20
MHz Indicator
VERT
MODE
Pushbuuon Selector
CH 1
ALT
CHOP
ADD
X-y
CH 2
INT TRIG
Pushbutton Selector
NORM
CH 1
CH 2
VOLTS/DIV Switch,
CHlandCH2
2-2
Controls illumination of internal
CRT graticule.
Compresses trace within graticule
area, regardless of position control
settings or amplitude of signal
applied.
limits to approximately 20 MHz
the bandwidth of the vertical ampli·
fier system and of the INTernal
TRIGger signaL
lights when bandwidth of vertical
amplifier
is being limited.
Displays
Channell only.
Dual trace display by switching
between channels at the end of A
sweep.
Dual trace display by switching
between channels every
0.5 /JS (1
MHz chopped display).
Algebraically adds
CH 1and CH 2
input signals.
Permits
X·Y operation. Displays CH
1signal on the vertical (Y) axis and
the CH 2signal on the horizontal
IX) axis, with a 4 MHz phase·
compensated bandwidth.
Displays
CH 2only.
The signal being displayed
is the
internal trigger source.
Selects
CH 1 as the internal trigger
source.
Selects CH 2as the internal trigger
source.
Selects 1X calibrated deflection
factors from 5mV/div to 5V/div
in
ten steps (1·2·5 sequence). Attenu-
ating probe tip deflection factors
for X, 0and Xl00 coded probes are
automatically indicated by the
three readout lamps at the circum·
ference of this knob.
All three
lamps are off when the channel
is
VARIABLE vohs/div
Control,
CH 1and
CH 2
GAIN Adjustment,
CHlandCH2
AEXT TRIG
Pushbutton
(momentary not on
485·' and 485-2)
,Mn/SO nSwitch,
CHlandCH2
(485·2 has RESET-
PUSH)
RESET Indicator
Input Selector Switch,
CH 1and CH 2
AC
GND
DC
Input CH 1 or Y
Connector
and CH 2
or XConnector
POSITION Controls,
CHlandCH2
not selected for display by the
VERT
MODE selector, or when
using probe IDENTify.
Provide continuously variable
deflection factors between calibra-
ted steps. Maximum deflection
factor range is extended to
12.5 V/div. Push·away control
provides calibrated deflection fac·
tor in the
CAL IN position.
Screwdriver adjustment allows cali·
bration of vertical deflection factor.
Overrides other vertical controls to
display the external signal being
used for Asweep triggering.
Illuminated push-push selection for
input impedance.
Is also used to
reset
SO noverload condition
(1 M1U50 nswitch must be pushed
twice
to return to 50 ninput
impedance). 485·2 is no n·
illuminated momentary switch for
RESET only.
When maximum input power is
exceeded in the 50 nmode causing
the overload disconnect to operate,
the RESET
indicat~ is illuminated,
and the 1 M
n/so nlamp is turned
off.
Capacitive!y couples input signal to
vertical amplifier.
Grounds the amplifier input and
permits precharging the
AC input
coupling capacitor.
SO ntermina·
tion disconnected
in the SO n
mode.
Signal
is directly coupled to the
vertical amplifier.
BNC connectors for applying exter·
nal signals. Included are concentric
coding rings for probes with scale
factor and identify switChing.
Vertically positions the display.
In
X-Y mode, CH 1controls positions
in the Y(vertical) axis and the CH
2control positions in the X(hori·
zontal) axis.

CH 2POLARITY
Switch +UP and INVERT slide selector
provides for inverting
CH 2display.
DC
Operating Information-485/R485 Service
DC (direct) couples all trigger sig·
nals to the trigger circuitry.
HORll DISPLAY
Pushbutton Selector
(Blue Panel
Background)
A
(Inoperative in the X·Y, VERT
MODE, and when timing knobs are
locked
in the 1, 2, and 5nsldiv
position.)
Displays Asweep.
EXT TRIG Input
Connector
SLOPE Switch, A
TRIGGERING and
BTRIGGERING
BNC connector providing input lor
external trigger signals.
Permits triggering the sweep on the
positive or negative·going portion
of the trigger signal.
INTEN
(A) Displays Asweep intensified (after
the delay time) for the duration of
Bsweep.
LEVEL Control, A
TRIGGERING and
BTRIGGERING
Selects amplitude
triggering signal
triggering occurs.
point on the
where sweep-
ALT
B lDL Y'D)
TRACE
SEPARATION
Control
Triggering Source
Switch, ATRIGGER·
ING and BTriggering
INT
LINE (A
TRIGGERING
only)
BRUNS AFTER
DELAY TIME
(B
TRIGGERING
Only)
EXT
EXT10
Triggering COUPLING
Switch, ATRIGGER-
ING and BTRIGGER-
ING
AC
LF REJ (AC
Coupled)
HF REJ lAC
Coupled)
Alternately provides INTEN (A)
and B(DLY'D) displays.
Displays B
(DL Y'D) sweep.
Provides additional positioning to B
(DlY'D) display in the AL Thori·
zontal display mode.
(Green Panel Background)
Uses the signal selected by the INT
TR IG switch as the triggering
signal.
Uses aportion of the line·frequency
voltage
as atrigger signal.
Bruns automatically after the time
selected by ATIME/DIV and the
calibrated DELAY TIME POSI·
TlON control.
Permits triggering on signals applied
to the EXT TRIG INPUT
connector.
Attenuates external trigger signal
by afactor of 10.
Trigger signals are
AC (capacitively)
coupled to the trigger circuitry.
Trigger signals below 16
Hz are
anenuated.
Attenuates triggering signals below
16 kHz.
Triggering signals below 16
Hz and
above
50 kHz are attenuated.
TRIG'D Indicator
SWEEP
MODE Switch,
ATRIGGERING
AUTO TRIG
NORM TRIG
SINGLE SWEEP
RESET
READY Indicator
ATRIGGER
HOLOOFF
DELAY TIME
POSITION
Aand BTlME/DIV
AND DELAY TIME
Switch
lights when Asweep is triggered.
Permits normal triggering on wave·
forms with repetition rates of at
least 20
Hz. Sweep free·runs in the
absence of an adequate triggering
signal.
Permits normal triggering. No CRT
display in the absence 01
an ade·
quate trigger signal.
Displays one sweep only until reset.
Amomentary-contact position of
the SWEEP
MODE switch that pro·
vides for re·arming the Asweep
generator during the SINGLE
SWEEP mode of operation.
Is illuminated when Asweep is
armed in the SINGLE SWEEP
mode.
Adjustable control of the lime
between sweep steps, permilS a
stable presentation of repetitive
complex waveforms. The control
covers at least the time of one
full
sweep for all but the two slowest
sweeps.
Ten·turn calibrated control delays
Bsweep start (or Btrigger arming)
from ato 9.9 times the Time
Base
ATIME/DIV setting after the start
of Asweep.
Selects calibrated Aand Bsweep
rates from 0.5 s/div 10 1ns/div
in
27 steps (1·2-5 sequence). Delay
Time operation extends from
0.5 s/div to 10 ns/div.
2·3

Operating Information-485/R485 Service
AVARIABLE
Control
DLY'D
SWEEP
PULL Knob
POSITION (hori-
zontall Control
Rear Panel
TRACE ROTATION
Adjustment
FOCUS and ASTrG
Adjustmen.t
Provides continuously variable A
sweep rate to approximately 2.5
times the calibrated setting (uncali-
brated sweep rate
is extended to
1.25 s/div). Push away control
provides calibrated rate
in the CAL
IN position.
Provides
for advancing the B
(DLY'D) sweep rate ahead of the A
(DELAYING) sweep rate.
(When A
rate
is 1, 2, or 5ns/div the concen-
tric knobs are locked together. and
only the Asweep
is displayed.)
Horizontally positions trace. In·
operative
in X- Ymode.
Screwdriver adjustment to align
trace with horizontal graticule lines.
Adjustment used
to obtain a well
defined display. ReQuires infre-
Quent readjustment.
ZAXIS INPUT
Connector
PROBE
POWER
LINE CORD
LINE VOLTAGE
SELECTOR
OPERATING TIME
Bottom Side
1 Mn BAL,
CHlandCH2
BNC input connector for intensity
modulation of the CRT display.
Two 4·pin connectors
at the rear of
the instrument provide power suit-
able for optional active probes such
as the P6201.
Power cord
is a3-wire. permanently
attached cable, approximately 7.5
feet
in length.
Recessed slide switch selects nomi·
nal operating line range. 115 V(90
to 136 VI or 230 V (l BO to 272 V).
Elapsed operating time indicator.
5000 hr. scale.
Adjustment provides for DC bal·
ance of 1
Mn Buffer amplifier.
B+GATE BNC
Connector providing a
positivelloing rectangular waveform
coincident with Bsweep.
A+GATE BNC
connector providing a
positivelloing rectangular waveform
coincident with Asweep.
A
SWEEP Connector BNC connector provides asample
of Asawtooth generator signal.
2·4
NOTE
Two types
of crt graticules have been used in some
Tektronix osciJfoscopes. One graticule has 0% and
100% risetime reference points that are separated by
6vertical graticule divisions. The other graricule has
the 0% and 100% rise rime reference points separated
by 5vertical divisions. In your manual, illusrrations
of the err face or risetime measurement instrucrions
may
not correspond with the graticule markings on
your
osciJJoscope.
REV.8.MAA.1976

Section 3-485/R485 Service
CIR CU IT DESCR IPTIO N
Introduction
This section describes the electrical operation and
relationship of the circuits
in the Type 485. The theory of
operation for circuits
that are used only in this instrument
are described
in detail in this discussion. Circuits that are
commonly used
in the electroniC5 industry are not
described in detail.
50 n ATTENUATOR
Two identical attenuators. one for each vertical channel.
are used to select the desired
... ertical sensiti ... ity of the 485.
Each
attenuator is an integral unit containing ... arious 50 n
attenuators, input overload protection circuitry and, where
applicable, a 1 megohm attenuator and 1megohm buffer
amplifier.
The 50 n
atlenuator is comprised of adelay tine.
DL410, and four
50 n attenuator sections. The';' 2 and';'
2.5 50 n attenuator sections are shared by a 1 M11 buffer
amplifier. The two +10
attenuator sections are used only in
the
50 11 mode. The delay line equalizes the delay
differences between the 1
Mn buffer amplifier and the
50 51 mode. R127 pro ... ides termination for the 1 Mn
buf1er amplifier when the instrument is operating in the
50 nmode.
1MEGOHM BUFFER AMPLIFIER
The 1megohm buffer amplifier pro ... ides ahigh input
impedance and unitY gain when terminated
in 50 n. It is
connected to the input through relay K151 onty in Ihe 1
megohm mode and when the RE5ET light
is on.
C33 and R33 pro ... ide input current limiting. CR33
clamps negati
... etransients in excess of 9.6 V to protect
FET
034. The gate to drain junction of 034 gi ... es clamping
for positive'going o
... erloads. The high frequency signal path
goes from the input FET
to 040, an emitter follower. R53
sets the high frequency gain.
060 dri ... es 062; CR62
balances
out Ihe base·emitter voltage of 062 and temper'
ature compensates
it to keep the standing current in 062
independent of temperature. 062 current dri ... es the output
for positi ... esignal swings. 060 also dri ... es 070, which pulls
the
output negati ... ethrough R69 and ser ... es as reverse
termination at high frequencies. CR64 and CA65 work
as
expanders for high amplitude signals. R55 and C55 are high
frequency adjustments which affect the first
fi ... enano·
seconds of the transient response. L56 and
L71 are integral
parts of the circuit board
pro ... iding high frequency peaking.
050 is alow impedance dri ... efor the low frequency
feedback path.
Low frequency feedback
is accomplished by U48. which
compares the voltage
le ... els at the base of 040 and the
output. J70. C46 matches the time constant of cn and
R75 with the time constant of A42 and parallel combina·
tion of C46 and C47. C48 sets the bandwidth of U48.
50 nOVER LOAD PROTECTION
In the 50 n. mode. K1 is closed by U80, which routes
the signat
10 the 50 11 atlenuator. 50 n. input protection is
pro ... ided by an RM5 detecting circuit in U80. A84 and
R107 attenuate the input signal 100X for U80. C86, C87.
and R87 integrate asignal proportional to the
RM5 ... alue of
the signal at pin 3of U80. When an overload
is detected,
Kl is de-energized. removing the signal from the 50 51
atlenualor section and applying it to the input of the 1
megohm buffer amplifier.
UOO also lights 0596. the RESET
light. To reset. 5125B
is switched to the 1megohm
position. causing the scope to be
in the 1megohm mode.
Return to the
50 11 mode is then accomplished by pushing
5125B (50
nil megohm mode switch) again.
VERTICAL PREAMPLIFIER
CH 2 is identical to CH 1except that CH 2has apolarity
switch. Only
CH 2 will be described here. The CH 2signal is
received from the CH 2 attenuator through J300. Diode
bridge CR301, CR302. CR303. and CR304 protects the
input from large signals
that occur too last for the input
protection relay to reacl. T305
is abalun. pro ... iding a
push·pull signal to U310 at high frequencies. The
50 51
input termination consists of R308, R305, and R309. R308
is adjusted to gi ... e 50 51 DC resistance at input connector
J1. CH 2offset control R306 adjusts the ... oltage at the
input connector
J1 to zero in the absence of an input
signal. Either R310 or R312
is selected by the POLARITY
switch 5310. allowing the variable control to be balanced
in
both in ... ert and normal modes. The output leads of U310
(pins 5,
6. 8. and 9) are crossed o ... er so that the polarity of
the signal can be
in ... erted. ROO. the variable control, and
the resistor network R317,
R3l9, R325. R326. R327.and
R328 determine the ratio of currents in pin 11 and pin 12,
hence the ratio of currents
in transistors connected to pins
6and 8to the current
in transistors connected to pins 5
and 9. Varying this ratio controls the gain of U3101•At
all
sensiti ... ities other than 5mV/di .... pins 11 and 12 of U350
are
shoned together by 595. Thus hall the signal current in
U350
is lost through R367 and R368. In 5mV/div, S95
contacts open, causing
all 01 the signal current to ftow in
the transistors connected to pins 5and 9, dOUbling the gain
I Gilbert. Barrie. ANew Wide·Bend Amplifier Technique, IEEE
Journel of Solid Stete Circuits, Vol SC.J number 4. December,
1968, P353.
3-1

Circuit Description-485/R485 Service
of U350. Other 595 contacts !shown on schematic 41 close
to maintain
the voltage at outer collectors pins 5 and 9 of
U350. R355, A356, and R357 Ire low frequeocy (thermal)
transient response compensations. The
CH 2gain control
R358
is used to at10w the CH 2gain control (front·panell
to be centered in its range.
SIGNAL CHANNEL SWITCH
Signal from U350 is connected to pins 13 and 1 of U430
and exits U430 at pins 16 and 14. The signal then continues
on to and is terminated by the trigger channel switch U530,
diminished
slig,tly in bandwidth by the energy extracted
by U430.
Apum'PUlI position control signal is injected between
U430 and U530 throug, R376 and R386.
If no currtf'lt flows through 0436, pin 12 is 0.3 V hig,er
than pin 11 and U430 is on. Under this condition, the
amplified signal leaves pins 5 and 9 on 100 nEC board
transmission lines, )oim with the signal from channell and
enters the AExt Trigger relay K410 on 50 n EC board
tran§mission lines.
lostrumenu without the A Ext Trigger option contain
iumper wires in place of the relay. The AExt Trigger signal
is injected by the relay at this location, which is considered
to be the interlace between
the preamplifier and the main
vertical amplifier. ARLC network consisting of C407,
L409. R407, etc. provides some
boost to compensate for
delay line losses
al high lrequeocies.
The signal
next passes into the delay line OL410 which
delays the vertical signal
75 ns so that the sweep may start
belore presentation of the triggering signal.
INTERNAL TRIGGER AMPLIFIER
Signals looped through U430 continue 10 trigger channel
switch U530 and are terminated
in R528 and R529 which
are the collector loads lor U350. If no current flows in
0538, voltage divider A534 and A535 holds pin 12 of
U530 above pin 11 and the CH 2trigger signal amplified in
U530 appears between pins 5
and 9 of U530. Trigger
channel switch U530 is turned off when 0538 conducts
4mA, pulling pin 12 down to 9.8 V(0.3 Vbelow pin 11).
Signal
current in this case is shorted out by the connection
between pins 6and 8. In either case, the total currenl in
U530
is unchanged so that the voltage on pins 5 and 9 of
U510 remains undunged at +12 V.
0544 and 0546 provide small additional gain for the
trigger signal. AT551 temperature compensates the gain of
3·2
stages U310 through 0564. 0552 and 0556 select the
bandwidth of the trigger signal.
In the full bandwidth mode 0556 is on and 0552 is off.
Filter
l559, R559, RS57, and C557 provides a constant
impedance at all frequencies as the collector load of 0556
and thus full bandwidth. In the 20 MHz mode 0552 is on,
0556 is off, and high frequencies are attenuated by C557.
Since
the bandwidth filter in the vertical amplifier is a
two·pole filter,
the 3dB point of the trigger's single pole
filter
is slig,tly less than 20 MHz. The low frequency phase
shift of the twO filters is identical, however, so that phase
match may occur in X·Y operation of the scope.
Signals
at the collector of 0556 are buffered by eminer
follower 0564 and sent to A and Btrigger amplifiers via
J568 and J566 respectively. CR562 matches the base
emitter junction of 0564, and A560 is adjusted so that the
zero signal voltage at 0564 's emitter is zero. A562 pulls
away the collector current of 0556 or 0552.
0572 and 0576 buffer the trigger signal, which becomes
the Xsignal in X·Y mode. C572 is part of the filter which
matches the delay of the vertical delay line OL410.
Trace Separation
Trace separation circuitry injects apositioning signal at
the main vertical interface during the operation of 8sweep
in the ALTernate sweep mode.
0440, 0456, 0444, and 0454 provide the push·pull
trace separation signal through R440
and R456. A458 and
C458 ensure that the output impedance of the trace
separation circuitry
is 100 n at all frequencies.
MAIN VERTICAL AMPLIFIER
The signal received Irom delay line Ol41 0 is terminated
and Tcoil peaked by the induclaoce 01 pins 1, 13, 14, and
16 of U620. The required additional inductance L621 and
L625 is achieved in the leads to pins 16 and 14 which are
lengths
of run on the etched circuit board. This inductance
plus C621 and C625, made from 3layers of elched circuit
board,
and R621 and R625 form the forward termination
of the delay line. The same method of termination and
peaking is used in each stage using the 151.()()78 integrated
circuit.2The network attached to pins 2 and 3 of U620,
provides delay tine and thermal transient response compen'
sation. The gain of the main vertical amplifier is adjusted by
R629. Vertical amplifier gain is temperature-compensated
by RT644.
2Addis, John "'Th," T.etlnolOlll~ On 0 ... Chip ~k. A8roHlHnd
Amplili ... ~. Etecuonics. Ju ... 5,1912, pt. 103.

The network connected between U650 and U660 is the
bandwidth limiter network. The network provides acon·
stant impedance of 50 nper side at all frequencies looking
from pins 4and 10 of U660.
In the full bandwidth mode,
signal from pins 5
and 9of U650 loops through U660 and
is terminated in the bandwidth limit network. In the
20 MHz mode, the signal is obtained from pins 6and 8of
U650.
In passing through the bandwidth limit network
signals above 20
MHz are attenuated at 12 dB/octave.
U660
is an FT doubler configuration with the input
Tcoil peaked.
Bias for the inner set of bases is set by R672
and R673. The
output transistors are high voltage devices
which drive the CRT's distributed deflection structure.
In the event of aloss of aCRT termination resistor, the
long tail
current through R657 pulls pins 2, 8, and 3of
U660 down. When the +27 Vsupply goes below 25
v.
0686 turns on, turning on 0688 which shorts the 25 V
supply. The loss of the +25 Vsupply
will shut down the
power supply inverter. See inverter current limiting.
SCALE FACTOR REAOOUT
Scale factor readout control circuitry is in U80. With a
, X probe or with an non·readout coded connector con·
nected to the input connector code ring surrounding J1. pin
11 of U80 is connected to +5 Vthrough RS1. The voltage
at pin 11 is sensed by U80 and pin 10 is pulled down,
lighting the Xl light
emining diode (LEDl. CR99. 11 kfl: to
ground from the code ring shuts off CR99, and turns on
CR98. the
Xl0 LED. A 6.8 kfl: from the code ring to
ground will shut off the X10 LED and light CR97, the
X100 LED. Ashort circuil on the code ring provides trace
ID which pulls down pin 7of UBO. shilts the trace upward
one·fourth
of adivision, and shuts off the scale factor light.
The trace 10 signal goes to R289 on
CH 1and R389 on
CH 2to provide trace shift in the vertical pre·amplifiers.
VERTICAL MODE CONTROL
Ul535B controls the vertical logic (diagram 3) via two
control leads from pins 8and 9. These leads also are
connected
via 01590 and 01594 to the trigger logic
(diagram 4) when the trigger
is NORM. The trigger logic is
independent of U1535B when CH 1or CH 2triggers are
called up.
U1535B
is controlled by the mode switch in CH I,
CH 2, ADD, or X·Y. In the ALT or CHOP mode, the IC's
control the input to pin 7of Ut535B to cause it
to eilher
alternate on command from AGATE or to CHOP. When
the HORIZ
is in ALT, and the vertical in ALT, Ul5358
changes state only every other AGATE. Note thai AGATE
is the signal that activates U1535 in AlT and the signal
Circuit
Description-485/A485 Service
from
Ul530B pin 6only allows every other AGATE
through.
Ul535A output is AGATE 2, and Ul535B is A
GATE 4.
In the CHOP mode, U1585 is on and Ul535B changes
state every time U1585 makes an output. While U1585 is
making an output asignal from Ul585 pin 4blanks to
prevent displaying the switching from CH 1and CH 2and
back.
ZAXIS
Ul560 output drives the ZAXIS amplifier. U1560 is
controlled by the Aand BGATES, the A·8 COfltrolvia pin
15, the INTENsity controls and the
EXT ZAXIS.
The circuit consisting of
01566 and 01568 disconnects
the BINTENSITY control during
X-Y or when A"'l, 2, Of
5 ns.
ATRIGGER AMPLIFIER
The Source Switch SW700 selects one of the four modes
which the
485 will trigger on: INT, LINE, EXT, or EXT-:-
10. Signals above approximately 1 MHz. except in HF REJ
are coupled throu!/l C705. A705 and R709 form a 1
MfI:
input resistance and a loss pad to compensate fOf the loss
across C705, matching high frequency and low frequency
gains. R708 protects
0712A input by limiting current when
CR711 or
0712A is forward biased. 0712A and 07128
form asource follower with the source voltage very close to
zero. The source follower drives
0716, an emitter follower.
which drives U730. The balun coil T719 produces a
differential signal. U730, U738. and U740 are cascode
amplifiers. The slope switch
SW72O, switches the collectors
of U740 to
give selection 01 the triggering slope.
AEXT TRIGGER IDENTIFY
The A EXT TRIG IDENTIFY function is driven by the
emitters of U730; this allows U830
to be coupled to the
trigger signal and the
level control. The center line crossing
point of the A EXT TA IG signal corresponds to the trigger
point of the sweep for timing information.
When the
EXT TRIGGER IDENTIFY switch is pushed,
U830
is turned on and its collectors are connected to the
vertical delay line
via relay K410. VR838 and VR839
provide
DC level shih.
The cables between JS42 and JS43 and relay Kl0 have a
time delay so
that the EXT trigger sigoal will be displayed
with the same time position as signals arriving through
CH 1
or CH 2input.
3-3

Circuit Oescription-485/R485 Service
ATRIGGER GENERATOR
The trigger amplifier drives aSchmitt Trigger circuit
based around CR751 and aconstant resistance load. This
circuit provides astandardized
output pulse that is time
related to the trigger event. This standardized pulse
and a
holdoff signal drive atwo tunnel diode circuit
to provide
stable triggering from
DC to at least 350 MHz with very low
trigger jiller.
CR762
is the arming tunnel diode and CR772 is the
output tunnel diode. After CR762 is fully armed and
CR772
is partially armed (at the end of holdoffl, apulse
from CR751 causes CR762
to go to its high state. This
increases the current through R764 to fully arm CR772,
lNhich goes to its high state lNhen the same pulse that fired
CR762
is received through the short delay line. The output
of CR772 drives differential pair 0792 and 0794. When
CR772
is in its high state pin 1of U7BO is high, turning on
the light DS781 (TRIG'D).
0792 provides the AGATE
through emitter follower
0810. The output of 0794
couples to adifferential pair 0802 and 0804, which
provide the sweep gates.
When the sweep
is completed, apositive signal from A
sweep turns on
0816 and 0822 to provide asignal to pin
16
if U780. This initiates the holdoff portion of U780 and
pin 17 goes positive. Pin 17 stays positive
as determined by
timing R's and C's on the timing switch
to pin 8. The
holdoff signal on pin 17 of U780 drives
0768 and 0778.
0768 and 0778 93te the current to CR762 and CR772.
lNhen 0768 and 0778 are off, CR762 and CR772 are
receiving the appropriate current as set by R765
(A arming
T.D.) and R775
(A output T.O.l. lNhen 0768 and 0778 are
on they reset CR762 and CR772
to their low states.
In AUTO TRIG pin 19 of U780 is grounded. The trigger
signals must recur
at least every 50 ms, which is the lowest
rate the circuit
will provide atriggered sweep in AUTO
MODE. The timing of the AUTO MODE is determined by
the rime constant of R787 and C787.
In SINGLE SWEEP mode +5.5 V is applied to pin 12 of
U780. There
will be only one sweep for each time U780 is
armed by pushing the RESET switch. The sweep will fire
v.tlen atrigger signal is received ........ ile or after the RESET
switch
is depressed. The READY light on pin 11 of U780
indicates
........ en the sweep is armed.
BTRIGGER GENERATOR
Is similar to ATrigger Generator except it has RUNS
AFTER DELAY TIME mode
that disconnects the trigger
amplifier. In addition, it uses ATrigger Gate and delay
pickoff gate as hold·off.
3-4
In the triggerable mode, the BTrigger Generator triggers
only after receiving both the delay pickoff and AGate
signals. Delay pickoff gate drives comparator
01092 and
01096. Both AGate and delay pickoff gate are required to
drive 01082. The output of Ql074 provides arming current
for CR1062 and CR1104.
\'Vhen the triggering signal causes
CRt052 to go to its high state, CR1062 is properly armed,
and
will fire arming CR1104, which fires approximately
1.5
ns after CR1062.
In 8RUNS AFTER OELAY TIME mode, the trigger
amplifier is turned off via R1038 and enough current is
flowing in the base of 01068 so that it is in saturation. This
saturation routes enough
current to CR1104 to turn it on
when AGate and delay pickoff gate arrive.
In both cases when CR1104 goes to its high state,
comparator
01106 and 01110 changes state giving a B
GATE
out thru 01114. 01106 is connected to comparator
01121 and 01124 ........ ich changes state, giving plus and
minus sweep 93te
out.
In A1, 2, and 5ns, B is in BRUNS AFTER DELAY
TIME.
In addition in 1and 2ns the voltage to v.tlich C1121
charges is varied by the HORIZONTAL POSITION control.
This gives atime position
to these two sweep speeds \o\'hen
using the HORIZONTAL POSITION control.
Aand BSWEEPS
Aand Bsweep generators each produce, upon receipt of
agate, ane93tive going sweep of 1.0 V/div of horizontal
excursion, starting at +13
V.
In ASweep, 0884 and 08BS are the miller amplifier.
Timing resistors R873 and R1409
to R1422 return to
+50 Vin calibrated settings. The values of the timing
resistors and capacitors C877 to C882, C1404 and C1405.
at various sweep speeds may
be determined on the Timing
Switch and Generator schematics.
Q862 compares the
sweep
output to +13 Vand sends the error signal to the
miller input via R866 and
0854. Receipt of a + gate at
TP851 turns off
0854, allowing the sweep to run. CR867
must not conduct during baseline, so
0866 sinks the higher
timing currents.
0934 turns on at the end of the sweep,
producing a + going stop pulse
that terminates the sweep
gate.
0856 and 0858 feed acurrent to the miller input at
the end of the gate to prevent large negative voltages and
long recovery times at low timing currents. R863 affects
linearitY at
10 and 20 ns/div.
The Delay Comparator
0902 enables BSweep when the
sweep voltage becomes less than the voltage on the Delay
Time position control,
\o\'here U910 is avoltage follower.
|
Correction by DCP 7 Sep 2016
Original: Q892 compares the

0908 disables BSweep in AHORIZ DISPLAY mode and
0914 introduces an offset at 10 and 20 ns/div. Switch A9
opens when ASweep speed
is 1, 2, or 5ns/div to start B
Sweep with minimum delay, since the BSweep Generator
produces these speeds regardless of the HORIZ DISPLAY
mode.
BSweep generator
is similar to A except that the
maximum
output rate is 5times faster. The sequence of
timing
components is different from ASweep. 01238
prevents large positive voltages that would occur at the
miller input after the end of the sweep but before the end
of the gate.
For ASweep speed settings of 10 ns/div
or slower, A
HORIZ DISPLAY mode displays ASweep. BHORIZ
DISPLAY mode displays
B. ALT, INTEN, and INTEN (A)
function. However, when ASweep speed
is set at 1. 2, or
5ns/div the Horizontal system displays BSweep which has
been gated on by Atrigger. Switch A9
is open so that the
Delay Time Position control
is effectively set to zero. At A
.. 5os/div, the displayed 8Sweep
is positioned horizontally
in the amplifier. At A '" 1or 2ns/div, the POSITION
control
is disconnected from the Horizontal Amplifier via
switches 819 and 820. The POSITION control is connected
to avariable delay circuit
01102 which time-positions 8
Gate for
about 30 ns after AGate.
HOR IZONTAL AMPLIFIER
The Asweep or 8sweep may be selected by saturating
01312 or 01318 respectively. This is done by the
horizontal control logic level at N4. If N4 is below -5 V.
01336 is off and 01338 saturates. This allows the node
R1313. R1314 to
go up and turns off 01318 and saturates
01312. The output of Asweep generator has been selected.
01338 being saturated also shunts to ground any signal
from 8sweep generator
that might be present in ALTernate
mode.
If N4 is -3 V. 8sweep is selected as 01336
saturates, turning off 01338 and pulling down on R1314.
saturating
01318 and turning 01312 off. CA1311 will
shunt any signal from Asweep.
A
+1 Vat N4 will turn 01342 on which turns off
01358, disconnecting Aand 8sweeps and saturating
01356, selecting the Xsignal for X·Y.
In A" 1,2, or 5ns, 818 closes and 01331 turns on.
saturating
01322, a2X magnifier. To provide offset to
enable viewing at sweep start, in 5ns 822 closes and offset
through R1327
is provided. In 1and 2ns, the base of
01328 is moved and a current from 01328 through 01331
and the base collector of 01332 is inserted to provide
offset.
Circuit
Description-485/A485 Service
01362 and 01364 compose a HI gain amplifier. The
feedback A
is A1364 and A1366. The input resistance in B
sweep
is A1321 and A1322. If Asweep is selected the input
resistance is Al307 and A1308.
8y selecting Bsweep and adjusting R1366, the hori-
zontal gain
is set. Then by selecting Asweep and adjusting
R1308. Asweep may be calibrated.
80th adjustments are
done in 0.1 ms/div. To get Aand Bsweep to start at the
same
point on screen, R1325 (8 REGISTER) provides
some offset
current for positioning Bsweep relative to A
sweep. This
is done in ALT horizontal mode. 01378 and
A1368 provide a
constant current for the operational
amplifier
01362, 01364. 01366 is saturated in Beam
Finder by
S6008. This cuts the feedback resistor value by
2/3 and Rf/AI is reduced. C1364 provides linearity control
for 1. 2. and 5ns/div.
01372 and 01374 are aparaphase amplifier to provide a
differential signal
to the horizontal amplifier. C1372,
C1374. A1372, and R1374 provides thermal compensation
for
01372 and 01374. Since the horizontal is symmetrical.
we will deal with one half only.
01172 provides isolation
between the CRT capacitance and the input amplifier
01154. 01172 and 01174 provide the current path to
drive the deflection plate.
01192 is'an emitter follower to
provide feedback to
01154. By using active feedback the
actual feedback resistance may be lowered without sacrific-
ing gain. This is accomplished by utilizing the impedance of
01192, emitter to base. and the divider circuit of A1177
and A1178.
CR 1156 provides a -5 Vreference for 01154
and CR1152. CA1153 limits the voltage swing to keep
either half of the horizontal from saturating. This allows
the horizontal to be very linear.
HORIZONTAL CONTROL
This circuit is activated by the HORIZ DISPLAY
controls. Several
output signals are derived; Bdisable,
horizontal control.
A·B control autofocus, trace separation
and vertical control drive.
The basic circuit consists of U1535A which is controlled
by the HORIZ DISPLAY. The HQRIZ DISPLAY switch
is
overridden when 8sweep is used in A" 1, 2, or 5ns or is
X-Yo Atruth table is shown on the diagram 12.
The main function of U1535
is to provide a HOR IZ
CONTAOL signal to Ihe horizontal Bpreamplifier. With the
X-Y control of 01520 (which saturates in X·YI, this signal
on pin
N4 has 3levels. In Ihe AL Tmode, U1535 switches
from one state
to another each time the AGATE goes
negative. U1530C inverts the AGATE
to get the positive
edge required by U1535A.
(It is important to maintain
correct timing with the sweep control since switching must
3-5

Circuit Description-485/R485 Service
occur during holdoll.)
Pin 5of U1535A alternates from HI
to LO and the sweep switches from A to B and back again.
When B
is called up and pin 5 is HI. the ZAXIS IC will be
in the Bmode via pin 15 signal and the Bintensity control
will function and Autofocus control is also provided. Also.
the trace separation signal will be LO, causing the trace
separation control to function. This
is via control of
U1530B.
All inputs must be HI to get LO outputs, and this
only occurs
in AL Twhen B is called up. The output of
Ul530B also drives the vertical logic when the
HOR IZ
DISPLAY is in ALT. Bdisable is controlled by A, the X·Y
signal. and switch 5WA2 (which is closed ...men A=1, 2, or
5ns).
POWER INVERTER/REGULATOR
General
The Power Inverter/Regulator circuit provides the opera-
ting power for this instrument from aline voltage source or
DC voltage source. This circuit also includes the line
Voltage Selector switch to allow for selection of a115 Vor
230 Vnominal operating voltage. Fig. 3·1 shows ablock
diagram of the Power Inverter/Regulator circuit.
Asche·
matic is shown on diagram 14 at the rear of this manual.
line Filter
The purpose of the line Filter is to prevent the
instrument from injecting power supply frequency inter-
ference into the power line, or power-line interference from
entering the instrument. 11812 and 11813 provide both
common-mode and differential filtering. R1B12 and R1813
are damping resistors. C1B12 and Cl
B13 are common·mode
filters and C1814
is adifferential filter. R1Bll discharges
C1814 when the power switch
is off.
Line Input Circuit
Thermal cutout 51802 provides thermal protection for
this instrument.
If the temperature exceeds asafe operating
level,
51802 opens to interrupt the applied power. When
the temperature returns to asafe operating level, S1802
automatically closes
to re-apply the power.
CR1821 contains the main power rectifiers for the
line
Input circuit. C1822 and C1823 are the line·storage
capacitors. With
51803 in the 115 Vposition. the circuit
acts as afull·wave voltage
doubler so that the voltage across
the series combination of C1822 and C1823 will be the
peak-to·peak value of the line voltage.
In the 230 V
position, CR1821 acts as afull,wave bridge rectifier.
Therefore the voltage developed across C1822 and C1823
wilt be approximately the same for either
115 Vor 230 V
operation.
RT1821 and RT1822 are thermistors which limit the
charging currents during turn on. When the
POWER switch
is turned off, the line 5top circuit SlOPS the Inverter and
C1822 and C1823 discharge thru R1822 and R1823.
[ WARNING'
Because the discharge is slow. dangerous potencials
will exist across capacitors C7822, C1823, and other
connected components for several minutes after the
POWER switch
is turned off. The presence of voltage
in the
circuit is indicated by relaxation oscillator
R1824, C1824, and D51824. Neon
bulb 051824
blinks until the potential drops to approximately
TOO V.
fl801
Lin.
(
Inp"t Voln,,,,
lint
fill ..
LiM Inp"t
CR1821.
C1822. C1823
Li ... T.igve<
St .... N.twO<lc
VR1831.C1829
In ......
01834.01844
Regulator &
Stop Monon."
01900.01902
/Mloin Pow .. T .. nslormw
fT1960
3 kV p..p
rTo H.V. Cif."iU
,-1--""
Low VOlt I---~""
R .. I'fl ...
f--~'"
f---~ ..,
L I---~"5
l
'~"'~"~'~O'~A~mt"~;'~"}~==~==
L
~ LiM StOP
& Protection +59.4
Ul910 Bel.""a N_
Fig. 3·1. Block diag.am of power inverte.lre9"IIIO' c',,:u;l.
3·6
REV. 8, AUG. 1975

Circuit Description-485/R485 Service
D51801 and D51802 are line voltage transient pro·
tectors. With $1803 in the 115 Vposition, only OS1801 is
connected across the line. If apeak voltage surge in excess
of approximately 230 V is present on the tine. 0$1801 will
break down and conduct sufficient current to open the line
fuse Fl801. In the 230 Vposition, 051801 and 051802
are in series across the line to protect against voltage surges
exceeding 460 V.
Transformer T1801 provides asample of the line \/oltage
lor triggering at line frequencies. It also provides asignal to
the Line Stop circuit to indicate the presence of line voltage
at the input to this circuit. T1825, C1825, and C1827
provide common-mode filtering. C1S26 and L1825 act as a
differential filter.
Start Network
Resistive divider R1828 and R1829 is connected be·
tween the input line and the negative side of C1823. When
the line voltage goes positive,
C1829 charges. At the same
time CR1842 conducts. charging C1835, and CR1843
conducts
to charge Cl848. When the voltage on C1829
reaches approximately
3,2 V, VR 1831 conducts, discharging
C1829 thru
the base of 01844. As 01844 turns on, C1835
discharges
thru L1835 to start the Inverter. After operation
has begun,
CRl842 discharges C1829 with each cycle of
the Inverter. This disables the start network.
Inverter
Refer to the simplified schematic shown in Fig. 3-2.
Once the Inverter has been started by the
Start Network, the
Inverter
is self-oscillating. Feedback necessary lor oscilla·
tion
is provided by base·drive transformer T1831. The
series-resonant circuit, consisting of
11835 and C1835. has
anominal resonant frequency of
about 25 kilohertz. To
provide regulation of the voltages induced
in the secondary
circuit, the action of regulator transistor
01900 varies the
frequency of oscillation on the low side of resonance by
holding
both 01834 and 01844 off for atime during each
half cycle as determined by the Regulator circuit (see
Inverter Regulator for amore detailed discussion of regula'
tion).
Transistors
01834 and 01844 are aswitching pair,
where only one transistor can
conduct at atime. The
direction of current flow
in the feedback winding of T1831
determines which transistor will conduct. Transistors
01834 and 01844 change states every half cycle. The
switching action provides asquare-wave voltage at the
emitter of 01834, which has apeak·to·peak voltage ax.ut
equal 10 the DC voltage from the line Input circuit. This
square-wave voltage supplies the drive necessary
10 main-
lain oscillation in the resonant circuit. When both
01834
and 01844 are being held off by 01900. resonant circuit
current flows through CR1834
or CR 1844. The resonant
circuit
current drives the primary of the power transformer
T1960 and thus supplies power to the Secondary circuit.
In normal operation, the sequence of events during one
cycle of operation
is as follows;
1. Assume the current in the resonant circuit is at a
ampere and beginning to increase in the direction to cause
conduction
in CR1834. At the time the current reaches a
ampere, regulator transistor
01900 is turned on by the
Inverter Control circuit. The Regulator transistor holds
both 01834 and 01844 off for acontrolled amount of
lime. During this time, resonanl circuit current flows
through CR1834.
+ -,,---------------,
01834 CR1834
II
INVERTER 1---,
<::....-.L_'""--.J'OC
T1848
CR1844
~~f;~==""=J'_'_""Jnl p ",w,,,,
r-1::J--~,1I C •J '''DO'
DC INPUT
VOLTAGE
119J..04l
Fig, 3·2, Simplified schematic of invetter.
REV.C.AUG.1975
3-7