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This manual applies to all instruments having serial prefix
1209U. Instruments with a serial prefix above 1209U may
have a Manual Change Sheet, to which reference should be
For Free Distribution Only
Copyright HEW LETT-PACKAR D LIMITED 1972
SOUTH QUEENSFERRY, WEST LOTHIAN, SCOTLAND
03720-95001 Printed: APRIL 1972
For convenience and ease of use, information on Model 3720A Spectrum Display
has been presented in two publications:
.OPERATING MANUAL hp Part Number 03720-95000
.SERVICE MANUAL hp Part Number 03720-95001
HOW TO ORDER
To order the above publications contact the nearest Hewlett-Packard Sales and
Service Office (addresses a t the rear of this manual). Give the hp Part Number of
the publication and the complete model number, name and serial number of the in-
SERVICE MANUAL CONTENT
Section I contains General Servicing Instructions and a Description of Controls
Section I gives information on Parts Identification, Maintenance and Repair
Section II contains a list of Replaceable Parts for the 3720A.
Section V gives the 3721A/3720A System Troubleshooting Procedure.
Section V contains the Functional Diagram, Assembly Location Diagrams and
Assembly Packages including Schematic and Component Location.
rhese publications are also available in 6in x 4in microfiche form. Each sheet contains
up to 60 pages, maximum, of text and illustrations. Order as above, quoting the
appropriatehpmicrofiche part number.
List of Contents
LIST OF CONTENTS
SECTION I INTRODUCTION
Paragraph 1.1 OPERATING INSTRUCTIONS . . . . . . . . . . . . .. Page 1-1
1-3 GENERAL SERVICING INSTRUCTIONS . . . . .... 1-1
Table 1-1 Accessories furnished with 3720A . . . . . . . 1-1
Figure 1-1 Model 3720A Spectrum Display . . . . . . . . . . . . . . 1-0
1-2 Front panel controls and indicators . . . . . . . . . . 1-2
1-3 Rear panel controls and connectors .. ... 1-4
SECTION II GENERAL MAINTENANCE
Paragraph 2-1 INTRODUCTION . . . . . . . . . .. Page 2-1
2-3 IDENTIFICATION . . . . . . . . . . . . . . . 2-1
2-4 Assembly Identification . . . . . . . . . . . . 2-1
2-6 Assembly Connection Identification .. . . . 2-1
2-8 Integrated Circuit Identification . . . ... 2- 1
2-15 INSTRUMENT DlSMANTLl NG . . . . . . . . . . . . ........ . . . 2-2
2-16 Removing Top and Bottom Covers . . . . ........ . . . 2-2
2-17 Removing Side Covers . . . . . . . . . . . ... 2-2
2-18 Front Panel Access . . . . . . . . . . . . . 2-3.
2-20 Locations of dc Power Supply Fuses . . . . . . . ... 2-3
2-23 Replacement of Display Sensitivity Lamps . . . ... 2-3
2-27 Cleaning the Air Filter . . . . . . . . . . . . . . ... 2-4
2-29 Cathode Ray Tube RemovaI/RepIacement . . . . . . . . . . . . . . ... 2-4
2-30 Line Transformer Removal/Replacement . . . . . . . . . . . . . . . . ... 2-5
2-31 Chassis Mounted Transistors Plate Removal/Replacement . . . . ... 2-5
2-32 High Voltage Tripler Assembly Removal/Replacement . . . . ... 2-5
2-33 REPAIR PROCEDURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... 2-5
2-34 Etched Circuits . . . . . . . . . . . . . . . . . . . . . . . ... 2-5
2-36 Integrated Circuit Removal/Replacement . . . 2-6
Table 2- 1 3720A Assem bl ies . . . . . . . . . . . . . . ... 2-1
2-2 Digital integrated circuit abbreviations .... 2-2
F igure 2- 1 A18/A19 Housing assembly retaining screws .. ... 2-3
2-2 CRT neck connections . . . . . . . . . . . . . . . . ... 2-4
SECTION 111 REPLACEABLE PARTS
Paragraph 3- 1 I NT R 0 DUCT I0 N . . . . . . . . . . . . . . Page 3-1
3-3 ORDERING INFORMATION .. ..... 3-1
3-7 REFERENCE DESIGNATORS .. .. 3-1
3-8 LIST OF ABBREVIATIONS ..
3-9 COLOUR ABBREVIATIONS ..
iable 3- 1 Replaceable Parts . . . . . . . . . . ........ .. 3-4
3-2 Code List of Manufacturers . . . . . ........ .. 3-50
Figure 3- 1 Location of cabinet components .. .. 3-49
SECTION IV SYSTEM TROUBLESHOOTING
Paragraph 4-1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . Page 4-1
4-3 USING TROUBLESHOOTING SECTION .. . . . . 4-1
4-8 TEST EQUIPMENT . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-2
4-10 Logic Probe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-2
4-12 SYSTEM TROUBLESHOOTING CONTROLS SET-UP .. 4-3
4-14 FLOW A . Control Faults . . . . . . . . . . . . . . . . . . . . . . .. 4-5
4-16 FLOW B . Display Sensitivity Faults . . . . . . . . . . . . .. 4-6 c
4- 19 FLOW C . Transfer Faults ,. . . . . . . . .
Data .. 4-7
4-21 FLOW D . Other Transferred Signals Faults . . . . . . . . .. 4-8
4-23 FLOW D(a) . Timescale Transfer Faults . . . . . . . . . . . . .. 4-8 k
4-25 FLOW D(b) . Correction Factors Transfer Faults ..
Gain .. 4-9
4-27 FLOW D(c) . Signal Recovery Transfer Faults . . . . . . .. 4-10 I
rable 4- 1 Flow charts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-1
3721A/3720A System repair procedure . . . . . . . . . . . .
Test equipment required for System Troubleshooting ..
4-4 Range transfer check . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-6
4-5 Range data logic check . . . . ... .. 4-6
4-6 Ti mescale transfer check . . . . . . . . . . . . . . .. 4-9
4-7 Gain correction factors check . . . . . . . . . . .. 4-10
4-8 Signal recovery transfer check .. ... .. 4-10
Figure 4- 1 750KHz . . . . . . . . . . . . . . . . . . .. 4-4
4-2 Transfer Data duration . . . . . . .. 4-4
4-3 Data Ready . . . . . . . . . . . . . . .. 4-4
4-'4 Transfer Data repetition rate .. .. 4-4
4-5 Multiplexed range outputs .. 4-6
SECTION V SCHEMATICS. COMPONENT LOCATIONS. DIAGRAMS
Paragraph 5-1 INTRODUCTION . . . . . . . . . Page 5-1
5-3 DRAWING SYMBOLS . . . . . . . . . . . ..... 5-1
5-5 ASSEMBLY PACKAGES . . . . . . . . . . . . ..... 5-1
5-7 Locating Assembly Packages . . . . .. 5-1
5-9 THEORY OF OPERATION . . . . . . . .. 5-1
5-10 Functional Diagram .. .. 5-1
Guide to schematics ..
Logic package outlines
.. 5-3 1'
. I WWW.HPARCHIVE.COM
List of Contents
5-3 Functional diagram .. .. .. .. .. .. .. .. .. .. .. .. 5-4
5-4 Guide to logic symbols .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 5-4
5-5 Location of components (front and rear panels) .. .. .. .. .. .. .. .. .. 5-5
5-6 Location of assemblies and chassis mounted components (top view) .. .. . .. 5-6
5-7 Location of assemblies and chassis mounted components (underside view) .. .. 5-7
5-8 Wiring of rotary switches .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . .. 5-61
5-9 Wiring of rear panel connectors 5-63
A1 Not Assigned
A2 Mantissa Range Assembly Package
Figure A2-1 Component location, A2 .. .. .. .. .. .. .. .. .. .. . .. .. Page 5-10
A2-2 Schematic A2 (part 1 ), Mantissa Range assembly . . .. .. 5-11
A2-3 Schematic A2 (part 21, Mantissa Range assembly 5-13
A3 Decade Range Assembly Package
Figure A3-1 Component location, A3 .. .. .. .. .. .. . .. .. .. .. .. .. .. .. . Page 5-14
A3-2 Schematic A3, Decade Range assembly . .. .. .. .. .. .. .. .. . .. .. 5-15
A4 Store 1 Assembly Package
Table A4-1 Component identification grid, A4 . Page 5 1 7
Figure A4-1 Component location, A4 .. .. .. .. .. ., .. 5-16
A4-2 Schematic A4 (part 11, Store 1 assembly .. _. 5-17
A4-3 Schematic A4 (part 21, Store 1 assembly .. .. 5-19
A5 Store 2 Assembly Package
Figure A5-1 Component location, A5 .. .. .. .. _. .. Page 5-20
45-2 Schematic A5, Store 2 assembly . _ . .. .. 5-21
A6 Buffer Assembly Package
Figure . A6-1 Component location, A6 .. .. .. .. . Page 5-22
A6-2 Schematic A6, Buffer assembly ._ . _ _ _ _ 5-23
A7 Switch Card Assembly Package
Table A7-1 Component identification grid, A7 . Page 5-25
Figure A7-1 Component location, A7 .. .. .. .. ._ . .. 5 2 4
A7-2 Schematic A7, Switch Card assembly .. .. .. 5-25
A8 Hybrid Multiplier Assembly Package
Table A8-1 Component identification grid, A8 .. . .. .. Page 5-27
Figure A8-1 Component location, A8 .. .. .. .. ._ .. .. ._ _. 5-26
A8-2 Schematic A8, Hybrid Multiplier assembly .. .. 5 2 7 ,
. , . ._
A9 Window Multiplier Assembly Package
Table A9-1 Component identification grid, A9 .. .. Page 5-29
F igure A9-1 Component location, A9 . . . . . . . . . . . . . . . . . . . 5-28
A9-2 Schematic A9, Window Multiplier assembly .. .... 529
A10 Integrator Assembly Package
Table A10-1 Component identification grid, A10 .. .. Page 5-31
Figure A10-1 Component location, A10 . . . . . . . . . . ........ . . . . 5-30
A10-2 Schematic A10, Integrator assembly .. ........ . . . . 5-31
A1 1 Polar Converter Assembly Package
Table A1 1-1 Component identification grid, A1 1 Page 5-33
F igure A1 1-1 Component location, A1 1 . . . . . . . . . . . . .... 532
A1 1-2 Schematic A1 1, Polar Converter assembly . . . . . 5-33
A1 2 Low Voltage Power Supply Assembly Package
Table A12-1 Component identification grid, A12 .......... .. Page 5-37
Figure A12-1 Component location, Chassis Mounted Transistors . . . . . . . . ........ . . . . 5-34
A12-2 Schematic, Line Module and Line Transformer . . . . . . . . . . ........ . . . . 5-35
A12-3 Component location, A12 . . . . . . . . . . . . . . . . . . . . . . . ... _. ._ 5-36
A1 2-4 Schematic A1 2, Low Voltage Power Supply assembly . . 5-37
A1 3 Control Logic Assembly Package
Table A13-1 Component identification grid, A13 .. .. Page 5-39
Figure A13-1 Component location, A13 . . . . . . . . . . . . . . . . . . . . .. _. 5-38
A13-2 Schematic A13 (part 11, Control Logic assembly .. . . . . 5-39
A13-3 Schematic A13 (part 2), Control Logic assembly .. .... 541
A14 Triangle Wave Generator Assembly Package
Table A14-1 Component identification grid, A14 .. .. Page 5-43
Figure A14-1 Component location, A14 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... 542 r
A14-2 Schematic A14 (part 11, Triangle Wave Generator assembly .. . . . . 5-43
. . . . 5-45
A14-3 Schematic A14 (part 2), Triangle Wave Generator assembly ..
A15 Sine Shaper Assembly Package
Table A15-1 Component identification grid, A15 .. Page 5-47
Figure A 1 5 1 Component location, A15 . . . . . . . . . . . . . . . . . . . 5-46
A1 5-2 Schematic A1 5 (part 11, Sine Shaper assembly .. . . . . 5-47
A15-3 Schematic A15 (part 21, Sine Shaper assembly .. ,. 5-49
List of Contents
A16 Deflection Amplifiers Assembly Package
Table A16-1 Component identification grid, A16 . Page 5-51
F igure A16-1 Component location, A16 .. .. .. .. .. .. .. .. _. .. .. .. .. 5-50
A16-2 Schematic A16 (part l), Deflection Amplifiers assembly .. .. 5-51
A16-3 Schematic A16 (part 21, Deflection Amplifiers assembly - .. 5-53
A1 7 High Voltage Power Supply Assembly Package
Table A17-1 Component identification grid, A17 .. .. Page 5-55
Figure A17-1 Component location, A17 .. .. .. .. .. .. .. .. .. .. .. . .. .. 5 5 4
A17-2 Schematic A17, High Voltage Power Supply assembly .. .. 5-55
A1 8 Range Indicator Assembly
Table A18-1 LED Truth Table _. .. .. Page 5-57
Figure A18-1 Component location, A18 .. _. .. .. .. .. _. , .. 5-56
A18-2 Schematic A18, Range Indicator assembly .. .. 5-57
i A19 Range Units Assembly Package
Figure A19-1 Component location, A19 .. .. .. .. .. . .. .. Page 5 5 6
A19-2 Schematic A19, Range Units assembly . ,. .. .. 5-57
A20 Storage Assembly Package
Table A20-1 Component identification grid, A20 .. .. .. .. ._ .. Page 5-59
Figure A20-1 Component location, A20 .. .. .; .. . .. .. .. 5-58
A20-2 Schematic A20,Storage assembly .. , .. 5-59
Figure 1-1 Model 3720A Spectrum Display
1-1 OPERATING INSTRUCTIONS 1-5 Remember, when servicing the 3721A/3720A
System, that a high intensity CRT display on the
3720A over an extended period of time may cause
1-2 Detailed operating instructions are contained
burn damage to the storage mesh or phosphors of
in the 3720A Operating Manual but, for conven-
ience, descriptions of the controls, indicators and
connectors on the front and rear panels of the
3720A are included in this section. 1-6 The storage CRT in the 3720A is mechani-
cally delicate and is easily damaged by violent
movement or by being inverted. To prevent
1-3 GENERAL SERVICING INSTRUCTIONS
accidental damage in use, remember the following
points when handling the 3720A:
1-4 When servicing the 3721A/3720A System,
Table 1-1 Accessories furnished with 3720A
3 Accessory hp Part Number
Detachable Power Cord Various
Rack Mounting K i t 5060-8742
Servicing Extender Board 03720-70021
3721A/3720A Interface Accessory 15549A
Trimming Tool 8730-0013
Operating Manual 03720-95000
Service Manual 03720-95001
For Free Distribution Only
2 1 27 26 25 28 29 30
1'1 1'2 13 14 1'5 16 ii is 21
1. DISPLAY: 8 y 1 0 division CRT, w i t h graticule 6. STORAGE TIME: potentiometer. Varies t h e time
calibrated in one division squares. Major axes cali- any display is stored, when in STORE mode.
brated in 0.2 division intervals. 1 division = 0.95cm.
7. FOCUS: potentiometer. Adjusts sharpness o f C R T
2. DISPLAY SENSITIVITY: illuminated digital readout. beam.
Scales the horizontal and vertical axes and the fre-
quency ..interval, Af, as selected b y Scale Factor
8. PERSISTENCE: potentiometer. Controls duration of
written trace, when n o t in STORE mode. F u l l y
SCALE FACTOR: lever switch. Selects axis calibra- clockwise position selects STORE mode.
t i o n t o be presented o n the Display Sensitivity panel.
9. INPUT: lever switch. Routes data f r o m the 3721A
i n t o Store 1 o r 2 as selected. In HOLD position retains
4. ERASE: pushbutton. Eradicates all traces displayed
data in stores.
o n storage C R T screen, when n o t in STORE mode.
5. INTENSITY: potentiometer. Controls brightness o f 10. LINE-ON: toggle switch. Controls ac supply t o t h e
CRT beam. instrument.
Figure 1-2 Front panel controls and indicators
11. DATA: lever switch. I n positions 1 o r 2 displays 21. HORIZONTAL: rotary switch. Selects function t o be
contents o f respective store on CRT. I n extreme right- presented on horizontal axis o f CRT. Used together,
hand position transfers control t o SPECTRUM switch. controls 20 and 21 enable the user t o select the
presentation format of the display.
12. WINDOW: lever switch. ON position - applies a
triangular window t o i n p u t data. OFF position - 22. CALIBRATION lever switch. Allows external X - Y
natural window occurs. Recorder p l o t size t o be set.
13. SPECTRUM: lever switch. Controls choice o f store 23. START: pushbutton. Initiates a single sweep of the
contents t o be transformed. pen across the X-Y Recorder chart.
14. REAL/COMPLEX: lever switch. 24. RECORD: Lever switch. Controls outputting o f trace
R E A L - calculates cosine components only. on t o X - Y Recorder.
COMPLEX - calculates cosine and sine components.
25. HORIZONTAL SHIFT: potentiometer. Moves trace
15. GAIN: rotary switch. Selects gain o f integrator. horizontally o n CRT.
16. GAIN VERNIER: potentiometer. Provides continuous 26. CAL: potentiometer. Adjusts horizontal size o f display.
gain adjustment between calibrated steps. I n CAL,
I gain selected b y G A I N switch.
27. HORIZONTAL GAIN: lever switch. Magnifies horiz-
mtal axis o f displayed trace b y a factor of 2.
17. OVERLOAD: lamp. Indicates if integrator becomes
overloaded at any time during processing.
28. PHASE SHIFT: lever switch. Adds 0. +goo, or
18. INTERPOLATION: potentiometer. I n the M A N U A L -90' phase shift t o any display axis representing
mode fractionally alters each o f the 100 frequencies phase information.
a t which the transform is computed (up t o A f max).
I n A U T O 10 equally spaced display points are pro-
duced i n each frequency interval d f . 29. VERTICAL SHIFT: potentiometer. Moves trace ver-
tically o n C R T screen.
19. MARKER: potentiometer. Puts a marker o n any
30. CAL: potentiometer. Adjusts vertical size o f display.
p o i n t o f the displayed function.
31. VERTICAL GAIN: lever switch. Provides magnifi-
20. VERTICAL: rotary switch. Selects function t o be cation o f vertical axis o f displayed trace b y a factor
presented o n vertical axis of CRT. of 2.
3 Figure 1-2 Front panel controls and indicators (continued)
1 2 4 5 3 6 7
1. POWER MODULE: contains ac line socket, fuse, 4. X-RECORDER: BNC connector. O u t p u t t o horizontal
and voltage selector switch. axis o f X-Y Recorder.
AC Line. 11 51230V. k 10%. 48 t o 65Hz. 250VA.
5. Y-RECORDER: BNC connector. O u t p u t t o vertical
Fuse. 1 1 5 V -2.5AT, 2 3 0 V -1.25AT. axis of X-Y Recorder.
C l i p i n fuse. Cannot be changed w i t h o u t first re-
moving power cable.
Selector. Lever switch, 115V or 230V line voltage 6. PEN LIFT: BNC connector. Provides control signal f o r
selector. Cannot be changed w i t h o u t first removing pen o f an X-Y Recorder w i t h remote pen control
power cable. facility.
VALID DATA: BNC connector. Provides p signal t o
indicate that valid data is present at X and Y Re-
2. EXTERNAL INTERFACE: 50-contact connector. corder output connectors.
Duplicates 3721A Data Interface rear panel connector.
3. 3720A13721A CONNECTOR: 50-contact connector.
Input socket f o r data f r o m 3721A. NOTE: - denotes alternating current ( a d .
Figure 1-3 Rear panel controls and connectors
L _... .
SECTION I I
2-1 INTRODUCTION 2-6 Assembly Connection Identification
2-2 This section contains information on Identi-
fication, Instrument Dismantling and Repair Pro- 2-7 Throughout this manual connections to printed
cedures for the 3720A. circuit assemblies are referred to by an abbreviated
form. For example, the connection to pin M on A5
assembly is written as A5(M).
2-4 Assembly Identification 2-8 Integrated Circuit Identification
2-5 Table 2-1 lists the designator, title and hp 2-9 Digital integrated circuits are referenced by
Part Number of all assemblies in the 3720A the physical location of each package (MC pre-
Spectrum Display. (Assembly is abbreviated to fix) on the circuit board, and by their function in
Assy in this Table). the circuit (G, FF, etc, as applicable).
Table 2-1 3720A Assemblies
Assemb Iy hp Part
A? Not Assigned -
A2 Mantissa Range Assy 03720-70002
4.3 Decade Range Assy 03720-70003
A4 Store 1 Assy 03720-70004
A5 Store 2 Assy 03720-70005
A6 Buffer Assy 03720-70006
A7 Switch Card Assy 03720-70007
A8 Hybrid Multiplier Assy 03720-70008
A9 Window Multiplier Assy 07320-70009
A1 0 Integrator Assy 03720-70010
A1 1 Polar Converter Assy 03720-70011
A12 Low Voltage Power Supply Assy 03720-70012
A13 Control Logic Assy 03720-70013
A14 Triangle Wave Generator Assy 03720-70014
A1 5 Sine Shaper Assy 03720-70015
A16 Deflection Amplifiers Assy 03720-70016
A1 7 High Voltage Power Supply Assy 03720-70017
A1 8 Range Indicator Assy 03720-70018
A1 9 Range Units Assy 03720-70019
A20 Storage Assy 03720-70020
.2-10The MC number gives the physical location of 2-15 INSTRUMENT DISMANTLING
each package in the form of a grid reference.
2-11 With the printed circuit board held compon-
115/230V AC AND +158V DC SUPPLY
ent side uppermost, and tongued connector near-
WIRES ARE EXPOSED WHEN ANY OF
est, numbering of the rows and columns of inte
THE INSTRUMENT COVERS ARE RE-
grated circuits starts from the top left hand corner
MOVED. USE EXTREME CAUTION DUR-
of the board. The first figure following the MC
ING TROUBLESHOOTING, ADJUSTMENT
prefix gives the row and the second gives the
OR REPAIR. AVOID DAMAGE TO THE
column. For example, MC52 will lie in the 5th
INSTRUMENT BY DISCONNECTING THE
row down and the 2nd column across, whilst
POWER CABLE BEFORE REMOVING OR
MC103 will be in the 10th row and 3rd column.
REPLACING COVERS, ASSEMBLIES OR
2-12 The second method of identification of the
individual 'component blocks' helps the circuit 2-16 Removing Top and Bottom Covers
description and signal flow indication.
1. Remove the four cross-head screws holding the
cover in position.
2-13 Each 'block' is given a prefix according t o 2. Slide the cover about l i n (2.5cm) t o the rear
its function (see Table 2-2 for a full list) and a and lift clear.
number indicating i t s position on the schematic NOTE: I t is not necessary to remove the feet or
diagram, in the same manner as resistors, capacitors, the tilt stand from the bottom cover before
etc, are referenced. remova I.
2-17 Removing Side Covers
Table 2-2 Digital integrated circuit abbreviations
1. Remove the four cross-head screws securing
I Abbreviation Function each cover.
2. Lift cover clear.
DD Decade Divider
2-18 Front Panel Access
IG Inverter Gate
Monostable 1. Remove the top and bottom covers from the
Sh if t Register instru ment.
2. Release the two screws securing the A l / A l l
retaining strip and lift this strip clear.
3. Remove assemblies A2 to A1 1 inclusive.
4. Release the two screws a t the base of each
2-14 All I C leads are referred to in the same way as
of the right hand and left hand board guides.
assembly connections. For example, lead 12 on
integrated circuit MC25, on A5 assembly, is 5. Withdraw the board guides to give access to
referred t o as A5MC25(12). Similarly when de- the rear of the front panel.
scribing schematics, lead 6 on Flip-flop 9 on 6. Re-assembly is a reversal of the above pro-
assembly A 2 would be referenced A2FF9(6). cedu re.
2-19 MAINTENANCE 6. Remove the four nuts holding the A18/A19
housing assembly to the front panel. If
necessary, gently move the wiring to gain
CAUTION access to the nuts.
7. Release screw B shown in Figure 2-1.
When handling the 3720A do not tilt the
8. Holding the A18/A19 housing assembly with
instrument onto its front panel.
one hand, gently press the Display Sensi-
tivity panel on the front of the instrument
with the other hand to release the assembly.
2-20 Locations of dc Power Supply Fuses 9. Replacement of the A1 8/A19 housing assem-
bly is a reversal of the above procedure.
2-21 Access t o the +158V, +50V, +15V, +5V, -15V,
-22V and -5OV power supply fuses is gained by
removing the bottom cover of the instrument.
2-22 The +12.6V power supply fuse is on the A20
Storage assembly. Access to the fuse is gained by
removing the top cover of the 3720A.
2-23 Replacement of Display Sensitivity Lamps
2-24 The lamps are mounted on the A18 and A19
assemblies. To replace the lamps the A18/A19
3 housing assembly must be removed from the
instrument and then dismantled.
2-25 Removal of A18/A19 Housing Assembly ( 103720 x O O 5 6 j
1. Remove the top cover, left side cover and Figure 2-1 A18/A19 Housing assembly
fluted aluminium plate from the instrument. retaining screws
Note that the fluted aluminium plate is self-
adhesive. 2-26 Dismantling the A18/A19 Assembly
2. Release the two screws securing the A l / A l l 1. Remove the A18/A19 housing assembly as
retaining strip and lift this strip clear. detailed i n Paragraph 2-25.
2. Release the two screws holding the range
3. Remove assemblies A2 to A6 inclusive. units lamp separator in place.
3. Slide the separator away from the assembly.
4. Release the two screws a t the base of the
left hand board guide and withdraw the
If the separator is forced sideways the units
5. Remove the left hand board guide support
lamps will be damaged.
by releasing the three screws A shown in
.3 Figure 2-1.
4. Pull out A18 and/or A19 assemblies as re 12. Remove the black light shield surrounding
quired and disconnect the A18/A19 inter- the CRT screen by gently squeezing i t a t the
connecting cables. top centre and bottom centre and pulling
5. Renew the faulty lamps. clear.
6. Re-assembly is a reversal of the above pro- 13. Unscrew and remove the bezel.
cedu re. 14. Slacken theclamp on the rear of the CRT neck.
15. Holding the screen of CRT with one hand,
gently press the rear of the tube with the
2-27 Cleaning The Air Filter other hand and slide the CRT forward and
2-28 The air filter on the fan cover of the 3720A NOTE: To prevent accidental damage to the
should be removed periodically for cleaning. Four storage mesh DO NOT TILT THE CRT FACE
screws secure the fan cover t o the back panel. DO WNWARDS.
16. Re-assembly is a reversal of the above pro-
2-29 Cathode Ray Tube Removal/Replacement cedure.
WHEN HANDLING THE CRT ALWAYS
WEAR GOGGLES AND GLOVES. HANDLE
WITH EXTREME CARE.
1 A16 DEFLECTION AMPLIFIERS ASSY I
1. Disconnect the power cable from the 3720A
2. Remove the top cover from the instrument.
3. Remove the back panel high voltage cover.
4. Unplug the CRT socket.
5. Release the two screws holding A20 in
place and lift out A20.
6. Unplug the wires connected t o the neck
of the CRT. The colours of the wires are
shown in Figure 2-2.
7. Remove AZO support block by releasing
the two screws which hold it in place.
8. Release the two screws securing the A l / A l l cu CR3
retaining strip and lift this strip clear.
9. Remove assemblies A2 t o A6 inclusive.
10. Remove the two screws holding the high
voltage connector A55J1 in place. Disconnect
the flexible wiring from this connector.
11. Unplug the wire from the collimator conn-
ector on the side of the CRT. Figure 2-2 CRT neck connections
2-30 Line Transformer Removal/Replacement 2. Remove the internal high voltage cover.
1. Tilt the 3720A onto its right hand side. 3. Remove the rear panel high voltage cover.
2. Remove the top and bottom covers from the 4. Unplug the five wires from A17 and then
remove A1 7.
3. 5. Unsolder the black and grey wires from the
Release the two screws holding A20 in place
and lift out A20.
6. Release the two screws holding A20 in place
.4. Remove the back panel high voltage cover.
and lift out A20.
5. Remove the internal high voltage cover.
1. Remove XA20 mounting plate.
6. Unplug the five wires from A17 and then
8. Release the two screws securing the A l / A l l
remove A I 7.
retaining strip and lift this strip clear.
7. Remove XA20 mounting plate and pull it
9. Remove assemblies A2 to A6 inclusive.
gently to the side.
10: Remove A20 support block. Two screws
8. Note the wire colours against the corres-
hold it in place.
ponding pin numbers on the top and bottom
of the line transformer and then unsolder 11. Remove the two screws holding the high
the connections. voltage connector A55J1 in place. Disconnect
the flexible wiring from this connector.
9. Remove the four screws holding the trans-
former in position. Retain the underside 12. Unsolder the wires from XA(20(L) and
nuts and safety mouldings and pull the trans- XA20(N) after noting their colours.
former out through the top of the instrument.
13. Remove the wires from the grommet.
10. Re-assembly is a reversal of the above pro-
14. Lift out the high voltage tripler assembly.
15. Re-assembly is a reversal of the above pro-
2-31 Chassis Mounted Transistors Plate cedu re.
1. Remove the top cover and right side cover
2-33 REPAIR PROCEDURES
from the instrument.
2. Release the two screws securing the A12/
A16 retaining strip and lift this strip clear. 2-34 Etched Circuits
3. Remove assemblies A12 to A16 inclusive. 2-35The etched circuit boards in the Spectrum
4. Release the two screws a t the bottom of the Display are of the plated-through type, consisting
of metallic conductors bonded to both sides of the
insulating material. The metallic conductors are
5. Withdraw the plate. extended through the component mounting holes
by a plating process. Solder connection can be
6. Renew the faulty transistors as necessary. made from either side of the board with equally
7. Re-assembly is a reversal of the above pro- good results. Following are recommendations and
cedure. precautions pertinent to etched circuit repair
2-32 High Voltage Tripler Assembly Removal/
Replacement a. Avoid unnecessary component substitution;
it can result in damage to the circuit board and/or
1. Remove the top cover from the instrument. adjacent components.
b. D o ' n o t use a high-power soldering iron on removing integrated circuit packages from a printed
etched circuit boards. Excessive heat may lift a circuit board:
conductor or damage the board. A 40 watt solder-
ing iron is sufficient. a. Solder Gobbler. This is the best method.
Solder is removed from the board by a soldering
c. Use a suction device, eg, Soldapult, or wooden iron with a hollow tip connected t o a vacuum
toothpick to remove solder from component source. This permits an IC to be removed intact
mounting holes. Care must be i3ken when clear- so that it may be replaced if an error has been
ing out component mounting holes; never use made in diagnosis.
sharp metal objects, such as an awl or a twist-drill,
which could damage the plated-throughconductor.
d. After soldering, remove excess flux from the b. Desoldering Tool. (Soldapult - hp Part No.
soldered areas with a soac and water solution. 8690-0060). This method is recommended if a
vacuum operated Solder Gobbler is not available.
e. When removing a multiple connection com- Remove the solder from each lead by using the
ponent, such as an integrated circuit, loosen soldering iron and the desoldering tool together.
it gradually, using a gentle side-to-side motion,
to avoid damage to the plated-throughconductors.
c. Clip Out. This is the least satisfactory of the
2-36 Integrated Circuit Removal/Replacement three methods. Clip the leads as close to the case
as possible. With a soldering iron and long nose
2-37 There are three recommended methods of pliers carefully remove the leads from each hole.
Section I I I
3-2 This section contains information for ordering replaceable parts. Table 3-1 lists parts in alpha-
numerical order of their reference designators and gives the following information for each part:
a. Hewlett-Packard stock number.
b. Description (abbreviations given in Paragraphs 3-7, 3-8 and 3-9).
c. Total quantities of each part used in the instrument (refer to TQ column).
d. Manufacturer and Manufacturer's part number (Code List of Manufacturers given in Table 3-2).
3-3 ORDERING INFORMATION
3-4 To order a replacement part, address the order or enquiry to your local Hewlett-Packard Sales and
Service Office (addresses are listed a t the rear of this manual).
3-5 Specify the following information for each part:
a. Model and full serial number of instrument.
b. Hewlett-Packard stock number.
c. Circuit reference designator.
3-6 To ocder a part not listed in the tables, give a complete description of the part including its function
and location in the instrument.
3-7 REFERENCE DESIGNATORS
A = assembly L = inductor S -= switch
C = capacitor MC = micro circuit SR = shift register
CR =diode MP = metal work/ T = transformer
DS =lamp miscellaneous part XA = printed circuit
F = fuse P = plug (male) board edge
IC = integrated circuit 0 = transistor connector
J ' = connector (female) R = resistor
3-8 LIST OF ABBREVIATIONS
A = ampere INCD = incandescent QUAD = quadruple
AC = alternating IND = inductor REC = rectifier
current INV = invert RH = right hand
AL = aluminium I/P = input
AMPL = amplifier K = kilo ( lo3) RND =round
ASSY = assembly LED = light emitting diode RTRY = rotary
BD = board LH = left hand side
SI = si I icon
BHD = bulkhead LIN = linear SPDT = single pole,
BKDN = breakdown LVR = lever
CBL = cable
SPST = single pole,
M = mega ( 1O6 )
COAX = coaxial MA = milli ampere
COMP = compensated MH = milli henry
T = throw
CONCEN = concentric MTG = mounting
TEMP = temperature
CONN = connector MW = milli watt
TGL = toggle
CONT = controlled NAND = not and
TQ" = total quantity
CRT = cathode ray tube NOR = n o t o r
UF = micro farad
DIO = diode NPN = negative-positive- (10-6)
DPDT = double pole, negative
V = volt
double throw NSR = not separately
repIaceable VAR = variable
FET = field effect
transistor OPER = operational W = watt
FXD = fixed PC = printed circuit
WVDC = working voltage
d irect cu rrent
GE = germanium PF = pic0 farad ( 1O-'*
XFMR = transformer
HV = high voltage PIV = peak inverse volts
= high speed