HP 492A Service manual

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HEWLETT-PACKARD COMPANY
4.2A/4.4A
TRAVELING-VVAVE
AMPLIFIER

)
)
)
00144·2
OPERATING AND SERVICE MANUAL
MODEL 492A
AND
MODEL 494A
SERIALS PREFIXED: 010-
TRAVELING-WAVE AMPLIFIERS
Copy,;gh' HEWLETT.'ACICA.D COM' ANY 1962
lS01 'AGE MILL ROAD, 'ALO ALTO, CALlFO.NIA, U.S.A.
Printed: JAN 1962

Model 492Aj494A
Table of Contents
Lists of Illustrations and Tables
TABLE
OF CONTENTS
Section
Page Section
Page
IGENERAL INFORMATION
1-1
IV PRINCIPLES OF OPERATIO
(cont'd)
1-1.
Description
1-1 4-10.
Traveling Wave Tube
4-1
1-6.
Instrument Identification
1-2
4-15. Grid Modulation.
4-4
1-8.
Traveling Wave Tube Warranty
1-2 4-17. Helix Modulation ·.
4-4
4-22. Constant-Amplitude, Linear
II
INSTALLATION
2-1
Sawtooth Generator
4-5
2-1. Mechanical Inspection
2-1
2-3.
Power Requirements.
2-1
VMAINTENANCE.
5-1
2-5.
Power Cable.
2-1 5-1.
Introduction.
·
5-1
2-8.
Installation
2-1
5-3.
Cleaning the Air Filter·
5-1
2-12.
Repackaging for Shipment
2-1
5-5. Test Equipment .
5-1
5-7. Repair. 5-3
III
OPERATING INSTRUCTIONS
3-1
5-8. Cabinet Removal
5-3
3-1.
Introduction .
3-1 5-10. Tube Replacement .. 5-3
3-3.
Preliminary Operating Procedure
3-1 5-12. Traveling Wave Tube Capsule
3-6.
Helix Control
3-1
Replacement
5-3
3-8.
Saturation Power Output
3-1 5-17.
Changing the Frequency Range
3-12.
Bandwidth Considerations .
3-1 of the 492A or 494A
5-5
3-14.
Constant Gain or Constant Output
5-21. Adjustments
5-5
Amplification
3-3 5-22.
Excessive Helix Current
5-5
3-18.
Buffer Amplifications
3-3 5-24.
Chassis Helix Control
5-5
3-20. Amplitude Modulation
3-3
5-26. Anode Voltage Control . 5-5
3-24. Pulse Modulation.
3-6
5-28. Regulated Power Supply 5-5
3-27. Limited Phase Modulation.
3-6
5-30. Troubleshooting.
·
5-6
3-29.
Unlimited Phase Modulation and
5-32. Performance Checks 5-7
Frequency Shifting .
3-6
5-33. Gain Check. 5-7
3-34. Homodyne Detection
3-7
5-36. Output Power Check 5-7
3-37.
Frequency Modulation
3-9 5-39. Noise Figure Check 5-7
5-42.
Hum and Spurious Modulation Check· 5-8
)
IV
PRINCIPLES OF OPERATION.
4-1
4-1.
Introduction .
4-1
VI REPLACEABLE PARTS .
6-1
4-3. Magnet Power Supply.
4-1
6-1. Introduction 6-1
4-5. Regulated Power Supply
4-1
6-4.
Ordering Information 6-1
LIST OF ILLUSTRATIONS AND TABLES
Number Illustration Title Page
4-3. CutawayView ofaTWT Capsule andMagnet
Showing the Important Elements· . . . 4-3
4-4. Cutaway View of an Encapsulated TWT· . 4-3
4-5. Simplified Circuit ofa Constant Amplitude,
Variable Slope Sawtooth Generator·· 4-5
5-1. Top View of Models 492A and 494A ., 5-2
5-2. Bottom ViewLooking Towards the Front
Panel of Models 492A and 494A . .. 5-4
5-3. Rear View of Models 492A and 494A" 5-4
5-4. Test Setup for Gain, Output Power, and
Noise Figure Performance Checks 5-7
5-5. Test Setup for Hum and Spurious
Modulation Performance Check . . 5-8
5-6. Models 492A and494A Schematic Diagram' 5-9
Number Illustration Title Page
1-1. Model 492A Traveling Wave Amplifier
1-1
1-2. Traveling Wave Tube Warranty
1-2
3-1. Operating Controls .
3-0
3-2. Typical Gain and Power Output
Characteristics
.
3-2
3-3.
Block Diagram of a Circuit used to
Maintain Constant-Level Output Power
from a TWT Amplifier
3-3
3-4.
Block Diagram of an Automatic Gain
Control to Maintain Constant Ampli-
fication from aTWT Amplifier
3-3
3-5. Typical Plot of Output Voltage vs Grid
Voltage of aModel 492A
3-4
3-6. Typical Plot of Output Voltage vs Grid
Voltage of aModel 494A.
3-5
3-7. RF Phase Shift Produced by Helix Mod
3-7
3-8. Offset Frequency Produced by Sawtooth
Modulation of the Helix . 3-7
3-9. Block Diagram of a Linear (Homodyne)
)
Detection System. 3-8
3-10. Block Diagram of a Circuit to Produce
an FM Signal with a TWT Amplifier 3-8
4-1.
Block Diagram, Models 492A and 494A
4-0
4-2.
TWT and How it Works .
4-2
00144-2
1-1.
3-1.
5-1.
5-2.
5-3.
6-1.
6-2.
Table Title
Specifications. .....
Maximum Operating Currents for
Models 492A and 494A . . .
Recommended Test Equipment
Tube Replacement List
Troubleshooting Chart . . •
Reference Designation Index
Replaceable Parts· . . . .
1-0
3-1
5-1
5-3
5-6
6-2
6-5
iii

Section I
Table 1-1
Model 492A/494A
Table 1-1, Specifications
~Model 492A
<Fj! Model 494A
Frequency Range: 4
gc to 8 gc
7 gc to 12,4 gc
Maximum Output Power: 20 mw minimum into 50 ohm load
20 mw minimum into 50 ohm load
Modulated
Pulse Delay:
Approximately 20 ns
Approximately 15 ns
Helix Modulating Voltage: Approximately 40 volts peak-to-
peak. Provides 3600 phase shift.
Input impedance lOOK
Approximately 50 volts peak-to-
peak. Provides 3600 phase shift.
Input impedance lOOK
Hum and Spurious Modulation: At
least 45 db below signal level
At least 45 db below signal level
Weight:
Power Supply:
66
lb net, 85 Ib shipping
US volts ±10%, 50 to 60 cps,
approximately 200 watts
63 Ib net, 84 Ib shipping
U5 volts ±10%, 50 to 60 cps,
approximately 225 watts
Accessories Furnished:
AC-16Q cable assembly
AC-l6Q cable assembly
For Both Models
Small Signal Gain: 30 db
minimum
Meter Monitors:
Cathode current, anode current, helix current, collector current.
Input Impedance: 50 ohms, swr less than 2
Output
Internal Impedance: 50 ohms, swr less than 3
Dimensions:
Cabinet Mount: 7-3/8 in.wide, U-l/2 in.high, 20 in.deep.
TypeN
Less than 30 db
t
:0: IU]
REAR ,~,,".
:0: '.1&
1.----19---.,
492AR/494AR
Rack Mount:
Noise Figure:
Connectors,
RF Input and Output:
Pulse Rise and Decay Time:
Amplitude Modulating Voltage:
Approximately 15 ns
Approximately 50 volts peak positive pulse will produce a40 db change
in rf power output. Sensitivity approximately 1 db/volt
1-0 00144-2

Model 492A/494A
SECTION I
GENERAL
INFORMATION
Section I
Paragraphs 1-1 to 1-4
1-1. DESCRIPTION.
1-2. The t$ Models 492A and 494A Traveling Wave
Amplifiers are broadband, linear amplifiers provid-
ing adjustable amplification up to at least 30 db, be-
tween 4and 12.4 gc, and have a maximum power out-
put of at least 20 milliwatts to an external load of 50
ohms. The frequency range of the t$ Model 492A is
4 to 8gc; the frequency range of the Model 494A is 7
to 12.4 gc. These traveling wave tube (twt) ampli-
fiers are designed to be used also as buffer amplifiers
or modulators for any signal within their frequency
range. As buffers, their input impedances remain
constant with any reasonable load change at the output
terminal; the attenuation between input andoutput sig-
nals is at least 60 db, minus the gain of the amplifier.
As modulators, they can be used to amplitude, fre-
quency, pulse, or phase modulate the signal being
amplified with no interaction on the signal source.
The gain and power output of the amplifiers are con-
tinuously adjustable by the front panelGRID BIAS con-
trol. Hum and spurious modulation generated within
the amplifiers are at least 45 db below the output sig-
nal level and the noise figure is less than 30 db.
1-3. The t$ Models 492A and 494A Traveling Wave
Amplifiers, amplify any type of rf signal: cw, swept,
sine-modulated, pulsed, multiple signals on different
frequencies, etc. Atwt amplifier used as a modulator,
in conjunction with a signal generator, can be used to
amplitude modulate an rf carrier to approximately 30%
with less than 2.5% harmonic distortion and up to 50%
with less than 5% distortion. Amplitude modulation
sensitivity is approximately 1 db/volt. Pulse modula-
tion is excellent; the rise time is less than 15 ns.
Phase modulation up to 3600with less than 1db
amplitude modulation is also possible. Wide-band
frequency modulation is simulated by a step-wise phase
modulation described in section Ill.
1-4. The front panel meter is provided for checking
and adjusting electrode currents in the traveling-wave
tube. The meter helps to obtain desired operating
characteristics during normal operation of the ampli-
fier and also assists with preventive maintenance and
troubleshooting. An anode voltage adjustment on the
instrument chassis prOVided to adjust the cathode
current of the twt back to normal due to tube ageing.
)
00144-2
Figure 1-1. Model 492A Traveling Wave Amplifier
1-1

Section I
Paragraphs 1-5 to 1-9
1-5. The ~ Model 492A and 494A are similar in that
one model may be changed to the other, by replacing
the twt, as covered in paragraph 5-17.
1-6. INSTRUMENT IDENTIFICATION.
1-7. Hewlett-Packard uses a two-section eight-digit
serial number (000-00000). If the first three digits
of the serial number on your instrument do not agree
WA •• ANTY CLAIM AND ADJUITMINT 'IOCIDUU
Model 492A/494A
with
those on the title page of this manual, change
sheets supplied with the manual will define differences
between your instrument and the Model 492A or 494A
described in this manual.
1-8. TRAVELING WAVE TUBE WARRANTY.
1-9. The Traveling Wave Tube Warranty is illus-
trated in figure 1-2. A sheet for your use is included
in the appendix of this manual.
MICROWAVE TUBE WARRANTY CLAIM
INFORMATION FORM
IMPORT ANT: Please ans .... er all questions fully -- InsuHlclent Information may delay processing
of your claim. -
for microwave tubes supplied by [he
HEWLETT· PACKARD COMPANY
for use In * Instruments
Microwave tubes supplied by the Hewlett-Packard Company. either 88 original or replacement.
for use in 8 instruments are actually warranted by the tube manufaclUrer and not by 9.
However, S will process warranty claims for you, and will promptly pass on all allowances
granted by the tube manufacturer.
In the event that your tube Is (ound to be repairable, the tube manufacturer reserves the right
to repair and return the tube in lieu of Issuing pro--Tata credil.
For your convenience, warrantyclalms(orall microwave tubes supplied by the Hewlett-Packard
Company may be made on this sIngle form; merely fJII out the lnfonnallon on the reverse side
and rerurn this fonn. along .... lth thedefectlve rube. to your 8 engineering representallve. or tO~.
Please be sure each space on the form is filled In··lack of complete infonnation may delay
processing of yourcTiim.
Each tube manufacturer has his own warranty policy. Copies of Individual Conditions of War-
ranty are available from your * engineering representative or from Ihe He .... len·Packard
Company.
SHIPPING INSTIUCTIONS
FROM: (rube Owner)
Company
Address
Tube type
Tube serial No. _
Tube mfr.
Use in ~ Model _
Instrument serial no. _
Date
FOR FURTHER INFORMATION CONTACT:
Name _
Title _
Company _
Address _
Tube purchased from
On P. O. number
The following instructions are included to aid you In preventing damage in transit. Package
your tube carefully •• no allowance can be made on broken tubes.
Tube is Original ( ) or Replacement (
1. Carefully wrap tube in 1/4 inch thick "klmpack". Cotton batting. or other sofl padding
material.
2. Wrap the above in heavy kraft paper.
3. Pack in a rigid container .... hlch Is at least 4 inches larger than the lube In each dimension.
4. Surround the tube with at least 2 inches of shock absorbing material. Be cenaln that the
packing Is tight all around the tube.
Date tube received _
Date first tested
Date placed in service _
Date of failure _
Hours use per day (average) _
Number of days In service _
Total hours filament operation
5. Tubes returned from outside the continental UnltedSrates should be packed In awooden box.
6. Mark container FRAGILE and ship prepaid via Air Freight or Railway Expreu. Do not
ship via Parcel POSt or AJr Parcel POSt since experience has shown that fragile Items are
more apt to be damaged when shipped by these means.
Tubes returned to the Hewlen·Packard Company should be addressed to:
SYMPTOMS: (Please describe conditions prior to and at time of failure. along with description of
lube's defect, If known) _
CUSTOMr. snvlcr
H.~".,.d.rd COfftpel'ly
]95 ,.~ Mil R_d
1'. Alto. C.lifomi., U.S.A.
01 II. W .... ,..I.r.,.J
H_lett·,.d.,dS.A.
kvedIiVifillli",dNo.l
G.I'l...... S ... itter!Mtd
Were there other circuit component failures at time of failure? Which ones?
Signature _
Title
9/12/61
1-2
Figure 1-2. Traveling Wave Tube Warranty
00144-2
)

Model 492A/494A
SECTION II
INSTALLATION
Section II
Paragraphs 2-1 to 2-13
/
2-1. MECHANICAL INSPECTION.
2-2. Unpack the instrument upon receipt and inspect
it for signs of phy ical damage such as scratched
panel surfaces, broken knobs, etc. If there is any
apparent damage, file a claim with the carrier and
refer to the warranty page in this manual.
2-3. POWER REQUIREMENTS.
2-4. The Model 492A and Model 494A require a power
source of 115 volts ±10%, single phase, 50 to 60 cps,
which can deliver approximately 225 watts.
2-5. POWER CABLE.
2-6. For the protection of operating personnel, the
ational Electrical Manufacturers' Association
(NEMA) recommends that the instrument panel and
cabinet be grounded. This instrument is equipped
with a
three-prong conductor power cable which, when
plugged
into an appropriate receptacle, grounds the
instrument. The offset pin on the power cable three-
prong connector is the ground pin.
2-7. To preserve the protection fearure when opera-
ting the instrument from a two-contact outlet, use a
three-prong to two-prong adapter and connect the
green pigtail on the adapter to ground.
2-8. I NST ALLATION.
2-9. The only special precaution necessary for in-
stalling the twt amplifiers is that they should not be
operated close to very large magnetic fields, such as
60-cycle fields, unless externally shielded. While
the twt amplifiers are shielded within the cabinet,
complete protection against large low-frequency fields
would require more shielding than is practical to in-
clude in the design.
2-10. To operate, connect the instrument to a 115-
volt ac power source, check and adjust rube operating
00144-2
currents as described in preliminary operating pro-
cedure, section 1Il, and connect to the external equip-
ment with coaxial cables terminated in standard UG-
21D/U, type connectors.
2-11. In section V, beginning with paragraph 5-32,
is a list of performance checks for this instrument.
These procedures make agood test as part of incom-
ing quality-control inspection following initial rurn-on.
2-12. REPACKAGING FOR SHIPMENT.
2-13. The following list is a general guide for re-
packaging an instrument for shipment. If you have
any questions, contact your authorized Hewlett-
Packard sales representative.
a. If possible, use the original container designed
for the instrument,
b. Wrap the instrument in heavy paper or plastic
before placing it in the shipping container.
c. Use plenty of packing material around all sides
of the instrument and protect the panel with card-
board strips.
d. Use heavy cardboard carton or wooden box to
house the instrument and use heavy tape or metal
bands to seal the container.
e. Mark the packing box with "Fragile", "Delicate
Instrument", etc.
Note
If the instrument is to be shipped to Hewlett-
Packard Company for service or repair, at-
tach to the instrument a tag identifying the
owner and indicating the service or repair to
be accomplished. In any correspondence be
sure to identify the instrument by model
number, serial prefix, and serial number.
2-1

Section III
Figure 1-3
POWER
1. Turns on all circuits of the amplifier (allow
IS
minutes warmup).
2. Meter selector switch is used for selection of
circuit to be checked on the front-panel meter.
3. Pin jack is for measuring grid bias (measures
Ek; grid grounded).
4. GRID BIAS control is used to adjust fordesired
tube current. Normal cathode current is indi-
cated on the meter plate. Do not exceed maxi-
mum operating currents, refer to table 3-1.
Model 492A/494A
~-~--;3
MAG C1RJD Il100
8
7
L
S. HELIX control is used to adjust for best broad-
band response, position 5, or for maximum
gain at anyone frequency.
6. INPUT rf connector is where the rf signal to be
amplified is coupled into the instrument.
7. OUTPUT rf connector is where the amplified rf
signal is coupled from the instrument.
8. GRID MOD. connector is the input for ampli-
tude modulation signals.
9. HELIX MOD. connector is the input for phase
and frequency modulation signals.
3-0
Figure 3-1. Operating Controls
00144-2

)
Model 492A/494A
SECTION III
OPERATING INSTRUCTIONS
Section III
Paragraphs 3-1 to 3-13
3-1. INTRODUCTION.
3-2. This section contains operating instructions for
theModels 492A and494A Traveling WaveTube Ampli-
fiers. Figure 3-1 gives basic operating instructions.
The remainder of this section supplements these in-
structions.
3-3. PRELIMINARY OPERATING PROCEDURE.
3-4. The front panel controls, indicator, and con-
nectors for the 492A and 494A are shown in figure 3-1.
This figure also shows the uses for the controls, indi-
cator, and connectors. Whenever the twt is turned
on, use the front panel meter to measure the current
to each electrode in the traveling-wave tube with the
GRID BIAS control set for zero bias. The safe maxi-
mum current for each electrode is shown in table 3-1.
ormal cathode current for your instrument is indi-
cated on the plate attached to the meter face. Cur-
rents are usually a little high when the instrument is
first turned on, but decrease to normal during warm-
up. Allow IS minutes warmup before making final
reading. The GRID BIAS control may be used to re-
duce the tube currents during warmup.
Table 3-1. Maximum Operating Currents for
Models 492A and 494A.
Adjusting the HELIX voltage control for maximum
gain at the upper frequency limit usually produces the
flattest frequency response over the band. Maximiz-
ing the gain at frequencies below the upper frequency
limit usually results in additional gain and power out-
put over that obtained when adjusted for flattest broad-
band operation. The final setting of the HELIX con-
trol is independent of the type of signal amplified.
3-8. SATURATION POWER OUTPUT.
3-9. Saturation power output is the maximum output
power obtainable with a given collector current
and optimized helix voltage. As the input signal is
increased from the noise level, the output-vs-input
characteristic is linear until saturation is approached
and the gain begins to decrease. Eventually the output
reaches a peak which is the saturation power output
and any
further increase in the input causes the
output to decrease. If the collector current is reduced,
the power output at which saturation occurs will be
decreased. The primary effects which lead to
saturation are: 1) the forces between the electrons
in the beam begin to limit the electron density in
the bunches 2) the energy transfer from the electron
beam to the helix causes the beam velocity to decrease
and gradually lose synchronism with the wave on the
helix. These effects become more pronounced as
saturation is approached, and cause the gain of the
twt to decrease.
3-6. HELIX CONTROL.
3-7. The HELIX voltage control on the front panel
maximizes the gain and power output of the twt at a
selected frequency or optimizes the gain and power
output over the entire band; see figures 3-2Aand 3-2B.
3-5. If, following tube replacement, or for some
other reason, the cathode current can be increased
with the GRID BIAS control to slightly above the safe
maximum current (see table 3-1), readjust the anode
voltage control, paragraph 5-26. The anode voltage
control is set to limit the twt to its normal cathode
current when the bias voltage on the twt is zero. If
the cathode current is limited to its normal value,
but the current to another electrode is excessive, the
amplifier requires service or adjustment, see para-
graphs 5-21 or 5-30.
)
CAUTION: DO NOT EXCEED:
Cathode current
Helix current
Collector current.
Anode current
3 ma
0.5 ma
3 ma
SO /la
3-10. Operating the 492A or 494A near saturation
power output will produce second harmonic content in
the output. To obtain maximum power output from the
twt without excessive second harmonic content in the
output, increase the input signal level until satura-
tion is reached, then reduce the input until the out-
put decreases approximately 6db (an input reduction
of 10 to 15 db). If second harmonic content is un-
important, operation at saturation is usually very
satisfactory.
3-11. One advantage of operating the twt near satura-
tion is the constant output characteristic exhibited by
atwt
at saturation. At saturation the gain of atwt
varies inversely with the input level and input varia-
tions of 10 to 15 db cause the output to vary only
4
to 6db. If a more nearly constant output is desired,
see paragraph 3-14.
3-12. BANDWIDTH CONSIDERATIONS.
3-13. The graphs in figures 3-2A and Bshow typical
gain vs frequency and saturation power output curves
for the 492A and the 494A amplifiers using two differ-
ent conditions of helix voltage. One set of curves
shows the gain and saturation power output when the
helix voltage is optimized at each frequency. The other
set of curves shows the gain and saturation power out-
put when the HELIX control is set to the broadband
00144-2 3-1