HP 204B Service manual

OPERATING AND SERVICE MANUAL
MODEL 204B
SERIALS PREFIXED: 416-
OSCILLATOR
I962
Copyright HEWLETT-PACKARD COMPANY
01369-4 Printed: JUNE 1964

Model 204B Index
SECTION 111 OPERATION
SECTION
I GENERAL INFORMATION
SECTION
II INSTALLATION
SECTION
IV PRINCIPLES OF OPERATION
SECTION
V MAINTENANCE
SECTION
VI REPLACEABLE PARTS
I
iii
01369-1

Model 204B
TABLE OF CONTENTS
Table of Contents
List of Illustrations
List of Tables
Section Page
I GENERAL INFORMATION
........ 1-1
1-1 . Introduction ........... 1-1
1-4 . Power Supply .......... 1-1
1-8 . Accessories Available ...... 1-1
1-10. Instrument Identification ..... 1-1
1-6 . MercuryandRechargeableBatteries 1-1
I1 INSTALLATION
............ 2-1
2-1
. Inspection ............ 2-1
2-4
. Rack/BenchInstructions ..... 2-1
2-6
. Combining Case ........ 2-1
2-8
. Adapter Frame ........ 2-1
2-12 . AC-Powered Oscillators ..... 2-1
2-16
. Repackaging for Shipment ..... 2-2
2-10 . Battery-Powered Oscillators ... 2-1
111 OPERATION .............. 3-1
3-1 . Introduction ........... 3-1
3-3
. Procedure ............ 3-1
3-5
. General Operating Considerations . 3-1
3-10
. Operation With Rechargeable
Batteries ........... 3-1
3-16
. Recharging Nickel-Cadmium
Batteries
.......... 3-2
3-20
. Cycle-Life of Nickel-Cadmium
Batteries .......... 3-2
3-26
. Mercury Battery Replacement ... 3-2
IV PRINCIPLES OF OPERATION ...... 4-1
4-1
. Introduction ........... 4-1
4-7
. Oscillator Circuit ........ 4-1
4-13
. Peak Detector .......... 4-2
V MAINTENANCE ............ 5-1
5-1
. Introduction ........... 5-1
5-3
. RequiredTest Equipment ..... 5-1
I
Section V (Cont'd) Page
5.5
. Performance Checks ....... 5-1
5.7
. Dial Accuracy Check ..... 5-1
Output Voltage Check
..... 5-2
5-9 . Distortion Check ....... 5-2
5-10
. Repair ............. 5-3
5-12
. Cover Removal ........ 5-3
5-14
. ServicingEtchedCircuitBoards . 5-3
5-16
. Troubleshooting ......... 5-4
5-18
. AdjustmentandCalibration .... 5-4
5-19
. AC Power Supply Checks .... 5-4
5-23
. Battery Charger Current 5-4
5-26
. DC Bal . Adjustment ...... 5-5
5-28
. Distortion Check ........ 5-5
5-29
. Frequency Calibration
5-31
. Replacement Procedures ..... 5-6
5-32
. Cam Cable Replacement .... 5-6
5-34
. Frequency Potentiometer
5-36
. Range Switch Assembly
5-38
. Battery-Pack Power Supply .... 5-8
5-40
. AC Power Supply ......... 5-8
5-42 Rechargeable Battery Power
5-45
. Replacement of Rechargeable
5-8. Frequency Response and
Adjustment
..........
Adjustments ......... 5-5
Replacement
......... 5-6
Replacement
......... 5-7
Supply
.............. 5-8
Batteries
........... 5-8
VI REPLACEABLEPARTS ........ 6-1
6-1 Introduction
.......... 6-1
6-2
. Ordering Information ...... 6-1
LIST OF ILLUSTRATIONS
Number Page
1.1 . Model 204B Oscillator .......... 1-0
2.2 .
Combining Case ............ 2-0
2.3
. Adapter FrameInstrumentCombinations 2-1
3.1
. Front Panel Controls, Connectors
3.2
. Rear View of Model 204B Showing
Mercury Batteries Installed
...... 3-2
4.1
. Block Diagram .............. 4-1
4.2
. Simplified Schematic Diagram ...... 4-2
4.3
. RC Network Characteristics ....... 4-2
5.1
.
Distortion Checks ........... 5-1
5.2
.
and Output Voltage Check ....... 5-2
2.1
. Combining Case ............. 2-0
Steps to Place Instrument into
2.4
. TwoSubmoduleUnits inRackAdapter ... 2-2
and Operating Instructions
...... 3-0
Test Setup for Dial Accuracy or
Test Setup for Frequency Response
Number Page
5.3
. Cover Removal .............. 5-3
5.4
. Model 204B. Exploded View ....... 5-4
5.5
. Cams and Cable Relationship ....... 5-6
5.6
. Range SwitchDetail ............ 5-7
Model 204B
............... 5-8
5.8 . AC Power Supply Installation ....... 5-9
Battery Power Supply
.......... 5-10
Pack Power Supply
........... 5-11
Rechargeable Batteries
......... 5-12
5.7
.
Rear View of Battery-Powered
5.9
. Rear Internal View of Rechargeable
5.10
. Oscillator and Amplifier with Battery
5.11
. Option 02 Power Supply with
5.12
.
Option 01 AC Power Supply ........ 5-13
LIST OF TABLES
1 Number Page Number Page
1.1. Specifications ............ 1-1 5.3 . Troubleshooting Summary ...... 5-5
5.1
. Required Test Equipment ....... 5-1 6-1 . Index by Reference Designator .... 6-2
5.2
. Dial Accuracy ............ 5-2 6.2 . Replaceable Parts .......... 6-5
01369-3
V

Model 204B Section I
Paragraphs 1-1 to 1-11
I'
SECTION I
GENERAL INFORMATION
1-1. INTRODUCTION.
1-2. The Model 204B Oscillator (figure 1-1) is a
transistorizedbattery-powered instrumentwhich pro-
duces sine-wave signals in the5-cps to560-kc range.
The output
is 10milliwatts into 600 ohms and is con-
tinuously adjustable over
at least a 40-db range. Into
the rated load of
600 ohms, frequency response is
within &3%.The oscillator output is floating, isolated
from power line ground and instrument chassis.
1-3. The600-ohm output impedance of the oscillator
makes it compatible withtransmission lines and many
distribution systems. When battery-powered, the
oscillator
is useful for isolated applications. The @
Model 403A/B AC Voltmeter, also battery-powered,
makes an ideal companion instrument for the Model
204B.
1-4. POWER SUPPLY.
1-5. Normally the oscillator is equipped with a bat-
tery pack which consists of four 6.75-volt mercury
batteries.
If operation from AC is desired, an AC
power supply (Option
1) or a power supply with re-
chargeable batteries (Option 2) may be used. Both
of these power supplies operate from115
or 230 volts,
and are
in kit form for field installation. Thebattery
pack
is also in kit form. All three types of power
supplies mount in the samelocation in
the instrument
and can be easily interchanged in the field.
1-6. MERCURY AND RECHARGEABLE
BATTERIES.
1-7. Mercury batteries used in the Battery Packare
vented to prevent excessive internal pressures from
Table
1-1. SI
FREQUENCY RANGE: 5cps to 560 kc in 5 ranges,
vernier control
DIAL ACCURACY: *3%
FREQUENCY RESPONSE: *3%, with rated load
OUTPUT IMPEDANCE: 600 ohms
OUTPUT: 10 milliwatts (2.5vrms) into 600 ohms,
5 vrms open circuit, completely floating
OUTPUT CONTROL: Continuously variablebridged
"TtTattenuator with
at least 40-db range
developing. The nickel cadmium batteries
are seal-
ed butpresent no problem ifproperly used (seeSection
111). The following precautions should be followedfor
either type of battery:
a. DO NOT short circuit either type of battery.
b. Dispose of batteries promptly when they have
exhausted
or will not operate equipment up to
specifications.
c. Never dispose of batteries by
fire.
d. Turn off battery powered equipment when not
in use.
e. Store batteries in a cool well-ventilated place.
1-8. OPTIONS AVAILABLE.
1-9. The following three power supplies are avail-
able in kit form for field installation:
Battery Pack Power Supply
- 204B-64B
AC Operated Power Supply - 204B-llA
Rechargeable Battery Power Supply
- 204B-11C
1-10. INSTRUMENT IDENTIFICATION
1-11. Hewlett-Packard used a two-section, eight-
digit serial number
(000-00000). If the first three
digits of the
serial number on your instrument do not
agree with those on the title page of this manual,
change sheets supplied with the manual will define
differences between your instrument and the Model
204B described in this manual.
xif ications
DISTORTION: Less than
1%
NOISE: Less than 0.05%, ac or battery operated
POWER SOURCE: 4 batteries
at 6.75 volts each,
7-ma drain, life at least 300 hours.
DIMENSIONS: Module 6-3/32 in. high, 5-1/8 in.
wide, 8
in. deep
NET WEIGHT:
6 lb
EQUIPMENT AVAILABLE: 204B-llA, AC Supply
Kit for field installation, 204B-llC Recharge-
able Battery Supply Kit for field installation,
204B-64B, Battery Kit for field installation.
01369-4
1-1

W
Model 204B Section I1
Paragraphs
2-1 to 2-14
2 -1.
INSPECTION.
SECTION II
INSTALLATION
2-2. Unpack the instrument upon receipt and inspect
it for signs of physical damage such as scratched
surfaces, broken knobs, etc. If damage
is apparent,
proceed
as described in the "Claim for Damage in
Shipment" section of the warranty in the rear of this
manual,
2-3. An electrical inspection should be performed
as soon as possible after receipt; see paragraph 5-5
for performance checks. These aregood test proce-
dures for incoming quality-control inspection.
2- 4. RACK/ BENCH INSTRUCTlONS.
2-5. The Model 204B is shipped with plastic feet and
a tilt stand attached, ready for use as a bench-type
instrument. To adapt the Model 204B
for rackmount-
ing, remove the plastic feet identifiedin figure 5-3 by
following the procedure given in paragraph 5-13
(c) .
2-6. COMBINING CASE.
2-7. The combining case shown in figure 2-1 is a
full-module unit which accepts varying combinations
of submodule units such
as the Model 204B. Being a
full-module unit, the combining case can be used as
a bench modelor it canbe rackmounted. Instructions
for use of the combiningcase aregiven graphically in
figure 2-2 for 1/2-module units. The Model 204B
is
a 1/3-module unit, therefore it is necessary to use
either the right or left divider latch when installing
the divider assembly.
2-8. ADAPTER FRAME.
2-9. The adapter frame
is simply a rack frame that
accepts any combination of submodule units (see fig-
ure 2-3);
it can only be rack mounted. Instructions
are
as follows:
a. Place adapter framevertically on edge of bench
as shown in step 1 , figure 2-4.
b. Stacksubmoduleunits in
frame as shown in step
2
, Place spacer clampbetween units, step 3
c. Place two end spacer clamps
as shown in step
4
, and push submodule units into frame.
d. Insert screws on either side of frame, step
5 ,
and tighten until submodule units are tight in
frame.
e. The complete assembly
is now ready for rack
mounting.
2-10. BATTERY-POWERED OSCILLATORS.
2-11, Power for battery-powered oscillators is sup-
plied from four 6.75-volt Mallory mercury batteries.
There are no special installation instructions for
placing battery-powered oscillators into operation.
To convert for operation from an acpower supply or
a rechargeable battery power supply, refer to para-
graph 5-35.
2-12. AC-POWERED OSCILLATORS.
2-13. POWER REQUIREMENTS. When the Model
204B has an ac power supply or the rechargeable
battery power supply,
a source of 115 or 230 volts
+lo%, 50 to 1000 cps will be required. When the
source
is 115 volts, set the switch, located on the
rear panel,
so that the number 115 appears. If a
230-volt source is used, place the switch so that the
number 230 appears.
2-14. POWER CABLE. For the protection of opera-
ting personnel, theNational Electrical Manufacturers'
Association (NEMA) recommends
that the instrument
panel and cabinet be grounded when being operated
from an acpower line. Instruments with the acpower
supply or rechargeable battery power supply are
equipped with
a detachable, three-conductor power
cable which, when plugged into an appropriate recep-
tacle, grounds the instrument. The offset pin on the
power cable three-prong connector
is the ground pin.
FILLER PANEL
Figure 2-3. Adapter Frame Instrument
Combinations
01369-3
2-1

Section I1
Paragraphs 2-15 to 2-17
2-15.
To preserve the protectionfeature when oper-
ating the instrument from
a two-contact outlet, use a
three-prong to two-prong adapter and connect the
pigtail on the adapter to ground.
2-16. REPACKING FOR SHIPMENT.
Model 204B
2-17. The following is a general guide for repack-
aging an instrument for shipment.
If you have any
questions, contact your local
@sales office or repair
facility. Proceed
as follows:
a. Placeinstrument inoriginal container if avai-
able.
If original container is not available, it can be
purchased from your nearest Hewlett-Packard Sales
and Service Office (refer to maps in Appendix).
b.
If original container is not used, wrap instru-
ment in heavy paper
or plastic before placing in
inner container.
c.
Use a heavy carton or wooden box to house the
instrument and inner container and usestrong tape or
metal bands to
seal the shipping container.
d. Mark shipping containerwith "Delicate Instru-
ment", "Fragile", etc.
as appropriate.
Note
If instrument is to be shippedto Hewlett-
Packardfor service
or repair, attach a tag
to instrument identifying owner,model
number, serial number, and indicating
service
or repairtobe performed. In any
correspondence identify instrument by
model number and serial number,and
serial number prefix.
ADAPTER
FRAME
T 1/2 MODULE
INSTRUMENT
CLAMP
204B-M-10A
Figure 2 -4. Two Submodule Units
in Rack Adapter
2-2 01369-3

Section 111 Model 204B
Figure 3-1
Controls and Connectors
1. For selecting frequency range. In OFF position
oscillator
is inoperative.
2. On indicator; glows in all positions of RANGE
switch except OFF.
3. For selecting frequency within desired range.
Dial calibration multiplied by RANGE switch
position gives output frequency in cycles per
second.
4. Provides fine frequency adjustment. Dial is
5. For selecting desired output level(10 mw max-
calibrated with arrow in vertical position.
imum into 600 ohms).
6. Output terminals, floating; impedance, 600
7. cabinet ground terminal.
Operation
a. Connect load to OUTPUT 60052(6) terminals.
b. Select frequency with RANGE (1) and FREQ.
(3). Example: If FREQ.
is set to 20 and
RANGE to XlK, the oscillator's output fre-
quency is 20 kc (20 cps x 1K = 20 kc).
c. Set AMPLITUDE
(5) for desiredpower level.
Not
e
If oscillator is equipped with AC PaverSup-
ply
or Rechargeable Battery Power Supply,
turning RANGE switch to OFF does not turn
AC Power Supply
or Battery Charger off.
Disconnect power cord to turn either
of these
two power supplies off,
Figure 3-1. Front Panel Controls, Connectors, and
Operating Instructions
3
-0 01369-3

Model 204B
3-1. INTRODUCTION
0
SectionI11
Paragraphs 3-1 to 3-15
SECTION 111
OPERATION
3-2. The Model 204B has four operating controls and
one output connector (see figure 3-1). Asine wave
signal in the 5-cps to 560-kc range is selected by
positioning the FREQ.
, RANGE, and VERNIER con-
trols. Output power level is controlled by the
AMPLITUDE control which hasatleast
a 40-db range.
A 115/230 volt switch is on the rear panel whenosci-
llators
are equipped with an ac power supply or re-
chargeable battery power supply,
3-4. Operating instructions for the Model 204B
are
given in figure 3-1.
3-3. PROCEDURE.
3-5. GENERAL OPERATING
CONSIDERATIONS.
3-6. To prevent damage to transistors in the oscil-
lator,
a blocking capacitor should be used in series
with the output when connecting to a load acrosswhich
there
is a dc potential difference. When the capacitor
is used, initial current surge must be limited to a
capacitance-voltage product lessthan 200 microfarad-
volts. For example,
if a 20-volt potential difference
exists across the load. use
a 10-uf capacitor (10uf x
20 v
= 200 !.l fv). CAUTION
DO NOT CONNECT THE OSCILLATOR
ACROSS
LOADS WHICH ARE REFERENCED
OFF GROUND BY MORE THAN rt25
VOLTS; HIGHERVOLTAGESWILL BREAK
DOWN COMPONENT INSULATION,
3-7. When the Model 204B
is initially turned on and
RANGE switch
is set to X1, allow 20 seconds for os-
cillator to stabilize;only a few seconds are necessary
for stabilization when switching to other RANGE po-
sitions.If FREQ. dial
is sweptat too fast a rate, out-
put voltage fluctuations will occur (since
a wirewound
resistor
is used as thevariable frequencydetermining
element).
If sweep speed is decreased or sweeping is
stopped, oscillator output will stabilize at its normal
level.
3-8. To obtain maximum amplitude stability from
battery-powered oscillators, wait 20 minutes after
turn-on topermit the battery voltage to stabilize.This
effect may cause
as much as a 1% change in amplitude
during the first 20 minutes of operation.
3-9. When operating from batteries (Mercury
or
Nickel-Cadmium) thebatteries require replacement or
recharging (Nickel-Cadmium batteries only) when
maximum oscillatoroutput becomes lessthan 5.0 volts
rms open circuit or less than 2.5 volts rms into 600
ohms. For mercury battery replacement data, refer
to paragraph 3-26. Charging instructions for nickel-
cadmium batteries
are given in paragraph 3-16.
3-10. OPERATION WITH RECHARGEABLE
BATTERIES.
3-11. There is no change in the operating procedure
for an instrument in which the rechargeable battery
01369-4
supply has been installed. However, this power sup-
plypermits eitheracoperationfor 115/230 vac
&lo%
(selected by switch on rear) 50 to 1000 cps power
sources
or batteryoperationforportable applications.
Battery operation only (with charger power cord dis-
cotnected)
is required at temperatures below 32'F
(0 $!) and is recommended at temperatures above
104
F (4OOC).
3-12. When the RANGE switch
is turned OFF, power
is removed from the oscillator portion of the instru-
ment and
a resistor is connected across the output of
the power supply. The charger will continueto
operate
as long as the power cord is connected to a
power source. The resistor, placed across the out-
put of the power supply, acts
as a load in place of the
oscillator section to maintain
a constant charge rate
through the batteries.
A diode in the output circuit
prevents battery discharge when the oscillator
is not
connected to
a power source.
3-13. It
is recommended that thepower cordbe con-
nected to
a power source whenever possible. This
will prevent self-discharge of thebatterycells and will
assure
a fully charged battery whenever portable
operation
is required. Turn the oscillator
OFF when
not in use, particularly
when operating with the power
cord disconnected.
3-14. When fully charged, the batteries will power
the oscillator for approximately 40 hours of contin-
uous
or intermittent operation provided they are at a
temperature of 81 F +lo . If the batteries are oper-
ated
at higher or lower temperatures their capacity
is reduced as the temperature extremes are approach-
ed;approximately28 hop
at 122 F (500C) or approx-
imately 20 hours at -4 F (-20 C). At temperatures
beyond these extremes the batteries are not capable
of supplying their characteristic stable discharge
voltage.
3-15. The
+122'F to -4'F temperature range is
adequate for most users, however, keep these limits
in mind when operating under field cotditions. Inter-
nal temperatures in excess of 122
F are easily
obtained
if the instrument is left in the sun, even with
a moderate ambient temperature. Good practice would
be to avoid storing, transporting,
or operating in
directsunlightotherthan for a very short period. Also
avoid placing the instrument in
a location where
surrounding equipment might excessively
raise the
internal temperature.
CAUTION
THE HERMETICALLY SEALED CELLS IN
THESE BATTERIES MAY BE PERMANENTLY
DAMAGED OR THEIR LIFE DRASTICALLY
REDUCED IF EXPOSED TO EXTREMELY HIGH
TEMPERATURES. THIS DANGER INCREASES
UNDER PROLONGED CONDITIONS.
3-1

Section I11
Paragraphs 3-16 to 3-27 Model 204B
3
- 16, RECHARGING NICKEL- CADMIUM
BATTERIES.
3-17. The batteries should be considered
as fully
discharged when maximum oscillator output drops
below
5.0 volts rmsopencircuit or less than 2. 5 volts
rmsinto 600ohms. The batteries
will not operate
much longer when this point
is reached and the osci-
llator probably will not meet specifications. Exces-
sive discharge of batteries may damage them
or
shorten their life.
3-18. Torecharge batteries, turnoscillator OFF and
connect power cord to
a suitable power source. The
batteries will be fully charged in approximately 60
hours at
a 5.5 miliiampere charge current and 30
hours at a 11 milliampere charge current if they were
fully discharged at the beginning. The instrument can
be turned on for use after an initial charge of
at least
20 minutes.
If the batteries were only partiallydis-
charged,the oscillator can be turned on for use
at any
time. The charge
rate is the same whether the osci-
llator
is OFF or operating; therefore, very little in-
crease in charging timewill be required
if the instru-
ment
is used while the batteries are being recharged
The charge circuit can be increased with the charge
current adjustment R102 (refer to figure 5-11) to
a
maximum of 11 milliamperes to provide a quick
charge. The
11 milliampere charge rate should be
used for quick charge only. Repeated chargingat
a
high rate will shorten battery life.
3-19. The batterJes can beocharged
at ang tempera-
ture between +32
F to +122 F (0 C to +50 C). How-
ever, to obtain optimum battery life, recharging
should be tone
at a temperature of 80.6'F *lO°F
(27OC *5.6 C). CAUTION
THE FOUR NICKEL-CADMIUM BATTERIES
ARE HERMETICALLY SEALED AND CAN BE
DAMAGED IF CHARGED AT A FAST RATE
AT
A TEMPERATURE EXCEEDING 122 F
(5OOC). IF THE CHARGE CURRENT ADJUST-
MENT HAS BEEN SET FORA QUICK CHARGE
RATE, PLACE THE INSTRUMENT
IN A
LOCATION WHERE THE AMBIEYT
TEMPERATURE DOES NOT EXCEED 104
F
(4OOC).
3-20. CYCLE-LIFE OF NICKEL-CADMIUM
BATTERIES,
3-21. As extremes in temperatures are approached,
the cycle-life (complete charge-discharge cycles) of
the batteries
is reduced. Storage at high tempera-
tures
will increase the self-discharge rate and also
decrease the cycle-life. Permanent battery damage
may result
if the batteries are stored at a high tem-
perature for
a prolonged period.
3-22. Battery cycle-life canbe extended by recharg-
ing before the batteries are completely discharged,
charging and discharging at
a rate which is slower
than the maximum ten-hour rate specified by the
manufacturer, and by not overcharging.
3-23. The cycle-life of the batteries is based, by
the manufacturerer,on
an endpoint of 80%of the rated
225 milliampere-hour capacity. This is with a ten-
hour charge and discharge current of
22. 5 milliam-
peres with discharge carried to the normal ten-hour
end voltage (1.10 volts/cell x
5 = 5.50 volts/battery)
on every cycle. Under these conditions
a cycle-life
in excess
of 100 cycles can be expected.
3-24. When used topowerthe
@ Model 204BOscilla-
tor, the batteries are discharged at approximately
a
40 hour rate. The batteries are not fully discharged
if they are recharged as recommended in paragraph
3-25. Optimum battery life can be obtained by fol-
lowing
a simple routine of 1) preventing complete
battery discharge by recharging before the oscillator
output level drops below 5.0 V rms into an open cir-
cuit
(2. 5 V rms into 600 ohms), 2) keeping the use of
a quick charge to a minimum, 3) operati.i,r at mod-
erate temperatures whenever possible and 4) by dis-
connecting power cord after 60 hours of continuous
charging with oscillator turned OFF(30 hours
if
charging batteries at a quick charge rate).
3-26. MERCURY BATTERY REPLACEMENT.
3-27. Battery replacement is required wnen the
oscillator output becomes lessthan
5.0 volts rms open
circuit
or less than 2. 5volts rms into 600 ohms pro-
vided the DC BAL
is adjusted correctly. Replace the
batteries with four 6. 75-volt mercury batteries
(Mallorytype TR235R
or equivalent). Refer to figure
3-2, and proceed
as follows:
a. Remove top cover retaining screw and slide
cover from instrument.
b. Remove wing nuts and battery clamps.
c. Remove and dispose of batteries according to
paragraph 1-6.
d. Install replacement batteries
so that battery
polarity matches polarity designation on battery
clamps.
e.
f. Replace top cover.
3-16.
Replace battery clamps and wing nuts.
POLARITY
DESIGN-ATION
WING
N UT
2046-S-I
I
BATTERY
CLAMP
Figure 3-2. Rear View of Model 204B Showing
Mercury Batteries Installed.
3-2 01369-2

Model 204B
\
I
FEEDBACK AMPLIFIERS EMITTER
RC
Section
Paragraphs 4-1 to 4-10
ATTENUATOR
SECTION
PRINCIPLES
OF OPERATION
BRIDGE
(RIOA,B,C2,C7)
4-1. INTRODUCTION.
FOLLOWERS !
,
,
4-2. The Model 204B consists of an RC bridge oscil-
lator circuit,
a peak detector circuit, and an output
attenuator (bridged-T type). These circuits and the
front panel controls associated with them are shown
in the block diagram, figure 4-1.
4-3. The RC bridge oscillator consists of an RC
bridge, atwo-stageamplifier, and twoemitter follow-
ers. The output of this circuit is a sine-wave signal
which
is 1) returned to the RC bridge asfeedback,
and 2) applied to the output attenuator.
4-4. The RC bridge consists of an RC frequency-
selective networkand
a resistive voltage divider net -
work. The RC frequency-selective network supplies
positivefeedback to the amplifier and determines the
frequency of oscillation. The resistive voltage divid-
ernetwork
suppliesnegativefeedbacktothe amplifier.
The output of the amplifier is proportional to the
difference between the feedback signals.
4-5. The peak detector detects changes in the RC
bridge oscillator output voltage and changes the divi-
sion ratio
of the resistive voltage divider network,
thereby changing the amount of negative feedback.
The peak detector with the divider networkmaintains
the RC oscillator output at
a constant level.
PEAK
DETECTOR
Q5 ,CR6
4-6. The attenuator is a bridged-T attenuator which
provides continuous control of the oscillator output
voltage while maintaining constant output impedance.
4-7. OSCILLATOR CIRCUIT.
4-8, TheRCbridge in the oscillator circuit consists
of an RC frequency-selective networkand a resistive
voltage divider network. The frequency-selective
network
is similar to one leg of a Wien bridge; the
resistive voltage divider, the other leg.
4-9.
As in any oscillator, an in-phase feedbackvolt-
age (from the oscillator circuit output) is necessary
to maintain oscillations. The proper phase relation-
ship at the desired frequency
is maintained by the RC
components in the bridge.
4-10. The frequency-selective network consists of
a
series branch, C1 and R1, and a parallel branch, C2
and R2,
as shown in figure4-2. For the frequency at
which X
= R in the series andparallel branches, the
positive
feedback voltagetothe amplifier is maximum
and is in phase with the oscillator circuit output vol-
tage (figure 4-3). Only that frequency
at which X =
Rwill be amplified; at frequencies where Xcdoes fiot
equal R, the positive feedback voltage
is not of the
right phase and
is insufficient in amplitude to sustain
oscillation. Figure 4-3 shows the positive feedback
curve and phase relationship for frequencies above
and below the frequency where X
= R.
C
01369-2
Figure 4-1, Block Diagram
4-1