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

\
-
FEEDBACK AMPLIFIERS EMITTER
,
RC
20
\
ATTENUATOR
-- I
FOLLOWERS
BRIDGE
(RIOA,B,C2,C71 QI , Q2 03, 04
SECTION IV
I
R40A,Bh
I
I
I
PRINCIPLES OF OPERATION
I
FEEDBACK
Section
Paragraphs 4-1 to 4-10
4-1. INTRODUCTION.
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-stage amplifier, and two emitterfollow
-
ers. The output of this circuit is a sine-wave signal
which
is 1) returned to the RC bridge as feedback,
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 tothe amplifier and determines the
frequency of oscillation. The resistive voltage divid-
ernetwork suppliesnegative feedbacktothe 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.
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. TheRC bridge 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-phasefeedback volt-
age (from the oscillator circuit output) is necessary
to maintain oscillations. The proper phase relation-
ship
at the desired frequency is maintained bytheRC
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
positivefeedback voltage tothe 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 Xc does got
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

Section IV Model 204B
Paragraphs 4-11 to 4-15
Figure
4-2. Simplified Schematic Diagram
4-11. The resistive voltage divider network consists
of R3 and
Z1 which provide negative feedback voltage
to maintain the oscillator circuit output
at a constant
level. The negative feedback
is developed in part due
tothe dynamic resistance
of CR4 and CR5 which is
controlled by the amount of forward bias applied to
the diodes by the peak detector circuit. If the oscil-
lator circuitoutput changes,
the peak detector detects
the change and converts
it to a new bias level for the
diodes. Diode conduction changes (seen
as an imped-
ance change in the bridge), and results in
a differing
amount of negative feedback, If the negative peak of
the oscillator output signalexceeds
-7 volts then, the
peak detector circuitwould decreasethe forward
bias
on CR4 and CR5, increasing diode dynamic resistance.
Increased impedance in
Z1 of the divider increases
the amount of negative feedback tothe emitter
of the
amplifier. Increasing the negative feedback results
in
a decrease inthe net input tothe amplifier and thus
the output signal decreases. The oscillator output
is
maintained at a constant 14 volts peak-to-peak.
4-12. Two amplifiers, Q1 and Q2, (figure 5-9) am-
plifythe signaland apply it to complementary emitter
followers Q3 and
Q4. The emitter followers are for-
ward biased by CR2 and CR3 and under
a no-signal
condition are conducting slightly to minimize cross-
over distortion. The oscillator circuit output
is
sampled by the peak detector and also coupled tothe
bridged-T attenuator.
4-14. The peak detector circuit consists of Q5 and
4-13. PEAK DETECTOR.
CR6. This circuit samples the oscillator circuit out-
put, and supplies
bias proportional tothe output signal
to control the dynamic resistance of the diodes in the
resistive voltage divider network.
4-15. Peak detector Q5 conducts only on the negative
peak of the output signal. At
7 volts peak, breakdown
diodeCR6 breaks down and the voltage
at the junction
of CR6 and C25through C29 decreases. This changes
the
bias to CR4and CR5, which affects the resistance
of these diodes. Capacitors C25 through C29 act to
averagethe
bias voltage applied tothediodes over the
period of one cycle.
FREQUENCY
0 A
0.3
F
0.2
0
R
LAG
PHASE
LEAD
RATIO
-
POSITIVE FEEDBACK
TO AMPLIFIER
FEEDBACK
TO RC NETWORK PHASE
204 0-S -13A
Figure 4-3. RC Network Characteristics
4-2 01369-2

Section V
Paragraphs 5-1 to 5-7
Model 204B
5-1. INTR U I
SECTION V
MAINTENANCE
N.
5-2. This section provides maintenance and service
information for the Model 204B Oscillator. Required
test equipment, replacement and adjustment proce-
dures, and
a troubleshooting summary are included
in the section. Also includedare performance checks
which verify proper instrument operation,
5-3. REQUIRED TEST EQUIPMENT.
5-4. Test equipment required for maintaining and
checking performance of the Model 204B
is listed in
table 5-1. Equipment having similar characteristics
can be substituted for the equipment listed.
5-5. PERFORMANCE CHECKS.
5-6. The performance checks verify that the Model
204B
is operating normally and is accurately cali-
brated. These checks may be used
after maintenance
or as part of incoming quality control inspection. In
the following checks be sure the test equipment used
has been recently calibrated.
5-7. DIAL ACCURACY CHECK.
a. Connect frequency counter and 600-ohm load to
oscillator output
as shown in figure 5-1.
b. Set Model 204B controls
as follows:
RANGE.,
............. XI
FREQ.. ............... 5
AMPLITUDE. ......... MAX.
VERNIER
.......... centered
Instrument Type
Oscilloscope
Distortion Analyzer
AC Voltmeter
DC Voltmeter
Clip-On
Milliammeter
Frequency Counter
01369-3
5232A/ S32A
FOR DIAL ACCURACY
@ 2040
I
I
I
I 0330 C/D
I
-6OOn
Figure 5-1. Test Setup for Dial Accuracy
or Distortion Checks
c. Set frequency counter controls
as follows:
Function.
, 100 PERIODS AVERAGED
SENSITIVITY,
............ 3
DISPLAY. maximum counterclockwise
d. Counter should read 200 i6milliseconds.
e. Set FREQ. dial to 20, counter should read 50.0
i1.5 milliseconds.
Table 5-1. Required Test Equipment
Required Characteristics
Passband: dcto 600 kc
Sensitivity:
0.1 volts/cm
Input Impedance:
1 megohm
Measure distortion to -40 db
Frequency Range: 5 cps to
Voltage Range:
1 mv to 5 volts
Accuracy:
*l%
Voltage Range: Positive and
1 kc
600 kc
negative voltages from 100
m
to 15volts
Input impedance: at least 10
megohms
Range: 5 to
11 milliamperes
Accuracy:
*3%
Counting Range: 5 cps to 600 kc
Accuracy: 0.03%
Us e
Waveform checking
Distortion measurement
AC voltage measurements
DC voltage measurements
Charge current
Frequency measurements
measurements
Recommended Model
@ Model 175A with
plug-in Model 1753A
@ Model 330B/C/D
@,I Model 400H
@ Model 412A
@ Model 428A
@ Model 5232A/5532A
5-1

Section V
Paragraphs 5-8 to 5-9
f. Set FREQ. dial to 50, counter should read 20.0
*O. 6 milliseconds.
g. Set RANGE toXlOand FREQ. dial to 5, counter
should read the same
as step f.
h. Repeat steps e and f withRANGE atX10. Coun-
ter should read 5.00 *O. 15 milliseconds and 2.00
f0.06 milliseconds respectively.
i. Set frequency counter function switch to FRE-
QUENCY
(. 1).
j. Complete check by setting RANGE switch and
FREQ. dial
as shown in table 5-2, columns one and
two. The counter reading should be
as shown in
column three .
Table 5-2. Dial Accuracy
Range
Switch
XlOO
XlOO
XlOO
X1K
X1K
X1K
X1OK
X1OK
X1OK
Freq.
Dial
5
20
50
5
20
50
5
20
50
500 cps
k 15 cps
2000 cps i 60 cps
5000 cps
f 150 cps
5 kc
i 150 cps
20 kc
i 600 cps
50 kc
f 1.5kc
50 kc
f 1.5kc
200 kc
i 6 kc
500 kc
f 15kc
5-8. FREQUENCY RESPONSE AND OUTPUT
VOLTAGE CHECK.
a. Connect ac voltmeter and 600-ohm load to oscil-
lator output as shown in figure 5-2.
to 25.
b. Set Model 204B RANGE to X1 and FREQ. dial
c. Adjust AMPLITUDE for 2.5 volt reading on
voltmeter.
d. Sweep FREQ.
dial by hand to read 50. As dial
is swept, voltmeter reading should not vary more
than &O. 075 volts.
e. Set Model 204B RANGE toX10.
f. Set FREQ. dial to 5 and repeat step d.
g. Complete check with RANGE switch
set to each
untested position while repeating step d.
Model 204B
0 2040
@400H
2040 -S -SA
Figure 5-2. Test Setup for Frequency Response
and Output Voltage Check
5-9. DISTORTION CHECK.
a. Connect distortion analyzer and 600-ohm load
to oscillator output
as shown in figure 5-1.
b. Set Model 204B controls
as follows:
RANGE.
............. XI00
FREQ..
........... 10 (1 kc)
AMPLITUDE. MAX.
.........
c. Set distortion analyzer controls as follows:
FREQUENCY RANGE.
..... XlOO
INPUT..
.............
AF
FUNCTION. ....... SET LEVEL
METER RANGE
......... 100%
d. On distortion analyzer:
(1) Adjust
INPUT SENSITIVITY for full scale
(2) Set FUNCTION to DISTORTION.
(3) Adjust FINEandCOARSE frequency controls
and BALANCE controlfordip
at fundamental
frequency
(1 kc); switch METER RANGE as
necessary tomonitor dip.
(4) Readjust controls untilabsolutedip
is located
e. Meter reading should be less than 1.0 on the 1%
reading (1.0).
range.
5-2 01369-3

Model 204B
Section V
Paragraphs 5-10 to 5-15
Figure 5-3. Cover Removal
5-10. REPAIR.
5-11. When repairing the Model 204B, use the ex-
ploded
view, figure5-4, as an aid to identifying parts.
5-12. COVER REMOVAL.
5-13. Remove covers prior to any check or adjust-
ment which requires power tobe applied. Refer to
figure
5-3, and proceed as follows:
a. Remove top cover screw.
b. Slide top cover to rear, and
lift to remove.
c. Slide rear foot assembly toward side while
d. Lift foot assembly to remove.
pushing foot release.
01369-2
e. Remove bottom cover screw and lift tilt stand.
f. Slide bottom cover to rear and lift to remove.
g. Remove side cover screws and side covers.
h. To replace covers and foot assembly, reverse
the order of steps
a through g.
5-14. SERVICING ETCHED CIRCUIT BOARDS.
5-15.
The etched circuit boards used in theModel
204B require that the soldering iron tip be applied to
the conductor side of the board when servicing. For
large components, such
as potentiometers, rotate the
soldering iron tip from lead to lead while applying
pressure to the part to
lift it from the board or use a
soldering tip such as Ungar #855 3/4 in. Cup Tip. In
addition tothe above information, the following should
be observed.
5-3

Section V Model 204B
Paragraphs 5-16 to 5-24
Figure 5-4. Model 204B, Exploded View
a. Do not apply excessive heat,
b. Apply heat to component lead on conductor side
of board, and remove leadwith
a straightupward pull .
c. Use a toothpick or wooden splinter to clean
holes.
d. Solder replacement components from the con-
ductor side.
5-16. TROUBLESHOOTING.
5-17. To locate trouble in the Model 204B, start with
a thorough visual inspection. Look for burned-out
or loose components, loose connections of any other
condition which suggests
a source of trouble. If a
visual inspection does not reveal the trouble, use the
block diagram, figure 4-1, and troubleshooting sum-
mary, table 5-3, ad aidsfor isolating the trouble. The
troubleshooting summary listsindication of and action
to
be taken for various troubles; the action should be
taken in the order given.
PROCEDURES.
5-18. ADJUSTMENT AND CALIBRATION
5-19. AC POWER SUPPLY CHECKS.
5-20. The
ac power supplyvoltagesare typically +13
and -13 volts, but may vary from 12 to 13. 5 volts. As
linevoltage isvariedfrom 103.5to 126. 5 volts, supply
voltages should remain with the 12 to 13. 5 volt limit.
5-21, Ripple voltage in the
ac power supplyshould be
less than 2 mv. To check ripple voltage, it
is nec-
essary to connect
a wire from circuit ground (h) to
the junction of RlOA and S1D. Measure ripple voltage
as the linevoltage is variedfrom 103.5 to 126.5 volts .
5-22. When trouble occurs, in the ac power supply
check CR15 and CR16
first. If any repair is per-
formed, adjust DC BAL control (paragraph 5-26).
5-23. BATTERY CHARGER CURRENT
ADJUSTMENT.
5-24. Thebatterycharger current
is setatthe factory
for5.5
*O. 5milliamperes toprovide maximum battery
life, Charge current adjustment R102 can be set to
provide a quick charge by increasing the charging
current to not more than
11 milliamperes.
5-25.
Use the following procedure to set charge
current adjust R102 for quick charge operation.
a. Remove top cover from oscillator chassis.
b. Connect power cord to
a power source.
c. Turn oscillator
OFF.
d. Attachcurrentprobe of clip-on milliammeter to
one input lead of battery charger circuit. (See figure
e. Set R102 for the desired charge current. .
5-11.)
1
5-4 01369-3

Model 204B Section
V
Paragraphs 5-25 to 5-30
i
Note
Do not exceed
11 milliamperes. A
charge current exceeding 11 milli-
amperes will shorten battery life.
Replace top cover.
5-26. DC BAL ADJUSTMENT,
5-27. When the DC BAL control R15
(rear panel) is
properlyadjustedthere shouldbe no dcvoltage present
at the oscillator output.
If dc voltage is present, pro-
ceed
as follows:
a. Connect a DC voltmeter to oscillator output.
(Withthe optionalRechargeable BatteryPower Supply,
connect DC voltmeter between floating ground and
the junction of L1 and C102.)
b. Set AMPLITUDE to MAX.
c. Adjust DC BAL on rear of instrument for a zero
volt reading. If necessary, change the value of R18
for proper range of R15.
If DC BAL will not adjust to
.zero volts, check power supply for proper voltages.
5-28. DISTORTION ADJUSTMENT,
a. Connect a 600-ohm loadacross the oscillator
output terminals.
b. Using an AC voltmeter, measure thegain con-
trol voltagebetween circuit ground and arm of poten-
Table 5-3. Troubleshooting Summary
Indication
No output signal; dc voltage
at output
greater than
k1 volt (dc voltage
measured at junction of L1 and
C102 on Option 02 PowerSupply
with Rechargeable Batteries)
No output on one or more ranges
tiometer R36. Voltage should be between 90 and 140
mv rms on all ranges.
If voltage is high, increase
resistance of R34;
if low, decrease resistance.
c. Set Model 204B controls as follows:
RANGE
............. XI00
FREQ..
.......... lO(1kc)
VERNIER.
......... centered
AMPLITUDE.
......... MAX.
d. Connect distortion analyzer across 600-ohm
e. Measure distortion, and adjust R36 for mini-
load on oscillator output terminals.
mum reading. Reading should be
less than 1%.
Note
Do not use coaxial cables (use twin lead
only) and disconnectall other equipment
from oscillator when making distortion
measurements.
5-30. Frequency calibration adjustments should be
performed, only
if necessary,after repairs are made
to frequency sensitive components.
5-29. FREQUENCY CALIBRATION ADJUSTMENTS.
a. Connect frequency counter and 600-ohm load to
oscillator output
as shown in figure 5-1.
No output signal;proper dc voltage
at
output (zero volts)
Output amplitude not correct and/or
distorted
If all ranges areaffected
If all ranges are NOT affected
Action
Check power-supply voltages (+13 and -13 volts)
Check Q3,
Q4, CR2, and CR3 for correct dc voltages
Check Q1, Q2, and CR1 for correct dc voltages
Check power-supply voltages (+13and -13 volts)
Check Wien bridge components
Check Q1 and Q2 for correct voltages
Check peak detector circuit (Q5, CR4, CR5, and CR6)
Check RANGE switch contacts
Check components connected to Wien bridge position when
(refer to schematic diagram)
(refer to schematic diagram)
RANGE switch
is inoperative position. For example:
If inoperative position is X1, check C2A and C7A.
RANGE switch
is in inoperative position.
Check components connected to peak detector circuit when
Check power-supply voltages (+13 and -13 volts)
Check components in upper and lower legs of Wien bridge
I
for proper value *percent of tolerance (refer totable 6-1
for tolerances). For example: When RANGE switch
is in
X1 position, check R24, R4, C2A, RlOA, RlOB, R11, andC7A
Check peak detector circuit (Q5, CR4, CR5, and CR6, forpro-
per operation. Refer towaveforms and voltages in schem-
atic diagram. Be sure CR6 breaks down at
7 volts peak.
respectively
Check for incorrect voltages
at Q3, Q4, Q2, and Q1
~~ ~~
Check components connected to peak detector circuit in
affected range
5-5
01369-4

Section V
Paragraphs 5-31 to 5-35
b. Set Model 204B RANGE to X100, VERMERto
center of its range, and AMPLITUDE to maximum
clockwise position,
c. Set FREQ. to 56 (5.6 kc).
d. Lock FREQ. dial shaft with
a number 8-32
socket set screw (dial shaft locking screw) whichin-
serts into threaded hole on top of dial shaft housing
.
e. Loosen potentiometer (pot) shaft locknut, fig-
ure 5-5a, and adjust pot by turning pot arm to obtain
a 5.6-kc output,
f, Tighten pot shaft locknut, and loosendialshaft
locking screw.
g. Set FREQ. to 5 (500 cps) and tightendial shaft
locking screw.
h. Loosen dial shaft locknut and adjustdial cam
by turning cam to obtain
a 500-cps output.
i. Tighten dial shaft locknut and loosendialshaft
locking screw.
j. Repeat steps c through i untilfrequencies are
within approximately rtl%.
k. Set RANGE to XlOK, FREQ. to 5 (50 kc), and
adjust C8 (figure 5-6) to obtain
a 50-kc output.
m. Set FREQ. to 56 (560 kc), and adjust C15
(figure 5-4) to obtain
a 560-kc output. If necessary,
change the value of C14 for proper range of C15.
Check gain control voltage according to
steps
a and b of paragraph 5-28.
Note
n. All frequencies across the band should be
within
*3%.
5-31. REPLACEMENT PROCEDURES.
5-32. CAM CABLE REPLACEMENT.
5-33.
If it is necessary to replace the cam cable,
order
it by @ Stock No. 8160-0003 and description.
For easier access to the cams, remove the screws
Model 204B
holding the power supplyand circuit board, then swing
the two assemblies to the side. Use figure 5-5
asa
guide, and proceed as follows:
a. Orient cams as shown in figure 5-5a.
b. Using
a hex socketwrenchand 3/8-inch wrench,
c. Remove terminal bolts from cable.
d. On replacement cable, place
a mark 6-7/8
inches from the end,
e. Slide replacement cable through one terminal
bolt
so that cable is orientated to terminal bolt as
shown in figure 5-5a, lower left detail.
f. With mark on the cable in center of terminal
bolt
as shown, install terminal bolt on dial cam.
g. Slidecable endsapproximately 1/4 inch through
second terminal bolt
so that cable is orientated to
terminalbolt
as showninfigure 5-5a, upper left detail.
h. Orientate the cams
as shown in figure 5-5band
use the figure
as a guide. Slide the cable onto the
cams, and install the second terminal bolt on the pot
cam. (Theshortercable length shouldpass over points
Aand
B; the longer length shouldpass over D, B , andC .
i. Tighten bothterminalbolts to remove all slack
in cable and allow the dial to cover its full range.
Do
not overtighten cable.
j, Perform Frequency Calibration Adjustments ,
(paragraph 5-29).
remove both cable terminal bolts.
5-34. FREQUENCY POTENTIOMETER
REPLACEMENT.
5-35. To replace the frequency potentiometer (pot),
RlOA and
B, it is necessary to remove the pot cam .
For easier access to the cams, remove the screws
holding the power supply and circuit board, then
swing the two assemblies to the side. Use figure 5-5
as a guide, and proceed as follows:
a. Position cams as shown in figure 5-5a. Some
instruments have aluminum cams which have the
counterweight shown in figure 5-5. Other instruments
SHORT
TERMINAL LENGTH
.
.
SHORT
TERMINAL LENGTH
--
LONG
LENGTH
COUNTERWEIGHT
PANEL BUSHING NUT
POT SHAFT LOCKNUT
POT ARM
CABLE
TERMINAL BOLT
POT CAM
DIAL SHAFT
LOCKNUT
DIAL CAM
SHOULD BE
IN CENTER CHASSIS TOP
WT
CPM
DIAL
CAM
....... LONG CABLE
LENGTH
b
Figure 5-5. Cams and Cable Relationship
5-6 01369-3

Model 204B Section V
Paragraphs 5-36 to 5-37
TO POWER SUPPLY
(OPTION 02 ONLY)
TO POWER SUPPLY
(OPTION 02 ONLY)
Figure 5-6. Range Switch Detail
have plastic cams which do not require
a counter-
weight. Ignore any references to counterweight in
following procedure when your instrument has the
plastic cams.
b. Loosen pot shaft locknutandpanel bushing nut,
c. Using
a hex socketwrench, turn cableterminal
bolt on pot cam counterclockwise 1/2 turn
or until
cable slips off cam.
d. Slide pot cam off pot arm. Note that one sec-
tion of cable
is shorter than the other.
e. Remove three screws holding pot to chassis.
f. Remove pot , and install replacement.
g. Orient the cams as shown in figure 5-5b and
use the figure
as a guide to help you string the cam
cable properly. (The shorter cable length shouldpass
over points
A and B; the longer length should pass
over points
D, B, and C.)
h. Turn cable terminal bolt clockwise approx-
imately 1/2 turn to remove some slack
in the cable,
i. Install counterweight and tighten panel bushing
nut. Counterweight
is keyed and should be positioned
as shown in figure 5-5. (omit
if no counterweight).
j. Align pot cam to the same plane as dial cam.
k. Remove all slack in cable by turning cable
terminal bolt clockwise. Do not overtighten cable.
01369-4
m. With cams aligned, tighten pot shaft locknut.
n. Perform Frequency Calibration Adjustments,
paragraph 5-29.
5-36. RANGE SWITCH ASSEMBLY REPLACEMENT.
5-37. To aidin replacingtheRANGE switchassembly
@ Stock No. 204B-95D, refer to the exploded view,
figure 5-4, and range switch detail, figure 5-6.
a. Removeoscillatorcovers(seeparagraph5-12).
b. Remove RANGE switch knob.
c. Remove indicator decaland the nut whichholds
the RANGE switch to front panel.
d. In order to remove leads from switch, it
is
necessary to slide switch away from panel and lift to
edge of chassis; clip leads at switch terminals.
e. Solder leads to replacement switch using fig-
ure 5-6
as a guide for correct lead connections.
f. Install switch as shown in figure 5-4 using nut
removed in step c.
g. Replace indicator decal
so that black portion
is adjacent to OFF position.
h. Replace switch knob.
i. Perform Frequency Calibration Adjustments
and DC BAL Adjustment, Paragraphs
5-29 and 5-26.
5-7

Section V
Paragraphs 5-38 to 5-46 Model 204B
5-38. BATTERY-PACK POWER SUPPLY.
5-39. To convert an AC-powered or rechargeable
battery-powered Model 204B for operation from
mercury batteries, obtain
a standard Battery Pack,
@ Stock No. 204B-64B. Install Battery-Pack Power
Supply according to instructions included with kit
,
Refer to paragraph 3-26 for battery replacement
instructions.
5-40. AC POWER SUPPLY.
5-41. To convert a battery-powered or rechargeable
battery-powered Model 204B for operation from AC
power, obtain AC Power Supply Kit, @Stock No,
204B-11A. (Refer to figures 5-8 and 5-12.) Install
AC-Operated Power Supply according to instructions
included with kit.
5-42. RECHARGEABLE BATTERY POWER SUPPLY.
5-43. The Rechargeable Battery Power Supply can
replace the standard Battery Pack
or the AC Power
Supply. This supply contains four 6.25 volt sealed
nickel-cadmium batteries which permit portable
operation and
a charge circuit which operates from a
115/230 volt (selected by switch on rear panel S101)
50 to 1000 cps power line, Power input to this power
supply
is approximately 3 watts. To convertan AC
powered
or a battery-powered Model 204B for oper-
ation from
a rechargeable battery power supply,
obtain Rechargeable Battery
Kit, @ Stock No. 204B-
11C. Install Rechargeable Battery Power Supply
according to instructions included with kit.
5-44. After Rechargeable Battery Power Supply has
been installed,turn oscillator on andcheck maximum
output voltage level. If level
is below 5.0 V rms
open circuit
(2. 5 V rms into 600 ohms), turn osci-
llator OFF and recharge batteries following the pro-
cedure
in paragraph 3-16.
CAUTION
BE SURE TO PLACE 115/230 VOLT
LINE SWITCH (S101) IN PROPER
POSITION BEFORE CONNECTING
POWER CORD TO A POWER SOURCE.
(SEE PARAGRAPH 2-12.)
5-45. REPLACEMENT OF RECHARGEABLE
BATTERIES .
5-46. When rechargeable batteries require replace-
ment, disconnect power cordandturn oscillator OFF.
The following procedure
is recommended:
a. Disconnect power cord and remove top, bottom
and side covers.
b. Remove sixscrewsholding rearpanel in place .
c. Remove four screws holding battery charger
circuit board in place.
REAR SIDE POLARITY REAR SIDE
SCREWS DES
I G NATION SCREWS
RE AR’PANEL
”
REAR PANEL
SCREWS
2048-6-17A
Figure 5-7. Rear View of Battery-Powered
Model 204B
d. Loosen two screws holding oscillator board
in place.
e. Slide power supply out of oscillator chassis.
f. Unsolder resistorR106 (4.7 ohms), red/orange
black, andvioletwiresfrombatteries (seefigure 5-9)
.
g. Remove two cap nuts and screws with fiber
washers from battery holder and rear panel,
h. Remove and discard old batteries.
CAUTION
DO NOT DISPOSE OF BATTERIES BY
BURNING, AS THEY EXPLODE IF
INCINERATED,
i. Place four new batteries in instrument
observing battery polarity
as indicated in figure 5-9.
j. Replace battery holder cap nuts, screws,and
fiber washers.
5-8
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