T.O 12R2-2BC-112

HANDBOOK

MAINTENANCE INSTRUCTIONS

RADIO RECEIVERS

BC-224-F BC-224-K BC-348-H

BC-348-K BC-348-L BC-348-R

PUBLISHED UNDER AUTHORITY OF THE SECRETARY OF THE AIR
FORCE
AF-WP-O-16 MAY 57
5,510

20 JULY 1945

REVISED 15 APRIL1957

TABLE OF CONTENTS
SECTION I
GENERAL DESCRIPTION
1. GENERAL.
2. EQUIPMENT SUPPLIED.
3. EQUIPMENT REQUIRED BUT NOT SUPPLIED.
4. RADIO RECEIVER BC-348-(*) OR BC-224-(*).
5. MOUNTING FT-154-(*).
6. MOUNTING FT-167-(*).
7. PLUG PL-P103, PL-Q103, AND PL-Q103-A.
SECTION II INSTALLATION AND ADJUSTMENT
1. UNPACKING THE EQUIPMENT.
2. BENCH TEST.
3. INSTALLATION.
4. ELECTRICAL CONNECTIONS.
5. ADJUSTMENTS.
6. AFTER-INSTALLATION TESTS.
SECTION III OPERATION
1. STARTING AND STOPPING THE EQUIPMENT.
2. OPERATION.
SECTION IV THEORY OF OPERATION
1. GENERAL.
2. FREQUENCY RANGE AND FREQUENCY BANDS.
3. DETAILED FUNCTIONING.
SECTION V MAINTENANCE
1. INSPECTION.
2. TROUBLE LOCATION AND REMEDY.
3. VOLTAGE AND RESISTANCE MEASUREMENTS.
4. ALIGNMENT
5. SPECIAL MAINTENANCE PROCEDURES.
6. REPLACEMENT OF FUSES AND LAMPS.
7. LUBRICATION DATA.
SECTION VI SUPPLEMENTARY DATA
1. TUBE COMPLEMENT.
2. LAMP COMPLEMENT.
3. FUSE COMPLEMENT.
4. FREQUENCY RANGE.
5. PERFORMANCE DATA.
SECTION VIII DRAWINGS

SAFETY NOTICE
This equipment employs high voltages which
can be fatal if contacted by personnel. Exercise caution when working with the
equipment when potentials are exposed.
SECTION I GENERAL DESCRIPTION

SPECIAL NOTICE
Radio Receiver BC-224-F, BC-224-K,
BC-348-H, BC-348-K, BC-348-L and BC-348-R are essentially alike. Therefore,
reference will be made throughout the book to these receivers as
follows:
"Radio Receiver BC-224- (*) or BC-348- (*)" Reference to the
dynamotors is mentioned in the same manner (Dynamotor DM-24-(*) or Dynamotor
DM28-(*), the asterisk indicating that the information applies to any model of
either dynamotor.
Mounting FT-154-(*) refers to any production model of this
mounting.
1. GENERAL.
(See
figure 1-1.)
a. Radio Receiver BC-224-(*) or BC-348-(*) is a locally
controlled, 8-tube, 6-band superheterodyne receiver for use in U. S. Army
Aircraft. The receivers are not intended for remote control and no features or
units have been provided for remote operation. These receivers are capable of
voice, tone and c-w reception with manual or automatic volume control. When
equipped with headsets, tubes, dial lights and fuses, and with the antenna,
ground and primary power source connections properly made, these receivers
become complete and operative equipments. All coils and high voltage power
supply units are built in the receivers.
b. Each receiver covers the
frequency range 0.2 to 0.5 megacycles and 1.5 to 18 megacycles.
c. The radio
receivers are essentially alike, electrically, the filament circuit and high
voltage supply units differ to permit Radio Receiver BC-224-*) to operate from a
14-volt power source and Radio Receiver BC-348-(*) from a 28-volt power source.
The total power consumed by each receiver is 56 watts. A dowel pin fastened to
the chassis of Radio Receiver BC-348-(*), prevents accidental installation of
the 14-volt dynamotor in the 28-volt receivers. There are minor mechanical
differences between the 12-volt receiver and the 28-volt receiver, incidental to
supporting electrical parts.
2. EQUIPMENT
SUPPLIED.
(See Table 1-1.)

TABLE 1-1. COMPONENT UNITS OF RADIO RECEIVER BC-348-(*) OR
BC-224-(*)

Quantity per Equipment

Name of Unit

Army Type Designation

Overall Dimensions (inches)

Weight (pounds)

1 ea.

Radio Receiver includes: Dynamotor (mounted on
receiver)

BC-348-(*) DM-28-(*)

18 x 10.5 x 9.5

35.5

Radio Receiver includes: Dynamotor (mounted on
receiver)

BC-224-(*) DM-24-(*)

18 x 10.5 x 9.5

35.5

1 ea.

Mounting

FT-154-(*)

18 x 8.5 x 1.375

3.81

1 ea.

Plug

PL-P103

2.3 x 2.15 x 2.56

0.34

Plug

PL-Q103

3 x 2.15 x 2.06

0.41

Plug

PL-Q103-A

3 x 2.15 x 2.06

0.41

1 ea.

Fuse (used with Radio Receiver BC-348-(*))

FU-35

Fuse (used with Radio Receiver BC-224-(*))

FU-23

2 ea.

Lamp

LM-27

1 ea.

Set of 9 Tubes

0.563

3. EQUIPMENT REQUIRED
BUT NOT SUPPLIED.
(See Table 1-2.)

TABLE 1-2. EQUIPMENT REQUIRED BUT NOT SUPPLIED WITH RADIO
RECEIVER

BC-348-(*) OR BC-224-(*)

Quantity per Equipment

Name of Unit

Army Type Designation

Required Characteristics

1

Signal Generator

I-72( )

Capable of generating r-f output from 100 kc to 32 mc,
modulated and unmodulated.

1

Jack

JK-34

1

Headset

HS-33

Low impedance headset.

1

Mounting

FT-167

Used when required in
installation.

4. RADIO RECEIVER
BC-348-(*) OR BC-224-(*).
a. CABINET.--The receiver cabinet is of
spotwelded aluminum construction with black wrinkle finish and is embossed on
the top and back to add rigidity and strength. The panel (front) end allows the
removal of the receiver chassis, which runs on the stainless steel strips
mounted in the end corners of the cabinet. Two tapped inserts are placed in the
rear to receive the thumb screw rods which secure the chassis in the cabinet. A
cutout in the rear bottom permits the entrance of Plug PL-P103, PL-Q103 or
PL-Q103-A. (See figure 1-2.) Four mounting studs are attached to the bottom of
the cabinet for securing the cabinet on Mounting FT-154-(*).

Figure 1-2 Bottom and Back View of Cabinet Radio Receiver
BC-348-(*) or

BC-224-(*)
b. CHASSIS.--The chassis consists of an
aluminum casting mounted between two end plates of sheet aluminum, which serve
as runners and guides when placing the chassis in the cabinet.

Figure 1-3 Radio Receiver BC-348-(*) or BC-224-(*) -- Front View
of Chassis with

the Tube Shell Cover Removed
c. PANEL.--The front panel
is attached to the chassis and to the end plates by screws. Two handles are
mounted on the panel. A cutout, covered by a plate, is provided to give access
to the wiring under the r-f tube shelf for servicing and maintenance. (See
figure 1-3.) The following panel items are mounted on the front of the panel:
Antenna and ground binding posts; antenna alignment control "ANT ALIGN"; dial
lights rheostat control "DIAL LIGHTS"; tuning control "TUNING"; band switch
control "BAND SWITCH": dial window housing which covers the dial lights; beat
frequency control "BEAT FREQ"; crystal filter control "OUT-CRYSTAL-IN"; volume
control "INCREASE VOL; AVC-OFF-MVC" control; "C.W. OSC" control; and two
telephone jacks "TEL."
d. DIAL AND MASK ASSEMBLY.--The dial and mask assembly
is mounted on the aluminum casting which carries the gearing of the tuning
capacitor drive and the detent. The dial is divided into six frequency bands.
The dial and tuning capacitor are so geared to the tuning control shaft that the
tuning capacitor covers the frequency range indicated on the dial for each band
in approximately 90 revolutions of the tuning knob. A mask with suitably located
and marked windows is mounted before the dial. The mask is controlled by the
band change switch and is positioned by the detent.
e. ILLUMINATION.--The
receiver tuning dial is illuminated by means of two dial lights, Lamp LM-27,
controlled by the "DIAL LIGHTS" rheostat. This rheostat has an off position when
the illumination is not desired. The dial lights are located beneath a readily
removable housing which permits the easy replacement of a dial lamp during
flight.
5. MOUNTING FT-154-(*).
(See
figure 1-4.)
The mounting is constructed of stainless steel except for the
aluminum base which carries the mounting holes and the bases of four shock
absorbers. To the top of the shock absorbers is fastened the stainless steel
support, which provides for the attachment of the receiver cabinet by means of
studs and snapslides. Grooves are provided in the stainless steel support to
facilitate the engagement of the studs of the cabinet with the mounting.

Figure 1-4 Mounting FT-154-(*) with plug PL-103 Attached
6. MOUNTING FT-167-(*).
A steel bracket holding a
rubber shock absorber may be used with some of the receivers where the sets are
to be subjected to excessive vibration. This bracket is attached at the top rear
of the cabinet and to the wall of the plane behind the set, thus arresting any
tendency toward horizontal motion. (See figures 1-5 and 8-15.)

Figure 1-5 Mounting FT-167
7. PLUG
PL-P103, PL-Q103, AND PL-Q103-A.
The plug, attached to the mounting
by screws, is provided with eight terminals which are accessible upon removal of
the rear cover of the plug housing. Plug PL-P103 is provided with a straight
outlet. A right angle outlet mounted in any of three positions, right, left or
back, may be used with Plug PL-Q103 or PL-Q103-A. The positions and uses of
these outlets with the plug are clearly shown on the outline dimensional drawing
in Section VIII (see figure 8-15).
SECTION II
INSTALLATION AND ADJUSTMENT
1. UNPACKING THE
EQUIPMENT.
Unpack the equipment as follows:
a. Remove Radio
Receiver BC-348-(*) or BC-224-(*) and Mounting FT-154-(*) from the packing
cartons.
b. Loosen the thumbscrews at the bottoms of the handles on the
receiver front panel and pull the chassis from the cabinet.
c. Remove the
cardboard packing filler from over the dynamotor.
d. Check to see that the
receiver is free from dust.
e. Check to see that the vacuum tubes are firm
in their sockets and the fuse and dial lights are correctly and securely
inserted.
f. Operate the controls on the front panel. Check for binding and
sticking.
g. Examine binding posts for proper spring tension.

Note
Do not change the factory settings of
potentiometers, couplings and screwdriver adjustments.
h. Slide
the receiver back into its case and fasten the thumbscrews.
2. BENCH TEST.
a. TEST EQUIPMENT REQUIRED.--The
following equipment is required for making a bench rest of the radio
receiver.

Name of Item

Remarks

Plug PL-P103, PL-Q103 or PL-Q103-A

Wired with terminals 3 and 4 positive and terminals 7 and 8 negative

Jack JK-34

Wired between terminals 1 and 5 on Plug PL-Q103

100 micromicrofarad condenser

To be used as a dummy antenna

Signal Generator I-72( ) or equivalent

Capable of delivering a 150 kilocycle- 18 megacycle, MCW and CW
signal

Single pole, single throw switch

Connected between terminals 2 and 6 of Plug
PL-Q103

* Any issue is applicable.
b. CHECK OF
RECEIVER OUTPUT IMPEDANCE CONNECTION.--The output of the receiver can be matched
to either a 4000-ohm or a 300-ohm load. The receiver is normally connected for a
4000-ohm load unless a decalcomania on the front panel indicates otherwise. To
change the output impedance of the receiver from 4000-ohms to 300-ohms proceed
as follows:
(1) Loosen the thumbscrews at the bottoms of the handles on the
front panel and pull the chassis from the cabinet.
(2) Remove the lead from
terminal 5 ("HI") on output transformer 123-A and solder the lead to terminal 6
("LO"). (See figure 8-17.)
(3) Return the receiver chassis to the cabinet and
tighten the thumbscrews.
c. SETTING UP THE EQUIPMENT.
(1) Connect Radio
Receiver BC-224-(*) to a 14-volt source of supply or Radio Receiver BC-348-(*)
to a 28-volt source of supply.
(2) Place the "AVC-OFF-MVC" switch in the
"OFF" position.
(3) Connect Plug PL-Q103 (wired as indicated in paragraph 2a,
this section) to Connector SO-104. Check to see that the single pole, single
throw switch connected across terminals 2 and 6 of Plug PL-Q103 is
closed.
(4) Connect Signal Generator 1-72-( )** to the antenna post "A" of
the receiver through the 100 micromicrofarad dummy antenna.
(5) Connect the
ground post "G" of the receiver to the signal generator ground connection.
d.
ADJUSTING THE SIGNAL GENERATOR.
(1) Turn on the signal generator.
(2)
Place the modulation switch on the signal generator in the modulation
position.
(3) Set the band selector switch on the signal generator to the
frequency band which includes 500 kilocycles.
(4) Adjust the signal generator
frequency dial to 500 kilocycles.
* Any issue letter is applicable.
**
Refers to any model except 1-72-F (25 cycles).
e. TUNING IN A SIGNAL.
(1)
Place the "AVC-OFF-MVC" switch in the "AVC" position. The dynamotor should
start. Allow one minute for the receiver to warm up.
(2) Set the "BAND
SWITCH" on the receiver to hand one (200 to 500 kc).
(3) Adjust the receiver
"TUNING" control to 500 kilocycles.
(4) Rotate the "INCREASE VOL" control on
the receiver fully clockwise.
(5) Reduce the output of the signal generator
until the signal is just audible.
(6) Place the "AVC-OFF-MVC" switch in the
"MVC" position and adjust the receiver "TUNING" control for a maximum
signal.
(7) Adjust the antenna alignment control for maximum volume in the
headset.
f. CHECKING DIAL LIGHT OPERATION.
(1) Rotate the "DIAL LIGHTS"
control on the receiver.
(2) Check to see that both lights are functioning,
that they can be adjusted to any degree of brilliance, and that they can be
turned off completely.
g. CHECKING CW OPERATION.
(1) Turn off the
modulation of the signal generator.
(2) Place the "CW. OSC." switch on the
receiver in the "ON" position.
(3) Rotate the "BEAT FREQ" control on the
receiver. The audio frequency sound should vary in pitch. Check to see that the
audio frequency sound becomes inaudible (zero beat) when the pointer on the
"BEAT FREQ" control points approximately straight up.
h. CHECKING CRYSTAL
OPERATION.
(1) Leave the "CW. OSC." control in the "ON" position.
(2)
Place the "CRYSTAL" switch in the "IN" position. The noise should be greatly
reduced.
(3) Rotate the "TUNING" control on the receiver slightly to each
side of the signal. The signal should be tuned in and out with a smaller
movement of the "TUNING" control knob with the "CRYSTAL" switch in the "IN"
position than with the switch in the "OUT" position. The volume of the signal
should be slightly less.
i. CHECKING CALIBRATION OF 200 TO 500 KILOCYCLE
BAND.--Adjust the signal generator for a signal of 500 kilocycles and tune in
the signal on the receiver as described in paragraphs 2d and 2e. The receiver
dial reading should fall between 504 and 496 kilocycles.

Note
The frequency of the signal generator should be
accurately determined for calibration of the receiver dial.
j.
CHECKING CALIBRATION OF OTHER BANDS.--Check the calibration on the 1.5 to 3.5
megacycle band as follows:
(1) Adjust the signal generator to any frequency
within the frequency band of 1.5 to 3.5 megacycles according to the procedure
for adjusting the signal generator to 500 kilocycles as described in paragraph
2d, this section.
(2) Tune the receiver to the signal of the signal generator
according to the procedure in paragraph 2e, this section. An audible note of the
frequency of the signal generator modulation should be heard with low signal
generator output into the receiver.
(3) Repeat the procedures described in
paragraph 2i above for the 6.0 to 9.5 megacycle band, the 9.5 to 13.5 megacycle
band, and the 13.5 to 18 megacycle band.
3.
INSTALLATION.
a. ANTENNA.--Satisfactory operation of the receiver can
be obtained with practically any type of mast, fixed, or trailing wire antenna.
The receiver is to be used with the particular type of liaison receiving antenna
already installed in the aircraft.
b. MOUNTING FT-154-(*).--Permanently
attach the mounting to the rigid members of the aircraft so that the following
conditions will be met:
(1) The receiver when installed on the mounting will
be as near as possible to the antenna leadin insulator.
(2) There will be
sufficient clearance on all sides of the receiver to allow free action of the
shock absorbers. (See figures 8-15 and 8--16.)
c. MOUNTING FT-167.--The use
of Mounting FT-167 is optional. If the mounting is furnished and it is desired
to use it in the installation, attach the mounting as shown in figure
8--15.
d. RADIO RECEIVER BC-348-(*) AND BC-224-(*).--Install the radio
receiver as follows:
(1) Place the receiver on the mounting with the studs on
the bottom of the cabinet entering the slots of the mounting.
(2) See that
the receiver is well down on the mounting and that all four studs are securely
seated.
(3) Push the cabinet towards the rear of the mounting. Be sure that
the connector fits securely into the plug. Be sure the plug has not been damaged
due to misalignment.
(4) Secure the receiver in place with the snapslides on
the lower front corners of the cabinet.
(5) Safety wire the holes of the
snapslides assemblies. Do not twist the wires too tightly.
* Any issue
letter is applicable.
4. ELECTRICAL
CONNECTIONS.
(See figure 8-1.)
Cut a length of power cable to
reach from Connector SO-104 on the rear of the receiver to the source of power
and make connections as follows:
a. Connect one end of the cable to Plug
PL-QI03 (if a right-angle connection is desired) or to Plug PL-P 103 (if a
straight connection is desired) as shown in figure 8-1.
b. Connect the other
end of the power cable to s 28-volt source of power for Radio Receiver
BC-348-(*) and to a 14-volt source of power for Radio Receiver BC-224-
(*).
c. Connect the ground binding post "G" by a short, direct,
low-resistance lead to some grounded metal portion of the aircraft and solder at
that point if practicable. Allow enough slack to prevent vibration being
transmitted to the receiver.
d. Connect a solid copper wire from the antenna
lead-in insulator to the antenna binding post "A" on the receiver. Allow enough
slack to prevent vibration being transmitted to the receiver.
e. Make sure
the engine ignition system, generator, and other possible causes of disturbance
are properly shielded and that bonding of metal parts is accomplished.
5. ADJUSTMENTS.
After the receiver has been bench
tested and installed as directed in paragraphs 2 and 3, this section, no further
adjustments are necessary before the equipment is put into operation.
6. AFTER-INSTALLATION TESTS.
Make an
after-installation check of the radio receiver as follows:
a. Plug headset
HS-33 into one of the jacks marked "TEL."
b. Place the "AVC-OFF MVC" switch
in the "MVC" position. The dynamotor should start.
c. Allow 30 seconds for
the tubes to warm up.
d. Set the "INCREASE VOL" knob until a slight
background noise is heard.
e. Set the "BAND SWITCH" to the frequency band in
which test signals are available.
f. Adjust the "TUNING" control to the
desired frequency.

Note
Tune in the signal with the "AVC-OFF-MVC" control in
the "MVC" position and with the "INCREASE VOL" control advanced only far enough
to give the desired signal strength. With the "AVC-OFF-MVC" control set at "MVC"
and with the "INCREASE VOL" control in the maximum position strong carrier waves
will block the receiver and intelligible signals cannot be
received.
g. Place the "AVC-OFF-MVC' switch in the "AVC" ,
position. The desired signal should still be heard.
h. Place the "BEAT FREQ"
control in the zero beat position (arrow on knob pointing up),
i. Place the
"CW. OSC." switch in the "ON" position. An audible beatnote should be heard
which varies in pitch when the beat frequency adjustment is changed.
j. Place
the "CRYSTAL" switch in the "IN" position. Noise should be greatly reduced.

Note
The signal can be tuned out by a much smaller
movement of the "TUNING" knob with the "CRYSTAL" switch in the "IN" position
than with the switch in the "OUT" position.
k. Turn the "DIAL
LIGHTS" control to see that both dial lights are functioning.
l. Turn on the
engine of the aircraft. An increase in background noise when the engine starts
indicates imperfect shielding, imperfect bonding, faulty generator regulator,
faulty generator open-filter capacitors, or combination of these faults.
m.
Turn off the engine of the aircraft and place the "AVC-OFF-MVC" switch on the
receiver in the "OFF" position.
SECTION III
OPERATION
1. STARTING AND STOPPING THE
EQUIPMENT.
a. To start the equipment place the "AVC-OFF-MVC" switch
in the "MVC" position.
b. To stop the equipment place the "AVC-OFF-MVC"
switch in the "OFF" position.
2.
OPERATION.
a. MODULATED SIGNAL RECEPTION.(1) Place the "AVC-OFF-AVC"
switch in the 'MVC" position.(2) Place the "C.W. OSC." Switch in the "OFF"
position.
(3) Place the "CRYSTAL" switch in the "OUT" position.
(4) Set
the "BAND SWITCH" to the desired frequency band.
(5) Adjust the "TUNING"
control to the desired frequency.

Note
Always tune in a signal with the "AVC-OFF-MVC"
switch in the "MVC" position and with the "INCREASE VOL" control advanced only
far enough to give the desired signal strength. With the "AVC-OFF-MVC" control
set at "MVC" and with the "INCREASE VOL" control set at maximum. Strong carrier
waves will block the receiver and intelligible signals cannot be
received.
(6) Adjust the "INCREASE VOL." control until the
desired signal is heard or the background noise becomes audible.
(7) Adjust
the "TUNING" control on the receiver until a maximum output for the desired
signal is obtained. The receiver is now properly tuned.
(8) Adjust the
"ANTENNA ALIGN" dial until the signal is loudest.
(9) If automatic volume
control is desired, place the "AVC-OFF-MVC" switch in the "AVC" position and
readjust the "INCREASE VOL." control for the desired output level
b. C-W
RECEPTION.
(1) Place the "AVC-OFF-MVC" switch in the "MVC" position.
(2)
Place the "C.W. OSC." switch in the "ON" position.
(3) Place the "CRYSTAL"
switch in the "OUT" position.
(4) Set the "BEAT FREQ" control near the zero
beat position (arrow on knob pointing up).
(5) Set the "BAND SWITCH" to the
desired frequency band.
(6) Adjust the "TUNING" control to the desired
frequency.
(7) Adjust the "INCREASE VOL." control until the desired signal is
heard or until the background noise becomes audible.
(8) Adjust the "TUNING"
control on the receiver for a maximum output of the desired signal.
(9)
Adjust "ANTENNA ALIGN" dial until the signal is loudest.
(10) Vary the "BEAT
FREQ." control to adjust the frequency of the beat note as desired.
(11) If
it is desired to use automatic volume control, place the "AVC-OFF-MVC" control
in the "AVC" position and readjust the "INCREASE VOL." control.
(12) If
extreme selectivity is desired, place the "CRYSTAL" switch in the "IN" position.
Readjust the "TUNING," the "BEAT FREQ.," and the "INCREASE VOL." controls, if
necessary, to secure the desired beat note frequency and volume level.

Note
The crystal bandpass filter is intended primarily
for use in C-W reception. However, the added selectivity may at times prove
helpful in receiving modulated signals through heavy
interference.
SECTION IV THEORY OF
OPERATION
1. GENERAL.
(See figure
4-1.)Radio Receiver BC-348-(*) comprises two stages of tuned radio frequency
amplification followed by a first detector, a temperature compensated heterodyne
oscillator, three intermediate frequency amplifier stages, a second detector and
one stage of audio frequency amplification with a transformer output circuit. A
crystal band-pass filter is included in order to increase the selectivity. The
receiver also includes a beat-frequency oscillator in order to make c-w signals
audible.

Figure 4-1 Radio Receiver BC-348-(*) or BC-224-(*) -- Block
Diagram
2. FREQUENCY RANGE AND FREQUENCY
BANDS.
The frequency ranges of 200 to 500 kilocycles and 1.5 to 18.0
megacycles are covered in six bands which are finder the control of a band
change switch. The frequency range for each of the six bands is given in the
following table:

Band

Frequency Range

1

200-500 KC

2

1.5-3.5 MC

3

3.5-6.0 MC

4

6.0-9.5 MC

5

9.5-3.5 MC

6

13.5-8.0 MC

3. DETAILED
FUNCTIONING.
a. INPUT COUPLING.--The antenna input circuit is
capacitively coupled to the first tuned grid circuit by means of the antenna
alignment capacitor 2. Sufficient range is available in this capacitor to permit
alignment for antenna capacities within the limits of 50 to 200
micromicrofarads. The antenna input circuit is designed for antennas whose
resistances are between 1 and 5 ohms.
b. INPUT PROTECTION.--The resistor 65-1
provides a leakage path for static charges which may collect on the antenna. The
input circuit will withstand the application of 250 volts d.c. without damage.
For protection against the application of radio frequency voltages up to 30
volts rms, the input circuit provides for the over shooting of the grid of the
first r-f tube and the building up of a protective negative grid bias across the
grid filter resistor.
c. RADIO FREQUENCY AMPLIFIER.--The radio frequency
preselector comprises three tuned circuits coupled by two super control pentode
amplifier tubes, tube JAN-6K7. Separate inductances are employed for each
frequency band. The r-f gain of each of the six bands is kept uniform by
selection of the turn ratio between the grid and plate circuit for each of the
respective bands. A relatively low signal level is maintained at the grid of the
first detector tube, thus insuring freedom from cross modulation
interference.
d. FIRST DETECTOR.--The first detector employs tube JAN-6J7
which has a sharp cutoff characteristic. The low signal level at the grid of the
first detector, together with the r-f preselection, insures a minimum of
undesired responses. The oscillator output is coupled into the cathode circuit
of this tube, and separate cathode coupling coils provide optimum oscillator
output for each frequency band.
e. HETERODYNE OSCILLATOR.--The heterodyne
oscillator employs a tuned grid, plate feedback circuit, utilizing a triode tube
JAN-6C5. Oscillator frequency stability over wide variations in ambient
temperature under service conditions has been obtained by the use of temperature
compensated ceramic fixed, capacitors (40, 41, 42, 43, 44, 45 and 46.)
Individual inductances and trimmers are employed for each frequency band. The
low impedance coupling to the cathode of the first detector insures frequency
stability with load variations or detector circuit tuning. On the four lower
frequency tuning bands the oscillator frequency is higher than the desired
signal by the intermediate frequency. On the two higher frequency ranges, bands
5 and 6, the oscillator is on the low frequency side of the desired signal. The
latter results in a more uniform tuning ratio over these bands and increases the
image rejection ratio.
f. INTERMEDIATE FREQUENCY AMPLIFIER.--The intermediate
frequency amplifier comprises three low gain amplifying stages coupled by four
high selective, double-tuned circuit transformers. The intermediate frequency
employed is 915 kilocycles. The i-f transformers are tuned by means of
adjustable iron cores and fixed capacitors. The increased permeability resulting
from the use of the iron cores contributes largely to the highly selective
transformer characteristics. The lowered tuned circuit impedance, secured by the
relatively large fixed tuning capacitors, provides an inherently stable
amplifier. Tube JAN-6K7 functions as the first i-f amplifier while the pentode
section of Tube JAN-6F7 is employed as the second i-f amplifier. Tube JAN-6B8 as
the third i-f amplifier, supplies a relatively high level signal to the diodes
of this same tube.
g. C-W OSCILLATOR
(1) The c-w oscillator employs the
triode section of tube JAN-6F7 (second i-f amplifier tube) in a tuned grid plate
feedback circuit. The adjustable iron core in the grid inductance 121 is
employed for rough frequency alignment. A small panel-operated beat frequency
control permits fine adjustment of the beat frequency within a range of
approximately 4,000 cycles each side of zero. The effects of ambient temperature
variations are minimized by the use of a temperature compensated tuned circuit.
The c-w oscillator operates at an extremely Iow level, minimizing harmonics and
stray oscillator pickup. The output is capacitively coupled to the plated
circuit of the second amplifier tube by the coupling lead connected to the
oscillator grid. Amplification by the third i-f amplifier stage, whose gain is
not controlled either by manual or a-v-c, provides sufficient output from the
c-w oscillator to the diode detector. This value of oscillator output is
somewhat below the level at which the a-v-c operates, thus permitting the use of
automatic volume control even for c-w reception.
(2) The C.W. OSC. switch 128
in the ON position supplies the oscillator plate voltage and increases the a-v-c
time constant by connecting the additional capacitor 123-C. Switch 128 supplies
the oscillator plate voltage by connection to the screen grids of the first and
second i-f and first r-f rubes. The same switching connects the loading resistor
58-4. This drops the screen voltage to the first and second i-f and first r-f
tubes to a value that reduces the sensitivity sufficiently to keep the overall
set noise essentially constant. This arrangement for supplying the c-w
oscillator has added advantages which are not obvious. For sufficient oscillator
excitation to handle high detector levels, encountered with a-v-c delay
operation and strong signal inputs (while still keeping the no-signal c-w
excitation below the a-v-c level,) the c-w oscillator Output should increase as
a strong signal input raises the a-v-c bias. This circuit arrangement, Figure
4-2, accomplishes this result, since with switch 129 in the a-v-c position,
resistors 57-6, 74, 70, and 79-A form a fixed bleeder supplying the screen grid
voltage to r-f, first detector and i-f tubes. A strong input signal building up
the a-v-c bias causes a considerable decrease in screen current and hence an
increase in the screen supply voltage. This increases the voltage supplied to
the c-w oscillator, and hence the excitation increases in proportion to signal
level at the detector.

Figure 4-2 C-W Oscillator Switching, Simplified
Diagram
h. CRYSTAL BAND-PASS FILTER. (See fig. 4--3)--Additional
selectivity is available by the use of the i-f crystal filter preceding the
first i-f amplifier tube. This crystal filter comprises a balanced capacity
bridge circuit which may be adjusted internally to provide a band width of 800
to 3,000 cycles at 10X down from resonance. The tapped tuned circuit (118 and
34-1) matches the impedance of the crystal bridge to the first i-f grid. The
crystal filter may be switched in or out of the circuit by the CRYSTAL OUT-IN
switch 127, actuated from the front panel. The symmetry of the resonance curve
is adjustable by the balancing capacitor 8. The filter band width is adjustable
by the secondary core of the first i-f transformer 117. As delivered by the
manufacturer, the band width is set at approximately 2,000 cycles.

Figure 4-3 Details of Crystal Filter Circuit
i. SECOND
DETECTOR.--Tube JAN-6B8 also functions as the second detector. A relatively high
level signal is supplied by the third i-f amplifier to the diodes of this tube.
One diode functions as the signal linear detector, while the other diode is
capacity coupled and provides high level, delayed a-v-c control bias.
j.
OUTPUT CIRCUIT.
(1) The high level signal diode supplies audio output for
driving the output tube, tube JAN-6K6, without additional audio amplifier stages
and high level detection results in a number of operating advantages. The high
level detection is relatively free from distortion, due to avoiding the
characteristic curvature at the lower end of the diode curve. The direct drive
of the output tube from the diode derector simplifies the dynamotor ripple
filtering and eliminates possible microphonics resulting from high audio
amplification. The high diode level further provides relatively high bias
voltage, insuring an unusually flat automatic volume control characteristic with
the desired time delay. The dual volume control comprises potentiometers 79-A
and 79-B. The latter, 79-B, functions only with the switch 129 in the AVC
position, permitting the desired adjustment of the audio level to the output
tube and load. For manual volume control with switch 129 in the MVC position,
the potentiometer 79-A becomes the active control operating on the cathode bias
of the r-f and the first and second i-f amplifier tubes. These potentiometers
have two linear resistance tapers providing a smooth variation of sensitivity.
Automatic load compensation is obtained by the method of biasing the output
tube. Here, the grid bias is obtained from the resistance drop across the
dynamotor filter reactor; thus, any tendency towards a decreasing load on the
dynamotor results in a slight decrease in the bias of the output tube with a
compensating increase in the load current. (See Figure 4-4.) Tube JAN-6K6
provides more power than necessary to operate a number of headsets in
parallel.

Figure 4-4 Load Compensator and Bleeder Circuit, Simplified
Diagram
(2) The characteristic increase of internal receiver noise, when
tuning from the Iow to the high frequency end of a band, has been corrected by
means of the variable resistor 78. The function of this potentiometer can be
more clearly understood by reference to Figure 4-4. Here the noise compensator
resistor 78 is mechanically connected to the shaft of the ganged tuning
capacitor with an electrical c0nnectiofi to give minimum resistance at the low
frequency end of the band. The cathode return lead of the second r-f amplifier
tube connects to the noise compensator resistor 78. The gain of this stage is
thereby decreased proportionally as the r-f tuned circuit impedance increases
(when tuning toward the higher frequency end of the frequency band). This
arrangement tends to keep the noise level and receiver sensitivity essentially
constant over the tuning ranges.
k. DYNAMOTOR DM-28-( * ) OR DM-24-( * ).The
dynamotor and associated r-f filter circuits are assembled in one unit. The r-f
filters are of the unbalanced type for use with a primary supply in which the
negative side is grounded. This dynamotor supplies all of the high voltage
direct current required for the operation of the receiver and, in addition, a
maximum of 20 milliamperes for use in operating accessory equipment.
SECTION V MAINTENANCE
1.
INSPECTION.
DELETED
2. TROUBLE LOCATION
AND REMEDY.
a. GENERAL.
(1) The normal sensitivity (number of
microvolts input to produce 10 milliwatts output in a 4,000 ohm resistance load)
of the receiver is 9 microvolts or less when measured under the following
conditions:
(2) AVC-OFF-MVC switch at MVC; 28-volts input for Radio Receiver
BC-348-(*) or 14 volts input for Radio Receiver BC-224-(*); c-w oscillator ON;
crystal filter OUT; output load 4,000 ohms non-inductive resistance if the
output transformer is connected for HI impedance, or 300 ohms non-inductive
resistance if the transformer is connected for LO output impedance; pure c-w
input from signal generator applied between antenna-ground terminals through a
100 mmf dummy antenna; volume control set to produce 0.3 milliwatt noise
output.
(3) This sensitivity will, of course, be subject to variation with
time, due to tube aging, etc. Therefore, it is recommended that no attempt be
made to retrain or realign the equipment unless the sensitivity is found to be
worse than 17 microvolts with new ú average tubes. The receiving equipment has
been carefully adjusted and aligned by the manufacturer.: before shipment and
should maintain these adjustments over reasonably long periods of time. Major
adjustments and repairs should be made only 'in an authorized repair shop
equipped with the necessary servicing tools and equipment.
(4) Any changes of
the adjustments of the radio frequency circuits should be done by trained
personnel only. The difficulties usually experienced are the result of external
deteriorating influences, such as worn out vacuum tubes, improper operating
voltage, blown fuse, external noises, etc.
Figure 5-1 graphically outlines
the procedure for trouble location, the numbers in each block referring to the
paragraph in this section.

Figure 5-1 Trouble Location and Correction Diagram
b.
WEAK OR NO SIGNALS ON ALL BANDS, MODULATED RECEPTION.
(1) CHECK OF DYNAMOTOR
VOLTAGES. --When all signals on all bands are weak or no signals are heard even
when known to be present, the procedure follows that shown in Figure 5-1. The
voltages checked at the dynamotor terminal board should closely approximate the
values shown in Figure 5-2. Conditions for measurement are as follows: Input
28-volts for Radio Receiver BC-348-(*) or 14volts for Radio Receiver BC-224-(*);
"CRYSTAL" Switch Set to "OUT"; 'AVC-OFF-MVC' Switch Set to "MVC"; "INCREASE
VOL." control set to maximum; "C.W. OSC." Switch Set to "OFF." Load 4,000 ohms
resistance (output transformer connected for HI output impedance). If these
readings do not approximate the values shown, check the fuse as well as the
dynamotor and filter circuits, wiring and components.
(2) TUBE CHECK.--If the
voltages at the dynamotor terminal board approximate the values given, check all
tubes for emission and characteristics or replace all tubes with those of known
average characteristics.

Note
All tubes of a given type supplied as spares must be
consumed prior to employment of tubes from general stock.
(3)
CHECK OF SOCKET VOLTAGES.--If tubes check satisfactorily, or if, after replacing
with tubes known to be good, the sensitivity is still low, check all tube socket
voltages. The average socket voltages are given in Figure 5-2.

Figure 5-2 Radio Receiver BC-348-(*) or BC-224-(*) -- Dynamotor
and Tube

Socket Voltages
(4) CHECK CIRCUIT WIRING AND
COMPONENTS.--If the tube socket voltages do not approximate the values shown in
figure 5-2, check the associated circuits and components for grounds, shorts,
and similar defects.
(5) TEST OF AUDIO-FREQUENCY AMPLIFIER.--Having checked
all socket voltages and found the values to be correct, test the audio frequency
amplifier. This can be checked by capacitively-coupling a 400-cycle voltage of
approximately 2 volts rms from the detector diode socket prong to ground using a
capacitor of 500,000 micromicrofarads. As an alternative, a modulated 91.5 kc
signal of 2 volts may be coupled to this point and ground. Proper functioning of
the audio amplifier will be indicated by an output well over 1.0 milliwatts for
the 2 volt audio input or approximately 1 milliwatt output for 915 kc input.
Check circuits, wiring and components if this order of response is not
obtained.
(6) TEST OF INTERMEDIATE-FREQUENCY AMPLIFIER.--FolIowing a
satisfactory test of the audio amplifier, check the intermediate frequency
amplifier by capacitively coupling the modulated test oscillator to the grid cap
of the first detector tube through a 100,000-micromicrofarad capacitor, with the
oscillator frequency being adjusted to 915 kc. A rough check of the proper
functioning of the i-f amplifier is indicated by a comfortable headphone output
level with low input from the test oscillator. (Approximately 30 microvolts
input for 10 milliwatts output.)
(7) I-F AMPLIFIER CIRCUIT CHECK.--If the i-f
amplifier does not respond as above or lacks sensitivity, a progressive check,
stage by stage, should be made. The test oscillator, 915 kc (modulated 30% with
400 cycles), is connected through a 100,000 micromicrofarad capacitor to the
second detector diode socket prong. A signal response indicates proper
functioning. Coupling the test oscillator to the grid of the third i-f should
indicate a decided gain in sensitivity. Proceeding similarly towards the first
detector, each stage should show a decided gain. Circuit check a faulty stage
for grounds, shorts, or defective components.
(8) CHECK AND ADJUSTMENT OF C-W
OSCILLATOR.---Check and adjust the c-w oscillator by coupling the 915 kc input
(modulation off) to the grid of the first detector tube and switch the c-w
oscillator ON. With the beat frequency control set at mid-position, the
oscillator inductance tuning core 121 is adjusted for zero beat. If no c-w beat
can be heard, check the c-w oscillator circuit for grounds, shorts, or defective
components. With the c-w oscillator ON the screen voltage at the first and
second i-f sockets drops to approximately 45 volts (measured to ground).
(9)
TEST AND ADJUSTMENT OF CRYSTAL BAND-PASS FILTER.--NormalIy the crystal bandpass
filter is adjusted at the factory for a band width of 1,500 to 2,000 cycles at
10X down from resonance. Test this filter by applying an unmodulated signal of
approximately 915 kc connected through a 0.1 mfd capacitor to the grid of the
first detector tube. The c-w oscillator should be off during this test. To
adjust the band width of the crystal filter, a signal generator or microvolter
having an expanded tuning scale in the vicinity of 915 kc and having also an
attenuator with a multiplier of 10 times (20db) is required. The following
procedure is recommended: Connect a microammeter with a range of approximately
200 microamperes in series with the cathode return of the AVC volume control
(79-B white lead). Throw the crystal switch to the IN position. With the
unmodulated 915 kc input from the test oscillator, find the resonance peak of
the crystal by slightly retuning the test oscillator until maximum deflection is
indicated on the microammeter. Adjust the phasing control 8 until the resonance
curve as indicated on the microammeter is symmetrical and free from dips or
peaks except for the main resonance peak of the crystal. Adjust the band Width
with an input voltage ratio of 10X to approximately 2 kc by slight realignment
of the secondary (top) tuning core of the first i-f transformer 251. After
concluding the above described tests, remove the microammeter and restore the
circuit to normal.
(10) CHECK OF HETERODYNE OSCILLATOR.--After checking the
functioning of the i-f and audio amplifiers, if signals are still not heard on
any band, check the heterodyne oscillator. This can be done by observing the
cathode voltages at the socket of the first detector tube, Tube JAN 6J7, when
grounding the stator of the oscillator section (l-D) of the tuning capacitor. If
no change in voltage is noted with this test, check the oscillator circuit for
grounds, shorts or defective components.
(11) TEST OF THE R-F
AMPLIFIER.--Having completed the test and alignment of the audio amplifier, i-f
amplifier and heterodyne oscillator, test the ú r-f amplifier as
follows:
With the band switch set on the band lacking sensitivity
capacitively couple a modulated signal from the test oscillator through a 100
micromicrofarads dummy antenna to the antenna post. Set this input signal
frequency accurately to the alignment frequency shown in table 5-5 for the band
under test. With the tuning control set for the approximate alignment frequency,
tune slowly around this point until the maximum response with the least signal
input is obtained. Capacitively coupling the test oscillator to the grid of the
first r-f and then to the second r-f tubes should show a progressive decrease in
output. This indicates the proper functioning of the preceding r-f stage or
circuits. If a decrease in response is noted when the signal input is
capacitively coupled progressively from the grid of the first detector to the
second and first r-f grid caps and finally to the antenna post (with the 100
micromicrofarad dummy antenna capacitor), check the stage which indicates a
decrease in response for circuit, ground, shorts, or defective components.
c.
WEAK OR NO SIGNALS ON ANY ONE BAND, MODULATED RECEPTION.--The condition of
satisfactory reception on several bands and weak or no signals on one or more
bands, indicates correct functioning of the i-f and a-f amplifiers and requires
checking only the r-f amplifier and heterodyne oscillator for the defective band
or bands. The procedure outlined in paragraphs 2b(10) and (11) should be
followed for the defective band or bands.
d. WEAK OR NO SIGNALS ON ALL BANDS,
C-W RECEPTION (MODULATED RECEPTION NORMAL) --Weak or no signals on all bands for
c-w reception with satisfactory modulated signal reception requires testing and
alignment of the c-w oscillator. Proceed as outlined in paragraphs 2b(7) and
(8).
e. NOISY OPERATION.--If during tuning, the receiver appears to be
erratic or noisy, clean the rotor plates of the gang tuning capacitor with pipe
cleaners or compressed air.

CAUTION
Do not apply excessive air pressure when using
air to clean the tuning capacitors. A strong blast of air will change the
tracking of the capacitor.
3. VOLTAGE
AND RESISTANCE MEASUREMENTS.
a. Voltage measurements to ground are
given in the following table. Make the following settings and adjustments before
measuring voltages:
(1) Set "AVC-OFF-MVC" switch to "MVC."
(2) Set the
"INCREASE VOLUME" control to the maximum position (fully clockwise).
(3) Set
tuning control to 200 kc.

TABLE 5-1. VOLTAGE TO GROUND, C-W OSCILLATOR OFF

Stage

Tube

Plate Volts

Screen Volts

Cathode Volts

Heater Volts

Plate Current (ma.)

Screen Current (ma.)

1 R-F

2 R-F

1 Det.

Osc.

1 I-F

2 I-F

3 I-F

Output

2 Det.

JAN 6K7

JAN 6K7

JAN 6J7

JAN 6C5

JAN 6K7

JAN 6F7

JAN 6B8

JAN 6K6

JAN 6B8 (Diode)

184

177

202

58

182

207

207

197

8.0

70

86

96

……

82

82

72

207

……

2.6

3.2

4.2

0.0

3.1

3.1

21.0

0.0

……

6.3

6.3

6.3

6.3

6.5

6.5

6.5

6.5

……

4.1

4.8

0.23

1.6

4.7

4.5

2.5

18.0

……

1.0

1.3

0.08

……

1.2

1.4

0.6

3.2

……

TABLE 5-2. VOLTAGE TO GROUND, C-W OSCILLATOR ON

Stage

Tube

Plate Volts

Screen Volts

Cathode Volts

Heater Volts

Plate Current (ma.)

Screen Current (ma.)

1 R-F

2 R-F

1 Det.

Osc.

1 I-F

2 I-F

3 I-F

Output

C-W Osc.

JAN 6K7

JAN 6K7

JAN 6J7

JAN 6C5

JAN 6K7

JAN 6F7

JAN 6B8

JAN 6K6

JAN 6F7 (Triode)

197

188

204

58

195

210

210

198

18.0

37

65

72

……

44

44

72

210

……

1.3

2.3

3.4

0.0

1.6

1.6

21.0

0.0

……

6.3

6.3

6.3

6.3

6.5

6.5

6.5

6.5

……

2.0

3.7

0.17

1.6

2.3

2.2

2.5

23.5

……

0.55

1.0

0.06

……

0.5

0.5

0.6

3.6

……

Note
The readings given above are average values taken on
receivers of this type using a 14-volt power supply or 28-volt power supply,
depending on receiver use. Meter indications within +/-10% of these values will
in most cases indicate correct operations. The readings are taken with the
tuning control set to the I-F end of the dial.
b. Resistance
measurements to ground are given in the following table. Make the following
settings and adjustments before measuring resistance:
(1) Disconnect the
power plug at the rear of the receiver cabinet.
(2) Set "AVC-OFF-MVC" Switch
to "MVC" unless otherwise indicated in the tables below.

TABLE 5-3. RESISTANCE TO GROUND (OHMS), C-W OSCILLATOR
OFF

Stage

Tube

Cathode

Plate

Screen

"AVC-OFF-MVC" SWITCH SETTING

"MVC"

"AVC"

Grid

Grid

1 R-F

2 R-F

1 Det.

Osc.

1 I-F

2 I-F

3 I-F

Output

JAN 6K7

JAN 6K7

JAN 6J7

JAN 6C5

JAN 6K7

JAN 6F7

JAN 6B8

JAN 6K6

490

480

15,000

0

520

470

6,200

0

5,200

5,200

5,600

41,000

5,600

500

500

1,080

80,000

75,000

75,000

………

70,000

70,000

180,000

480

100,000

100,000

0

100,000

500,000

500,000

5,000

700,000

1.8 meg.

1.8 meg.

0

100,000

1.8 meg.

2.25 meg.

5,000

………

TABLE 5-4. RESISTANCE TO GROUND (OHMS), C-W OSCILLATOR
ON

Stage

Tube

Cathode

Plate

Screen

"AVC-OFF-MVC" SWITCH SETTING

"MVC"

"AVC"

Grid

Grid

1 R-F

2 R-F

1 Det.

Osc.

1 I-F

2 I-F

3 I-F

Output

C-W Osc.

Det. Diode

AVC Diode

JAN 6K7

JAN 6K7

JAN 6J7

JAN 6C5

JAN 6K7

JAN 6F7

JAN 6B8

JAN 6K6

JAN 6F7

JAN 6B8

JAN 6B8

490

480

15,000

0

520

470

6,200

0

………

………

………

5,200

5,200

5,600

41,000

5,600

500

500

1,080

82,000

180,000

380,000

80,000

75,000

75,000

………

70,000

70,000

180,000

480

………

………

………

100,000

100,000

0

100,000

500,000

500,000

5,000

700,000

500,000

………

………

1.8 meg.

1.8 meg.

0

100,000

1.8 meg.

2.25 meg.

5,000

………

………

………

………

Note
The readings tabulated above are average values
taken on receivers of this type with the storage battery disconnected. Meter
indications within plus or minus 10% of the values will in most cases indicate
correct operation.
4.
ALIGNMENT
a. ALIGNMENT OF I-F AMPLIFIER.--When all stages have been
tested, the i-f amplifier alignment is checked by capacitively coupling a low
level input signal of 915 kc to the first detector grid and adjusting the i-f
tuning cores of both primary and secondary windings of the first, second and
third i-f transformers and the tuned circuit of the crystal filter assembly for
maximum output. The fourth i-f transformer is slightly over-coupled with the
resultant double response peaks symmetrically located approximately 5 kilocycles
each side of the 915 kc i-f alignment frequency. In general, it will not be
necessary to realign this transformer because of its broad characteristic.
However, if realignment becomes necessary, the following procedure should be
followed.
Connect the modulated test oscillator through a 100,000
micromicrofarads capacitor to the grid of tube JAN 6B8 (third i-f amplifier
tube) and to ground. Set the input frequency at 910 kc and adjust alternately
first the primary tuning core, then the secondary tuning core of the fourth i-f
transformer until maximum output is obtained when both cores are turned in a
right-hand screw direction. The modulated test oscillator should then be turned
slowly through the 915 kc setting and to approximately 920 kc where a second
response peak of approximately equal amplitude as that at 910 kc should be
obtained. This indicates correct alignment. Slight supplementary adjustments of
the primary and secondary tuning cores may be necessary to secure a symmetrical
double peak response with the slight hollow between response peaks located at
the 915 kc alignment frequency. If a sweep frequency-modulated test oscillator
is available, this fourth i-f transformer can be conveniently aligned by visual
observation on a cathode-ray oscilloscope. In this case, the tuning core
adjustments are made to obtain a double-peak response pattern centered at 915
kc.
b. ALIGNMENT OF R-F AMPLIFIER.

Note

For a general alignment start with the 200-500 kc band No.
1.

Band

No.

Freq. Range

Alignment

Frequency

Trimmers*

Osc.

Det.

R-F

Ant.

1

1

2

3

4

5

6

200-500 kc

200-500 kc

1.5-3.5 mc

3.5-6.0 mc

6.0-9.5 mc

9.5-13.5 mc

13.5-18.0 mc

500 kc

200 kc

3.5 mc

6.0 mc

9.5 mc

13.5 mc

18.0 mc

6-1

10

6-2

6-3

6-4

6-5

6-6

3-5

…

5-5

3-6

5-6

7-2

4-3

3-3

…

5-3

3-4

5-4

7-1

4-2

2**

…

5-1

3-1

5-2

3-2

4-1

*Refer to figures 6-1, 6-3, 6-4, 6-6, 6-9, and
6-10 for location. The alignment controls for the various bands are numbered on
the chassis adjacent to the control. Controls for band 1 are marked 1, those for
marked 2, etc.
** Antenna alignment control. In the r-f alignment for any
particular band adjust the tuning control for the alignment frequency (see Table
5-5) and couple the modulated test signal at this alignment frequency from the
test oscillator to antenna post through the 100 mmf dummy antenna. Adjust the
three trimmers (Ant.; R-F; and Det.) for this band, for maximum output. A
similar procedure is followed in the alignment of each band.
c. ALIGNMENT OF
HETERODYNE OSCILLATOR.

Note

For a general alignment start with the 200-500 kc band No.
1.

The alignment of the heterodyne oscillator is
necessary only when the tuning dial frequency calibration is in error by more
than 0.5 per cent. To align the oscillator follow the same general procedure
described for the alignment of the r-f amplifier. With the BAND SWITCH on band
No. 1 (200-500 kc) and the TUNING control set to the alignment frequency, couple
the output of the modulated test oscillator (set at the alignment frequency) to
the antenna post through the 100 micromicrofarad dummy, antenna. Adjust the
oscillator trimmer for this band for maximum audio output. Note that in band No.
I there are two such adjustments, one at the i-f and the Other at the h-f end of
the dial. A similar procedure is followed in the alignment of each
band.
5. SPECIAL MAINTENANCE
PROCEDURES.
a. CARE AND SERVICING OF DYNAMOTOR DM-28- (*) or DM-24-
(*).(1) The dynamotor and filter assembly is removable from the receiver chassis
without disturbing other parts, provided the procedure outlined below is
followed.
( 2 ) Loosen the two thumbscrews on the receiver panel, and draw
the chassis from the cabinet. Lay the chassis with top upwards on a smooth, fiat
surface, with the rear toward the operator. Loosen the five connector screws on
the dynamotor terminal strip and withdraw the spade terminals beneath
them.
(3) Loosen the four captive screws 211, Figure 5--3; which hold the
dynamotor unit to the chassis. Remove the dynamotor and filter assembly from the
receiver by grasping the dynamotor and lifting vertically.
(4) The filter
portion of the dynamotor unit is made accessible by the removal of the filter
unit cover located at the base of the dynamotor. (See fig. 5--3.)

Figure 5-3 Dynamotor DM-28-(*) or DM-24-(*) and View Showing
Filter Open
(5) Lubricate thc dynamotor at 1,000 hours or approximately
six months of ordinary service. For ordinary and LOW temperature conditions use
a mineral oil grease AN-G-15. For unusually high temperatures which are present
in tropical climates use AN-G-5 grease. The directions for lubrication are
stamped on the inside of the end-bell dust covers. Access to the bearings of the
dynamotor is obtained by removing the dust covers after first cutting the safety
wires and removing the retaining screws, then unscrewing the bearing end plates.
Do not PACK the lubricant in these bearings.
(6) When necessary to replace
the ball bearings or turn down the commutators, first remove the brushes from
their cartridges. (See fig. 5-4.) Remove the nuts from the tie rods which hold
the bearing end-bells, and pull the end-bells away from the field coil assembly.
The armature can now be taken out. Examine the brushes to see that they have
worn properly and are free from hard spots. Should such spots be apparent (they
generally cause grooves in the commutator surface), replace the brush and smooth
the commutator. The ball bearing retainers and the shaft are machined for very
snug fits, but a slight tapping will loosen them. To remove the bearing
retainers from the end-bells, use two small screwdrivers as wedges between the
outer ball race and the end-bell. If the grease slinger becomes bent during
removal, straighten it and replace it on the shaft before replacing the
bearing.

Figure 5-4 Dynamotor DM-28-(*) or DM-24-(*) --
Disassembled
(7) To smooth down the commutator, rotate it in a lathe
holding a fine grade of sandpaper (not coarser than size 00), lightly against
the commutator surface. Do not use emery cloth. Wipe away all residue of dust,
sand and dirt to leave a clean, smooth, polished commutator surface. Never sand
a commutator having a smooth or polished surface or turn it down simply because
it is discolored. If the commutator is turned down in a lathe, the mica segment
separators must be undercut.
(8) Re-assemble the dynamotor in the reverse
process of the disassembly procedure. The use of the screwdrivers as wedges is
not necessary. In replacing the brushes, check to see that the + and - markings
on the brushes correspond with those on the brush holder supports, and that the
marked side of the brush is towards the top of the dynamotor. The commutator
must be given a final inspection for free running, cleanliness and absence of
grease or oil. Wipe the end-bells clean and dry them before replacing on the
dynamotor.
(9) The nominal ratings of Dynamotors DM-24-(*) are: Input, 2.45
amperes at 13.8 volts; output, 70 milliamperes, at 220 volts; regulation 12 per
cent. The nominal ratings of Dynamotors DM-28-(*) are Input, 1.23 amperes at
27.9 volts; output, 70 milliamperes at 220 volts; regulation, 12 Per cent.
b.
REMOVAL OF FRONT PANEL.
(See figure 8-14.)
(1) For adjustment of dial or
mask, or for servicing certain parts, it may be necessary to remove the panel.
Take the chassis, with panel attached, completely out of the cabinet. Place it
with the panel facing upwards. Unsolder the lead to the antenna binding post and
the lead from the chassis to the dial lights. Remove the dial light housing
cover, the two thumbscrew rods, the handles, and all knobs and retaining nuts of
all controls except the DIAL LIGHTS control. Remove the retaining nuts of the
TEL jacks. The panel may be lifted off after the removal of the end plate
holding screws, the chassis holding screws, and the dlal casting holding
screws.
(2) In replacing the BEAT FREQ. control knob, turn the flexible shaft
until the set screw in the coupling at the internal end, points away from the
panel. Now mount the knob so that the arrow points vertically towards the top of
the receiver. This knob has two set screws. In replacing the other knobs on the
shafts, note that flats on the shafts provide for proper location. Tighten all
set screws securely, and give a second tightening to the set screw on the band
switch knob after the shaft has been rotated a few times.
c. DIAL AND MASK
ASSEMBLY.
(1) Attached to the front panel, and to the main frame or chassis
casting used as the frame for the switch drive shaft, dial mask and detent, for
the tuning dial, tuning shaft, reduction gears, and stop. All of these parts are
assembled, and can be moved as a unit.
(2) The switch drive shaft passes
through a hole in an adjustable plate, located in the dial lamp homing, and
through a clearance hole in the panel. The hole in the adjustable plate is
purposely given a larger clearance than bearing requirements would dictate. The
bearing is used to support the shaft against forces which might spring it or
damage the internal bearing. The shaft extends through a long bushing pressed in
the dial housing. At the inside end of the shaft the detent (star wheel) and the
driving portion of the coupling member are pinned in place by means of taper
pins. The dial assembly runs on the outside surface of the long bushing referred
to above. Just inside the panel, the hub of the mask is attached to the shaft by
a taper pin.
(3) A stop arm, on the back of the dial frame casting, engages a
pin in the detent and limits its rotation to 6 positions spaced 60 degrees
apart. Attached to the casting is the assembly, consisting of pivot pins, arms
with rollers, and spring which positions the detent. This assembly is locked
with two dowel pins after the correct location is made.
(4) On the front
(panel) end of the bushing (through which the switch shaft passes) there is a
narrow shoulder which supports the lower end of the dial index plate. This index
plate is attached to the casting at its upper end with means for removing any
slack and keeping it straight and taut. The inner end of the mask hub and the
outer end of the dial hub turn and are held against opposite sides of the index
plate.
(5) The dial is attached to a flanged hub which runs on the outside
surface of the bushing through which the switch drive shaft passes. This hub
also carries a large gear driven by a pinion combined with a split idler gear.
This split idler gear and pinion has adjustment in the clearance holes for the
mounting screws to enable the backlash to be reduced to the smallest practicable
amount between the pinion and the large dial gear.
(6) The tuning shaft has,
in addition to the pinion referred to above, a stop, and a worm which meshes
with a split worm gear on a cross shaft at the back of the frame. Both this
cross shaft and the tuning shaft run in bearings which are integral in the
casting. Both shafts have spring thrust washers to remove end play. The cross
shaft carries a pinion which is meshed with a split gear on the tuning capacitor
shaft. The degree of mesh of this pinion and gear is adjustable by moving the
tuning capacitor toward or away from the panel after it is placed in position
and before the holding screws are finally tightened.
(7) The overall gear
ratio between the tuning shaft and the capacitor shaft is 200 to 1. The design
of the tuning capacitor permits but a small amount of rotation at either end of
its travel beyond the 180 degrees required to give its complete range in
electrical capacity. However, means are provided to stop the tuning shaft at
either end after a total of approximately 100 revolutions. This is accomplished
by a cam on the outer edge of the tuning dial which operates an arm pivoted on a
pin on the frame casting. One end of this arm has a roller which runs on the
outer edge of the dial. The roller is held in contact with the dial by a spring.
The dial edge is cut away in such a manner that when the end of the tuning scale
is reached, the roller can move toward the center of the dial, being forced in
that direction by the spring. Thus the opposite end of the stop arm is moved so
that the hook at the end of the arm engages the rotating stop on the shaft, and
thereby prevents further rotation. When the direction of rotation of the tuning
shaft is reversed, the roller and arm are pushed outward against the spring by
the cam edge of the dial and the stop disengages.
(8) Since a definite
relation must be set and maintained between the dial position and the angular
position of the rotor of the tuning capacitor, adjustment is provided at the
pinion on the cross shaft at the rear of the frame. (This is the shaft which
also carries the worm gear.) This pinion is held in place by two set screws
which bear in a groove on the shaft. By loosening these two set screws, either
the dial or the tuning capacitor can be rotated while the other part remains
fixed. The correct relation between these is that the tuning capacitor rotor
plates are fully meshed with the stator plates (maximum capacity) when the dial
is set with the isolated index mark at the low frequency end of the 13.5 to 18.0
mc band.
d. REMOVAL AND REPLACEMENT OF DIAL MECHANISM.
(1) To remove the
dial mechanism from the chassis for servicing, first remove the front panel in
accordance with instructions given in paragraph 5b. The frame of the mechanism
is attached to the chassis by the bracket holding the fuse, and by two slotted
hexagon head screws through the flange on the under side of the chassis. One of
these screws is beneath the removable shield which covers the terminal of the
first i-f transformer. When these screws are removed, the complete unit can be
removed. The center disc of the flexible coupling is loose and will drop out of
engagement.
(2) To remove the index and dial, remove the taper pin holding
the mask hub to the switch drive shaft. Support the shaft when driving out this
pin, so that excess stress will not be placed on the center bushing. After the
removal of the mask and the index, the dial and its gear are free to slide off
the bushing. In replacing these parts, the thrust washer behind the dial hub
must be turned in the position to give maximum thrust, which is convex side
outward.
(3) If the dial is removed and replaced, it is possible that in
meshing the dial gear with the idler gear the stop relationship may not be
correct. In this case it may be necessary to change a tooth at a time to correct
the relationship. The stop relationship must be such that the roller arm hook
and the tuning shaft stop arm engage fully at the end of the last revolution.
However, on the previous revolution the arm must not start to move until the
rotating arm has passed under the roller arm hook. This adjustment can only be
made by trial and inspection, but can be secured in one or two trials.
Substitution of a different stop arm may also require readjustment. In this case
the adjustment should be made by lengthening or shortening the roller end of the
arm. This is done by loosening the two nuts on the arm, and adjusting the arm by
means of the slotted holes provided.
(4) When the dial mechanism is replaced,
the flexible coupling must be properly positioned so that the position of the
mask corresponds to the switch position, since it is possible otherwise to get
the band switches to an operative position. The correct relative positions are
obtained when the mask is set to band 1 and the set screw, locking the flat
switch to the large level gear hub, is vertical.
(5) If any of the gear
trains including split gears have been unmeshed in disassembly, the split gears
must be reset to put tension on the loose section when they are again meshed.
Normally, a displacement of one tooth between the two sections is sufficient.
Trial will show whether this will remove the backlash.
(6) When the dial
mechanism is reassembled to the chassis, carefully adjust the relation between
the dial and the tuning capacitor in order to maintain the calibration and
prevent over-running the capacitor (refer to paragraph 3-1h). This is done by
loosening the two set screws in the pinion on the cross shaft on the back of the
dial assembly. Before putting the mechanism in place, this pinion can be moved
along the shaft toward the worm gear to clear the capacitor split gear. This
facilitates assembly, since the gears can be meshed after the dial mechanism is
bolted in place, making it easier to get the tension on the split gear in the
capacitor assembly.
e. REMOVAL OF ANTENNA, R-F, DETECTOR, AND OSCILLATOR
UNITS.--In many cases servicing of these units will require only the removal of
the top or bottom cover of a particular unit; however any unit may be removed
and replaced independently as follows:
(1) Unsolder the lead to the main
tuning capacitor at the capacitor by first removing the capacitor shield.
Unsolder all other leads at the unit.
(2) Disconnect the band switch drive
shaft and withdraw same from the antenna unit end.
(3) In case of antenna
unit, disconnect the antenna alignment control shaft.
(4) Remove screws
holding unit to the tie strips at the bottom.
(5) Remove screws holding the
unit to the chassis.
(6) Lift the unit from the receiver, taking care that it
comes out freely.
(7) When replacing a unit, reverse the above procedure. Do
not screw the chassis holding screws tightly until the drive shaft has been
replaced and the band change switch knob has been rotated a number of times.
This will insure the self-alignment of the unit and the proper action of the
detent.
6. REPLACEMENT OF FUSES AND
LAMPS.
a. Remove the dial light housing cover by unscrewlng the two
thumbscrews at each side of the dial. The lamps are accessible with this panel
off.
b. To replace fuse, remove the chassis from the cabinet and turn upside
down. The fuse is located on a panel in the center of the bottom side of the
chassis.
7. LUBRICATION DATA.
This
dynamotor requires lubricating after 1,000 hours or approximately 6 months of
ordinary service. Lubricate it with Air Corps Grade 375 grease only. The
directions for lubrication are stamped on the inside of the end-bell dust
covers. To gain access to the dynamotor bearings, remove the dust covers after
cutting the safety wires and removing the retaining screws, then unscrew the
bearing end plates. Do not PACK the lubricant in these bearings.
SECTION VI SUPPLEMENTARY DATA
1. TUBE COMPLEMENT.

TABLE 6-1. TUBE COMPLEMENT

Quantity

JAN Type

VT-type

Function

3

JAN 6K7

VT-86

RF amplifier

RF amplifier

IF amplifier

1

JAN 6J7

VT-9!

1st Detector

1

JAN 6F7

VT-70

2nd i-f; c.w. oscillator

1

JAN6B8

VT-93

3rd i-f; 2nd detector, a.v.c.

1

JAN 6C5

VT-65

Oscillator

1

JAN 6K6

VT-152

Output amplifier

1

JAN 991

Voltage regulator

(Neon bulb)

2. LAMP
COMPLEMENT.

TABLE 6-2. LAMP COMPLEMENT

Quantity

Type

Rating

2

LM-27

6.3 volts; 0.25
amperes

3. FUSE
COMPLEMENT.

TABLE 6-3. FUSE COMPLEMENT

Quantity

Type

Rating

1* or

1**

FU-35

FU-23

5 amperes; 25 volts

10 amperes; 25
volts

* Used with Radio Receiver BC-348-(*) only.
**
Used with Radio Receiver BC-224-(*) only.
4.
FREQUENCY RANGE.

TABLE 6--4. FREQUENCY RANGE

Band

Frequency Coverage

1

200- 500 kilocycles

2

1.5- 3.5 megacycles

3

3.5- 6.0 megacycles

4

6.0- 9.5 megacycles

5

9.5-13.5 megacycles

6

13.5-18.0
megacycles

5. PERFORMANCE
DATA.
a. SELECTIVITY.--The selectivity of a radio receiver is that
characteristic which determines the extent to which it is capable of
differentiating between the desired signal and disturbances of other
frequencies. This characteristic may be determined as follows:
(1) With Radio
Receiver BC-348-(*) or BC224-(*) and a signal generator, both tuned to 200 kc,
adjust the generator output to obtain a receiver output of 10 milliwatts into a
300 ohm resistive load. Use 30 percent modulation, 400 cycles.
(2) Increase
the voltage of the signal generator to twice (2X) the value obtained
above.
(3) Increase the generator frequency until the receiver output
decreases to 10 milliwatts. Record the frequency change.
(4) Without changing
the generator output, decrease tile generator frequency, passing through 200 kc,
until the receiver output is again I0 milliwatts. Record the difference between
this frequency and 200 kc.
(5) The sum of the two values obtained in steps 3
and 4 should approximate 2.5 kc, as indicated in the table below. Additional
data on the selectivity characteristic of Radio Receiver BC-348-(*) or BC-224(*)
may be obtained by using generator outputs of 10 times (10X), 100 times (100X),
1,000 times (1,000X), etc., the value obtained in step 1 and repeating 3, 4 and
5.
(6) Repeat steps 1 through 5 substituting 500 kc in place of 200 kc. Data
on Radio Receiver BC348-(*) or BC-224-(*) is obtained as outlined above, using
the proper reference frequencies as given below. (These measurements were made
with "CRYSTAL" switch in the "OUT" position and the "AVC-OFFMVC" switch in the
"MVC" position.)

TABLE 6-5 SELECTIVITY DATA

Band No.

Ref. Freq.

MCW Selectivity

2X

10X

100X

1000X

1

1

2

3

4

5

6

200 kc

500 kc

1.5 mc

3.5 mc

6.0 mc

9.5 mc

13.5 mc

2.5 kc

5.0 kc

7.0 kc

7.0 kc

7.0 kc

7.0 kc

7.0 kc

7.0 kc

10.0 kc

15.0 kc

15.0 kc

15.0 kc

15.0 kc

15.0 kc

12.0 kc

15.0 kc

25.0 kc

25.0 kc

25.0 kc

25.0 kc

25.0 kc

15.0 kc

22.0 kc

35.0 kc

35.0 kc

35.0 kc

35.0 kc

35.0 kc

Table 6-5 is presented for use as a guide in
servicing receivers. It applies to undamaged and perfectly aligned receivers,
under reasonable climatic conditions. These values are to be regarded as
average, to be approximated when adjusting the equipment after overhaul or long
service. Departures from these values are not necessarily cause for major
operations on the equipment. The values should be employed with caution and
discretion, particular in the case of measurements carried out under extreme
conditions of temperature or humidity, or with a signal generator whose accuracy
is not definitely known.
b. SENSITIVITY.--The r-f input required from a
directly connected signal generator to obtain 10 milliwatts output (6.3 volts)
with a 300 ohm resistive load is shown for three points in each radio receiver.
The load resistance shall be connected to the test receiver only, and the output
circuit of this receiver shall be isolated from the headsets and output circuits
of other receivers. The frequencies at which the measurements must be made are
listed in the table below. Input voltage 14 or 28 volts. Sensitivity values are
in microvolts, modulated 30 percent at 400 cps.

TABLE 6-6 SENSITIVITY DATA

Band

Freq. Range

Antenna Binding Post

Mixer Control Grid (Top Cap)

Audio Grid

1

2

3

4

5

6

200-500 kc

1.5-3.5 mc

3.5-6.0 mc

6.0-9.5 mc

9.5-13.5 mc

13.5-18.0 mc

7 microvolts

5 microvolts

5 microvolts

5 microvolts

5 microvolts

5 microvolts

35 microvolts

5 microvolts

5 microvolts

5 microvolts

5 microvolts

5 microvolts

2 volts *

2 volts

2 volts

2 volts

2 volts

2 volts

* Audio input 400 cps.
This table of sensitivities is for use
as a guide in servicing the receivers. It applies to undamaged and perfectly
aligned receivers under reasonable climatic conditions. Microvolt values shown
are to be regarded as average; they are to be approximated when adjusting the
equipment after overhaul or long service. Departures from these values are not
necessarily cause for major operations on the equipment. The values should be
employed with caution and discretion, particularly in the case of measurements
carried out under extreme conditions of temperature or humidity. A signal
generator whose accuracy is not definitely known and a set of vacuum tubes which
are not average may produce results varying considerably from those shown in the
table.
c. DYNAMOTOR PERFORMANCE DATA.

TABLE 6--7. DYNAMOTOR PERFORMANCE DATA
Average
performance data on Dynamotor DM-24-D, DM-24-F, DM-28-O, DM-28-L, DM-28-H and
DM-28R is as follows: (dynamotor and filter disconnected from receiver and
negative high voltage connections made to case of unit).

DM-24-(*)

DM-28-(*)

Input

Output

Input

Output

Volts

Amperes

Volts

milliamperes

Volts

Amperes

Volts

Milliamperes

12

12

12

14

14

14

1.4

2.0

2.5

1.4

2.2

2.8

213

210

190

255

236

226

0

30

60

0

40

75

24

24

24

28

28

28

0.7

1.1

1.3

0.8

1.1

1.5

215

210

202

258

246

236

0

30

60

0

40

75

Figure 6-1 Radio Receiver BC-348-(*) or BC-224-(*) -- Top View
of Chassis

Figure 6-2 I-F Transformers, C-W Oscillator and Crystal Filter
Assemblies

Figure 6-3 Antenna Unit

Figure 6-4 R-F Unit

Figure 6-5 Detector Unit

Figure 6-6 Oscillator Unit

Figure 6-7 Radio Receiver BC-348-(*) or BC-224-(*) -- Resistor
Boards, View A

Figure 6-8 Radio Receiver BC-348-(*) or BC-224-(*) -- Resistor
Boards, View B

Figure 6-9 Radio Receiver BC-348-(*) or BC-224-(*) -- Bottom
View of Chassis

Figure 6-10 Radio Receiver BC-348-(*) or BC-224-(*) -- Rear View
of Chassis
SECTION VIII DRAWINGS

Figure 8-1 Plug Connections

Figure 8-2 Radio Receiver BC-348-(*) or BC-224-(*) -- Wiring
Diagram, Tube Shelf

Figure 8-3 Radio Receiver BC-348-(*) or BC-224-(*) -- Wiring
Diagram, Antenna

Unit (does not include Radio Receiver BC-348-R)

Figure 8-4 Radio Receiver BC-348-(*) or BC-224-(*) -- Wiring
Diagram, Antenna Unit

Figure 8-5 Radio Receiver BC-348-(*) or BC-224-(*) -- Wiring
Diagram, R-F Unit

Figure 8-6 Radio Receiver BC-348-(*) or BC-224-(*) -- Wiring
Diagram, Detector Unit

Figure 8-7 Radio Receiver BC-348-(*) or BC-224-(*) -- Wiring
Diagram, Oscillator Unit

Figure 8-8 Radio Receiver BC-348-(*) or BC-224-(*) -- Wiring
Diagram,

I-F Transformers

Figure 8-9 Radio Receiver BC-348-(*) or BC-224-(*) -- Wiring
Diagram, Crystal Filter

Figure 8-10 Radio Receiver BC-348-(*) or BC-224-(*) -- Wiring
Diagram, Beat

Frequency Oscillator

Figure 8-11 Radio Receiver BC-348-(*) or BC-224-(*) -- Wiring
Diagram, Resistor Boards

Figure 8-12 Radio Receiver BC-348-(*) or BC-224-(*) -- Wiring
Diagram,

Dynamotor Assembly

Figure 8-13 Radio Receiver BC-348-(*) or BC-224-(*) -- Dial and
Mask Assembly

Figure 8-17 Radio Receiver BC-348-(*) or BC-224-(*)

-- Wiring Diagram

Figure 8-18 Radio Receiver BC-348-(*) or BC-224-(*)

-- Schematic Circuit
Diagram