T.O AN 16-40BC348-3

HANDBOOK

MAINTENANCE INSTRUCTIONS

RADIO RECEIVERS

BC-348-J BC-348-N BC-348-Q

THIS PUBLICATION REPLACES T.O. AN-08-10-112 DATED 18 DECEMBER
1943

PUBLISHED UNDER AUTHORITY OF THE SECRETARY OF THE AIR
FORCE

REVISED 21 JUNE1948

OCR by Bill, NJ7P

From a partial Manual from

Fair Radio Sales
This manual is missing the comprehensive
parts lists and some of the pictures from the original manual. The full text and
key data is present.

TABLE OF CONTENTS
I. GENERAL DESCRIPTION
1. GENERAL.
2. COMPONENT UNITS.
3. CABINET.
4. CHASSIS.
5. DIAL AND MASK ASSEMBLY.
6. DYNAMOTOR.
7. I-F TRANSFORMERS, CRYSTAL FILTER, C-W OSCILLATOR.

8. MOUNTING.
9. PANEL.
10. PLUG.
11. R-F AND OSCILLATOR UNITS.
II. INSTALLATION AND ADJUSTMENT
12. INITIAL PROCEDURE.
13. INSTALLATION.
14. PREPARATION FOR USE.
III. OPERATION
15. PROCEDURE.
IV. MECHANICAL AND ELECTRICAL CHARACTERISTICS
16. CIRCUITS.
17. FREQUENCY RANGE AND BANDS.
18. INPUT COUPLING.
19. RADIO FREQUENCY AMPLIFIER.
20. FIRST DETECTOR.
21. HETERODYNE OSCILLATOR.
22. INTERMEDIATE FREQUENCY AMPLIFIER.
23. C-W OSCILLATOR.
24. CRYSTAL BAND-PASS FILTER.
25. SECOND DETECTOR.
26. OUTPUT.
27. DYNAMOTOR DM-28-(*).
V. MAINTENANCE
28. INSPECTION.
29. DYNAMOTOR SERVICE ANDMAINTENANCE.
30. REMOVAL OF FRONT PANEL.
31. DIAL AND MASK ASSEMBLY.
32. SERVICING THE DIAL AND MASK ASSEMBLY.
33. REMOVAL OF ANTENNA, R-F, DETECTOR AND OSCILLATOR UNITS

34. TROUBLE LOCATION AND REMEDY.
VI. SUPPLEMENTARY DATA
35. GENERAL.
36. CERAMIC CAPACITORS.
37. HEATER SHUNT RESISTOR.
38. STATIC DRAIN RESISTOR.
39. OUTPUT CONNECTIONS.
40. MOISTURE-FUNGUS RESISTANCE.

Handbook of

MAINTENANCE INSTRUCTIONS

For

RADIO RECEIVERS

BC-348-J, BC-348-N and BC-348-Q

SPECIAL NOTICE
Radio Receivers BC-348-J, BC-348-N and
BC-348-Q are essentially alike. Therefore, reference will be made throughout the
book to these receivers as follows: Radio Receiver BC-348-(*). However, all
photographs and diagrams will be labeled for the "J" equipment, since it applies
equally well to the other two models.
Reference to the dynamotors is
mentioned in the same manner. The asterisk indicates that these units are
mentioned collectively.
I. GENERAL
DESCRIPTION
1.
GENERAL.
Radio Receivers BC-348-J,
BC-348-N and BC-348-Q are locally controlled, eight-tube, six-band
superheterodyne receivers for use in U. S. Army aircraft. They cover a frequency
range of from 200 to 500 kc and 1.5 to 18.0 megacycles. These receivers are
designed for operation on a 28-volt power supply. Their power consumption is 60
watts with no power supplied through the output plug to external equipment. All
controls are 1ocared on the front panel where they may be easily operated by
aircraft personnel. Antenna, ground and headphone connections are made on the
front panel. Power and interphone connections are made through a connector plug
at the back of the receiver. Each receiver is capable of voice, tone and c-w
reception. Either manual or automatic volume control may be selected by a switch
on the front panel; likewise normal or extreme selectivity is provided by means
of an i-f crystal filter that may be switched in or out of the circuit as
desired. A beat frequency oscillator is employed for c-w reception. The
receivers are not intended for remote control and no provision has been made for
this operation.
2. COMPONENT
UNITS.

Description

Size in Inches

Weight in Pounds

1 Radio Receiver BC-348-(*) Complete (Includes Dynamotor
DM-28-(*) and Mounting FT-I54-J, FT-154-Q or FT-154-AA)

18 x 10 1/2 x 9 1/2 high

38.0

1 Mounting FT-I54-J, FT-I54-Q or FT-154-AA (Does not
include Plug PL-P103 or PL-Q103)

18 x 8 1/2 x 1 3/8 high

3.843

1 Plug PL-P103 or Plug PL-Q103

l 5/s x 2 1/8 x 2 7/8 high

2 x 2 1/8 x 3 high

.312

.375

1 Set of 8 Vacuum Tubes, 2 Pilot Lights, 1
Fuse

.563

3. CABINET.
Radio
Receiver BC-348-(*), is housed in an aluminum cabinet 18" long, 8 15/16" deep,
and 8 5/8" high and is of spot-welded construction. The outside has a black
wrinkle finish. An opening is provided at the rear of the cabinet for the
connector plug. A plate is attached to the bottom, which reinforces the cabinet
and provides a mounting for four mounting studs, A flange on the front of the
plate mounts two snapslides which are used to lock the cabinet to the receiver
mounting. The top and back of the cabinet are strengthened by embossed grooves
that run nearly the length of the cabinet.
4. CHASSIS.
The
chassis of Radio Receiver BC-348-(*) is an aluminum casting with plates mounted
on both ends. These end plates have cutouts to facilitate servicing. They serve
as a mounting for parts and as runners for the chassis when it is installed or
removed.
5. DIAL AND MASK
ASSEMBLY.
The dial and mask assembly
is a casting which mounts a dial scale calibrated in six frequency ranges, a
dial mask with windows, a band switch shaft, a detent wheel and mechanism, and
drive gears. A stop arm provides a positive stop at each end of the frequency
range. The use of split gear tuning minimizes backlash. The drive ratio is such
that approximately 100 revolutions of the tuning control are required to turn
the tuning capacitors through the frequency range. The assembly is mounted on
the front panel, and indicates the range and frequency to which the receiver is
tuned.
6. DYNAMOTOR.

Mounted at the left (from front of
chassis) rear of the chassis is the dynamotor assembly. It consists of Dynamotor
DM-28-(*) and an r-f filter unit that supplies all the high voltage direct
current for the operation of Radio Receiver BC-348-(*). In addition to the power
supplied by the dynamotor to the receiver, there is available at the connected
plug 20 milliamperes at approximately 200 volts for the operation, when
necessary, of accessory equipment. To remove the unit for servicing or
replacement, disconnect the connections at the terminal strip and loosen the
four mounting bolts at each corner of the mounting plate.
7. I-F TRANSFORMERS, CRYSTAL FILTER, C-W
OSCILLATOR.
The i-f transformer
assemblies are mounted on the left (from front of chassis) front section of the
chassis near the i-f tubes. Fixed capacitors are used across the primaries and
secondaries. They are adjusted by varying the transformer cores. On the same
section of the chassis, but on the underside in front of the 2nd i-f tube
socket, is the crystal filter coil. The crystal and switch may be seen from the
upper side of the chassis, mounted on the front panel above the coil. With the
switch in the IN position, sharp selectivity is obtained on all bands. Near the
crystal filter coil, but mounted on the front panel in back of the beat
frequency control knob, is the cw oscillator coil. The position in which the
coil is mounted allows direct control of the variable core from the front panel
for adjusting the beat frequency.
8. MOUNTING.
The
mounting base is a metal plate upon which four shock absorbers are mounted. A
second metal plate, with grooves and cutouts to fit the mounting studs on the
bottom of the cabinet and studs to fit the snapslides, is attached to the top of
the shock absorbers. A metal stiffener is attached to the bottom of this plate
to strengthen the assembly, and provision is made at the rear of the plate for
mounting the connector plug.
9. PANEL.
The front
panel is attached to the chassis. The controls, the input and output
connections, and the handles are located on it. Through the lower part of these
handles pass thumbscrew rods that fasten the chassis to the cabinet. On the
right side of the panel is a small metal plate held in place by six screws and
covering an opening that gives access for servicing to the underside of the r-f
tube shelf. Both the front panel and the r-f tube shelf plate have a black
wrinkle finish. White lettering identifies each control and indicates the switch
positions.
10.
PLUG.
The eight-contact connector plug
which provides power connections at the rear of the receiver is mounted by
screws on the receiver mounting. The wiring terminals are accessible by removing
the rear cover on the plug housing. The connector, when provided with a straight
fitting, is identified as Plug PL-P103. When it is provided with a right angle
fitting, it becomes PL-QI03, regardless of whether the fitting is mounted toward
the right, left or back of the receiver.
11. R-F AND OSCILLATOR UNITS.
The antenna, r-f, 1st detector and oscillator units are
sub-assemblies consisting of a shield can, coils, trimmer condensers, band
switch section, resistors, fixed capacitors and in the oscillator unit, the 1st
detector and oscillator tube, Tube VT-150. The four units are mounted at the
right (from front of chassis) rear of the chassis and bonded together by common
ground straps. The band switch sections are ganged by connecting the arms on the
switch shafts together with slotted bars and tension springs, and are controlled
as a unit from the front panel.
II. INSTALLATION
AND ADJUSTMENT
12.
INITIAL PROCEDURE.
a.
UNPACKING.
Radio Receiver BC-348-(*) is packed in a wooden box with Mounting
FT-154-J, FT-154-Q or FT-154-AA. To unpack the receiver preparatory to
installation, the following steps should be taken in the order given:
(1)
Pull out the nails in the top of the box and remove the cover.
(2) Remove
the large cardboard filler in the top of the box.
(3) Remove the cardboard
carton containing Mounting FT-154-J, FT-154-Q, or FT-154-AA.
(4) With the
wooden packing box placed on the floor, stand at one end, place the palms of the
hands against each side of the cardboard carton containing the receiver and lift
the carton from the packing box.
(5) Break the seal on the top of the
cardboard carton.
(6) Remove the filler in the top of the carton.
(7)
Grasp the handles on the front of the receiver and lift the receiver out of the
carton.
(8) Break the seal and open both ends of the carton containing
Mounting FT-154-J, FT-I54-Q, or FT-154-AA.
(9) Slide the mounting out of the
carton and place with the receiver.

Figure 2 – Radio receiver BC-348-J, Tube
Positions
(10) Loosen the thumbscrews in the lower part of the
handles on the front panel and pull the chassis from the cabinet.
(11)
Remove the cardboard filler over the dynamotor and replace the chassis in the
cabinet.
b. INSPECTION.
(1) Figure 2 shows the position of each tube,
dial lamp and fuse. Before the receiver is installed, loosen the thumbscrews in
the lower part of the handles on the front panel and pull the chassis from the
cabinet. Make certain the tubes are well pushed down and firmly seated, and that
the fuse and dial lamps are correctly and securely inserted.
(2) The
dynamotor and the electrical connections to the dynamotor must be securely
fastened in position. The dynamotor is fastened by four mounting screws at the
corners of the mounting plate and the electrical connections are made at a
terminal strip accessible through a cutout in the left (from front of chassis)
chassis end plate.
13.
INSTALLATION.
a. MOUNTING.
The
receiver should be mounted as near as possible to the antenna lead-in insulator
with sufficient clearance on all sides to allow free action of the shock
absorbers. A permanent installation should be made and Mounting FT-154-J,
FT-154-Q, or FT-154-AA, securely fastened to a rigid section of the aircraft.
The drilling plan for the mounting is shown in Figure 29.
b. ELECTRICAL
CONNECTIONS.
The connector plug mounted on the rear of Mounting FT-154-J,
FT-154-Q, or FT-154-AA ,(Figure 19), provides electrical connections between the
receiver and the other aircraft equipment (Figure 32). On the rear of this plug
is a cover held in place with two screws. After these screws have been removed
the cover may be taken off and eight terminals exposed. Four of these terminals
(numbers 3, 4, 7 and 8) are used for connections to the aircraft power supply.
Two terminals, numbers 2 and 6, carry the screen voltage circuit out of the
receiver to terminals on the transmitter relay so that the screen circuit may be
opened and the receiver protected while the transmitter is operated. Should the
transmitter be removed or the receiver be used in an installation with no
transmitter, terminals 2 and 6 must be connected together in order to have the
receiver operate The aircraft interphone system may be connected to terminals 1
and 5 at which the receiver output is available. These terminals may be left
open if output is desired only for headphones, and the headphones plugged into
jacks on the front panel. See Section II, paragraph 14b(9). Should it be
necessary to supply power to additional external equipment, a maximum of 20
milliamperes at approximately 200 volts is available from terminals 2 and 5. All
leads should be carefully measured and cut to the correct length, inserted
through the fitting on the bottom of the connector plug and soldered to the
correct terminals. After all connections have been made, check carefully to make
certain that the terminals are wired correctly, and replace the cover on the
rear of the connector plug. The plug may now be mounted in position. The two
pair of leads carrying the power to the receiver through terminals 3, 4, 7 and 8
may be replaced with one pair of leads, each of which must have a cross
sectional area equal to the two replaced leads.
c. ELIMINATION OF ELECTRICAL
INTERFERENCE.
Electrical disturbances are set up by the aircraft ignition
system and electrical devices. This interference must be either eliminated or
lowered to a level below the strength of the signals to be received. This is
accomplished by shielding the entire electrical ignition system and devices,
filtering the electrical connections between the devices, and the bonding of all
metal parts to a common ground.
d. MOUNTING THE RECEIVER.
(1) After
Mounting FT-154-J, FT-154-Q or FT-154-AA is located and the electrical
connections are made, the receiver is ready to be installed.
(2) Make certain
the thumbscrew rods holding the chassis to the cabinet are tight, and push both
snapslides toward the center of the cabinet. Lift the receiver onto the mounting
so that the rear studs on the bottom of the cabinet are in the groove on the
mounting plate.
(3) Push the cabinet towards the rear of the mounting and at
the same time lower the front of the cabinet. The studs will drop into holes
provided for them in the mounting.
(4) Push the receiver still farther
backwards (more pressure will have to be exerted, as the studs fit into the
slots provided for them), until it will go no farther. Now push the snapslides
on the front of the cabinet over the studs on the mounting plate, thus locking
the receiver to the mounting.
(5) Insert safety wire through the holes in the
snapslides. Twist the ends together and turn the ends in so there will be no
danger of scratches or minor injuries to the personnel.
e. ANTENNA AND GROUND
CONNECTIONS.
The antenna and ground binding posts are on the front panel at
the lower right-hand side where they may be identified by the letters "A' and
"G." Connections to these binding posts should be made with short, low
resistance leads having sufficient slack to prevent the transmission of
vibration to the receiver. The antenna lead should connect to the antenna
insulator, and the ground lead to some metal part of the aircraft where it
should be soldered, if practicable. The receiver may be used with any type of
antenna. However, the most efficient antenna is one which has the greatest
effective length away from the grounded metal fuselage.
14. PREPARATION FOR USE.

a. INSPECTION.
After the installation
has been completed, a final check on the points listed below should be made
followed by an operating test:
(1) Check the connections at the aircraft
power supply.
(2) Check the connections at the transmitter relay.
(3)
Check the connections at the interphone system, if used.
(4) Check the power
connections to auxiliary equipment, if used.
(5) Check the antenna and
ground connections.
(6) Make sure the thumbscrew rods holding the chassis in
the cabinet are tight.
Make sure the snapslides are locked and secured with
safety wire.

Figure 3 – Radio Receiver BC-348-J, View of front
Panel
b. CONTROLS.
The operator should become thoroughly familiar
with the controls on the front panel (see Figure 3) and their function before
beginning the operating test.
(1) AVC-OFF-MVC.-- Power to the receiver is
controlled by the AVC-OFF-MVC receiver switch. With this switch in the OFF
position, no power is supplied to the receiver. When switched to either the MVC
or AVC position, power from the primary source is supplied to the tube heaters
and dynamotor, placing the equipment in operation. The screen grid voltage
supply leads are carried out of the receiver through the power plug to the
keying relay of the associated transmitter where the circuit is opened when
actually transmitting. (See Section II, paragraph 13b).
(2) DIAL LIGHTS.--The
brilliancy of the dial illumination is controlled by this knob. The dial lamps
may be adjusted for any desired degree of illumination or turned off
completely.
(3) BAND SWITCH. -- The frequency band in which the receiver is
operating is indicated on the dial mask visible through the dial window. This
band may be changed to any desired frequency band by turning the band-switch
control.
(4) TUNING.--Reception is accomplished by tuning the receiver to the
desired signal with this control. The frequency to which the receiver is tuned
is indicated at all times on the dial scale.
(5) INCREASE VOL.--The volume
level of the audio signal is controlled by this knob. When the receiver is
operated with manual volume control, the sensitivity of the receiver is
controlled. When automatic volume control is desired, the level of the audio
signal fed into the output tube is controlled.
(6) C-W OSC.--The c-w
oscillator is turned ON for c-w, or OFF for voice reception with this knob. The
a-v-c time constant is also changed at the same time to conform with the type of
signal to be received.
(7) BEAT FREQ.--This knob adjusts the frequency of
the c-w oscillator and allows the operator to adjust the tone of the received
signal to the pitch he considers the most suitable.
(8) CRYSTAL--This control
inserts a crystal filter into the circuit when turned to the IN position. This
filter increases the selectivity, of the receiver, enabling reception through
heavy interference.
(9) TEL. JACKS.--Dual output is provided through two open
circuit phone jacks. These jacks are connected to the output circuit of the
receiver and permit headphone reception by the operator.
(10) ANTENNA AND
GROUND BINDING POSTS.--The antenna is connected to the binding post marked "A",
while the ground lead is connected to the binding post marked "G."
III. OPERATION
15. PROCEDURE.
a.
OPERATING TEST.
When the receiver has been completely installed, an operating
test should be made as follows:
(1) Plug a headset into one of the jacks
marked "TEL." Set receiver switch to MVC. Start the dynamotor. After the tubes
have warmed up (approximately 30 seconds), advance volume control knob until a
slight background noise is heard. Set band switch to the frequency band in which
test signals are available.
(2) Using the tuning knob with reference to the
calibrated scale on the dial, tune in the desired signal.

NOTE
All tuning should be done on MVC switch with the
volume control advanced only enough to give the desired signal strength. In the
absence of a signal the setting of the volume control can be judged by the
loudness of the background noise. On MVC with the volume control set at maximum,
very strong carrier waves will block the receiver and intelligible signals
cannot be received.
(3) Set the receiver switch to AVC. The
desired signal should still be heard.
(4) With the beat frequency adjustment
at zero beat position (arrow on knob pointing up), turn the c-w oscillator
switch to the ON position. An audible beat note should be heard which should
vary in pitch when the beat frequency adjustment is changed.
(5) With the c-w
oscillator still on, throw the crystal filter switch to IN. Noise should be
greatly reduced and the signal can be tuned out by a much smaller movement of
the tuning control knob than when the crystal filter switch is in the OUT
position.
(6) Turn the dial light rheostat and observe if control of
illumination is secured with both dial lights functioning.
(7) A check
should be made before flight with the airplane engines running. An increase of
background noise when the engine starts, indicates imperfect shielding,
imperfect bonding, faulty generator regulator, faulty generator, open filter
capacitors, or a combination of these faults.
(8) Always turn the receiver
switch to the OFF position when the receiver is not being used.
b.
RECEPTION.
(1) MODULATED SIGNAL RECEPTION.-For the reception of modulated
signals in the frequency bands covered by this receiver, turn the AVC-OFF-MVC
switch to MVC, the C-W OSC. control to OFF and the CRYSTAL control to the OUT
position.

NOTE
Tuning should be done in the MVC position with the
volume control advanced only as far as required for a comfortable output level.
While waiting for the tubes to warm up, adjust the dial light control for the
desired dial illumination and turn the band switch to the frequency band in
which the signals to be received are transmitted.
After the tubes
have warmed up (approximately 50 seconds), adjust the volume control until the
background noise can be heard. Turn the tuning control until the frequency of
the desired signal is reached and the signal is heard in the headphones. Turn
the tuning control slowly back and forth until the position at which the signal
is received the strongest is found. After the signal is tuned in, if automatic
volume control is desired it may be used by switching the AVC-OFF-MVC control to
the AVC position and readjusting the volume control for the desired output. In
the event interference is encountered, the crystal filter may be switched into
the circuit, increasing selectivity and permitting reception that would be
exceedingly difficult otherwise.
(2) C-W RECEPTION.--For the reception of c-w
signals, turn the c-w oscillator control to ON and the beat frequency control to
the zero beat position (arrow on knob pointing up). Proceed as instructed for
the reception of modulated signals and when the signal is tuned in, adjust the
beat frequency control to the position producing the most satisfactory tone.
Automatic volume control may be used when desired by switching to the AVC
position and readjusting the volume control. The crystal filter should be used
to increase the selectivity of the receiver if objectionable interference is
encountered. A slight readjustment of the tuning, beat frequency and volume
controls may be required to secure the desired beat note frequency and volume
level after the crystal filter is switched in. The sensitivity may sometimes be
slightly improved by readjusting the positions of the beat frequency knob and
the tuning control.

NOTE
The crystal band pass 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.
IV. MECHANICAL AND
ELECTRICAL CHARACTERISTICS
16. CIRCUITS.
Electrically, the receiver comprises two stages of tuned radio frequency
amplification preceding the 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 and beat-frequency oscillator are also included. The
former is for increasing selectivity and the latter for receiving c-w signals.
The schematic and wiring diagrams are shown in Section VIII.
17. FREQUENCY RANGE AND
BANDS.
Six bands controlled by a band
change switch are covered. The frequency range for each of the six bands is
given in the following table.

Band

Frequency Range

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

18. INPUT
COUPLING.
The antenna input circuit is
designed to operate properly with antennas having capacities ranging from 50 to
250 mmf. A one megohm resistor is connected across the antenna and ground posts
to discharge static charges.
19. RADIO FREQUENCY AMPLIFIER.
The radio frequency preselector comprises three tuned circuits
coupled by two Tubes VT-117. Separate inductors are employed for each frequency
band.
20. FIRST
DETECTOR.
The first detector employs
Tube VT-150 which also functions as the heterodyne oscillator. The low signal
level at the grid of the first detector, together with the r-f preselection,
insures a minimum of undesired responses.
Fixed bias is provided by returning
the control grid circuit through a filter resistor to the low potential end of a
25 ohm resistor (Refs. 108-1 and 108-2 in parallel). The bias consists of the
drop across this resistor which is in the negative plate supply
line.
21. HETERODYNE
OSCILLATOR.
The heterodyne oscillator
employs a tuned grid circuit. Excitation is secured by means of a cathode
winding tightly coupled to the grid winding. The high value grid resistor and
the low grid coupling capacity used, together with the inherent stability of
Tube VT-150, makes a voltage regulator unnecessary. The effects of wide
variations in ambient temperatures under service conditions on the oscillator
frequency have been reduced to a minimum by the use of a highly stable tuning
capacitor and temperature-compensation with ceramic fixed capacitors (35-1, 3%2,
42-1, 42-2, 45, 48, 49-2).
Individual inductors and trimmers are employed for
each frequency band. 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 bands 5 and 6, the oscillator is on the low frequency
side of the desired signal. The latter results in an improvement in the image
rejection ratio.
22.
INTERMEDIATE FREQUENCY AMPLIFIER.
The
intermediate frequency amplifier comprises two low-gain amplifying stages
coupled by three highly selective, double-tuned circuit transformers and one
resistance coupled stage. The intermediate frequency employed is 915 kc. The i-f
transformers are tuned by means of adjustable iron cores and fixed capacitors.
The lowered tuned circuit impedance, secured by the ' relatively large fixed
tuning capacitors, provides an inherently stable amplifier. Two Tubes VT-117
function as the first and second i-f amplifiers and Tube VT-Il6 is employed as
the third i-f amplifier. A relatively high level signal is supplied to the
second detector diodes Tube VT-233 which also functions as the c-w
oscillator.
23. C-W
OSCILLATOR.
a. The c-w oscillator
employs the triode section of Tube VT-233 (second detector) in a tuned grid,
plate feedback circuit. The variable iron core in the grid inductance 15 t is
used for frequency adjustment, and is so mounted that about one turn of the beat
frequency knob on the front panel will vary the frequency of the c-w oscillator
approximately 4,000 cycles each side of the zero beat position. (Arrow on knob
pointing up.)

Figure 4 – C-W Oscillator Switching
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 low
level, minimizing harmonics and stray oscillator pickup. The output is
capacitively coupled to the plate circuit of the second amplifier tube by a
twisted wire capacitor. Amplification by the third i-f amplifier stage, the gain
of which is not controlled either manually or by 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.
b. C-W oscillator
switch 168 in the ON position supplies the oscillator plate voltage and
increases the a-v-c time constant by connecting the additional capacitor 64.
Switch 168 supplies the oscillator plate voltage by connecting to the screen
grids of the first and second i-f tubes. The same switching shunts resistor
101-2 across plate resistor lOl-1, which drops the gain in the first i-f tube to
a value that reduces the sensitivity by an amount sufficient to keep the overall
set noise essentially constant.
24. CRYSTAL BAND-PASS FILTER.
Additional selectivity is available by the use of the i-f
crystal filter following the first i-f amplifier tube. Of interest in connection
with this filter is the bridge circuit composed of auto transformer 150, a
neutralizing capacitor and the capacity of the crystal holder. See Figure 5.
Undesired signals transmitted through the capacity of the crystal holder to the
grid of the second i-f tube are neutralized by an opposite voltage developed in
the auto transformer, and made equal to the undesired grid voltage by the
neutralizing capacitor. The crystal filter may be switched in or out of the
circuit by the crystal "Out-In' switch 167 which is actuated from the front
panel. The filter bandwidth is adjusted by the neutralizing capacitor. See
Figure 5. This capacitor consists of two wires and two lugs extending from the
neutralizing coil. See Figure 11. For adjustment, see Paragraph 34e(10).

Figure 5 – Crystal Filter Circuit
25. SECOND DETECTOR.
Tube VT-233 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.
26.
OUTPUT.
a. DESCRIPTION.
The high
level signal diode supplies audio output for driving output Tube VT-152 without
additional audio amplification. The design choice of three i-f 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 of the output tube from the diode detector 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 Il0 comprises the 350,000-ohm
audio control potentiometer and the 20,000-ohm bias control potentiometer. The
audio volume control functions only with switch 169 in the a-v-c position and in
this position it permits the desired adjustment of the audio level to the output
tube and load. See Figure 6. The bias volume control also varies to some extent
the r-f and i-f tube bias. For manual volume control with switch 169 in the
m-v-c position, the bias volume control becomes the active control operating on
the cathode bias of the first and second r-f and the first and second i-f
amplifier tubes. These potentiometers provide a smooth variation of
sensitivity.
b. CONSTANT INTERNAL RECEIVER NOISE.
The flat gain
characteristics of the r-f and detector coils insures a uniform noise level over
the bands. This is obtained by a combination of inductive and capacitive
coupling in the coils.
27.
DYNAMOTOR DM-28-(*).
The dynamotor and
associated r-f filter circuits are assembled in one unit (Ref. 400) which
supplies all of the high voltage direct current required for the operation of
the receiver. In addition, a maximum of 20 milliamperes at approximately 200
volts d-c is available at contacts 2 and 5 on the connector plug at the rear of
the receiver for the operation of external accessory equipment.

TABLE A – CHARACTERISTICS OF TUBES, LAMPS AND FUSE

TUBE CHARACTERISTICS

Tube Type

RMA Equivalent

Heater

Volts/Amps

Plate

Volts

Screen Volts

Grid Volts

Plate Ma.

Screen Ma.

Mu

Plate Resistance

Transconductance Micromhos

VT-116

6SJ7

6.3/0.3

250

100

-3.0

3.0

0.8

2500

1,500,000

1650

VT-117

6SK7

6.3/0.3

250

100

-3.0

9.2

2.4

1600

800,000

2000

VT-150

6SA7

6.3/0.3

250

100

-2.0

3.4

8.0

800,000

VT-162

6K6

6.3/0.4

250

250

-18.0

32.0

5.5

150

68,000

2200

VT-233

6SR7

6.3/0.3

250

-9.9

9.5

16

8,500

1900

DIAL LAMPS

Type

RMA Equivalent

Volts

Amps.

LM-27

No. 44

6.3

0.25

FUSE

Type

Volts

Amps.

FU-35

25

5.0

Figure 6 – AVC Connections

Figure 7 – MVC Connections
V.
MAINTENANCE

NOTE
A standard signal generator, a phantom antenna, a
Test Set 1-56-A and other like equipment should be used when servicing and
aligning Radio Receiver BC-348-(*)
28. INSPECTION.
a.
DAILY.--Turn on receiver. Check dial lamps. Check for operation on all bands
with the c-w oscillator "ON". This test can be made by observing the noise level
with the volume control at maximum.
b. TWENTY HOURS.--Repeat above. Check
antenna, ground and cable connections for effects of vibration.
c. FORTY
HOURS.--Repeat above. Check all dial lamps and vacuum tubes with Model.685 Tube
Checker in Test Set 1-56-A.
d. MAJOR OVERHAUL.--Repeat above. Inspect and
replace dynamotor brushes if necessary. Lubricate dial and tuning capacitor
drive mechanism. Check dynamotor and tube socket voltages as described, in
Paragraphs 34e(1) and 34h(I.).
29. DYNAMOTOR SERVICE ANDMAINTENANCE.
a. REMOVAL FROM CHASSIS.
The dynamotor and filter assembly
(Figure 8, reference 400) may be removed easily from the receiver chassis by
repeating the following steps in the order given:
(1) Loosen two thumbscrews
299 on the front panel, and remove the chassis from the cabinet.
(2) Loosen
the five screws on the dynamotor terminal strip 422, and withdraw the five leads
with spade terminals from under them. Retighten the five screws partially in
order that they will clear the chassis when the dynamotor is removed.
(3)
Push the left (from front of chassis) thumbscrew rod stop spring towards the end
plate with a screwdriver, and withdraw thumbscrew rod 2~) until the second stop
position is reached.
(4) Loosen the four captive screws 426 which fasten the
dynamotor to the chassis, and lift the dynamotor vertically from the
receiver.
The parts in the dynamotor filter are made available for servicing
by removing the metal cover and fibre insulator on the bottom of the unit.
b.
LUBRICATION
Lubricate dynamotor at 1,000 hours or approximately six months of
ordinary service. For ordinary and LOW temperature conditions use mineral oil
grease AN-G-15. For unusually high temperatures, as in tropical climates, use
AN-G-5 grease. The directions for lubrication are stamped on the inside of dust
covers 415.
To reach dynamotor bearings 425 for lubrication, it is only
necessary to cut the safety wires, remove retaining screws 416 and dust covers
and then take out the screws holding retaining plates 414. Take off the
retaining plates, gaskets 413 and washers 412. Care should be taken not to lose
or interchange the parts The bearings are now accessible for lubrication. Do
not pack the lubricant in the bearings, merely add a small quantity so that no
pressure is built up.

Figure 8 – Dynamotor, DM-28 (*)
c. COMMUTATOR
When
necessary to replace the ball bearings 425, or to turn down the commutators,
first remove the brushes from the brush holders. Remove the nuts from tie bolts
411 which hold bearing support brackets 408 and 409 and pull one bracket away
from the frame.

NOTE
The bearing support bracket is more readily
removable from the frame by a slight tilting back and forth of the frame with
respect to the bracket.
Armature 403 and the other bracket
support may now be removed from the frame. Examine the brushes to see that they
are free from hard spots and are wearing properly. Should hard spots be apparent
(they generally cause grooves in the commutator surface), the brush should be
replaced and the commutator smoothed down. To smooth down the commutator, rotate
it in a lathe and hold a fine grade of sand paper, not coarser than size 0o,
preferably either 5/0 or 6/0, against the commutator surface. Do not use emery
cloth. All residue of dust, sand and dirt should be wiped away to leave a clean,
smooth, polished commutator surface. If air is available, the unit should be
cleaned by air pressure. A commutator having a smooth or polished surface should
never be sanded or turned down simply because it is discolored and well
seasoned. If the commutator is turned down in a lathe, the mica segment
separators must be undercut but not more than 1/32".
d. BEARINGS.
In
changing ball bearings 425, it will be necessary to use a bearing puller since
the shaft is machined closely to the dimension of the ball bearing inner race.
Actually, the inner race is a light press fit, and the bearing cannot be removed
from the shaft without considerable force. The outer races of the ball bearings
are merely snug fits in bearing support brackets 408 and 409, and in the
disassembly process it should be easy to remove the bracket from the ball
bearings. If the grease slinger becomes bent during removal of the ball
bearings, it should be straightened and replaced on the shaft before replacing
the bearing.
e. REASSEMBLY.
ReassembIy of the dynamotor is accomplished in
substantially the reverse of the disassembly procedure. 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. Armature 403 must be given a final inspection for free
running, cleanliness and absence of grease or oil. Bearing support brackets 408
and 409 should be wiped clean and dry before replacing them on the
dynamotor.
TABLE B -- DYNAMOTOR DM-28-(*) RATINGS

INPUT

OUTPUT

Volts

Amperes

Volts

Amperes

24

0.7

215

0.

24

1.1

210

30.

24

1.3

202

60.

28

0.8

258

0.

28

1.1

246

40.

28

1.5

236

75.

f. POWER RATING.
The nominal rating of
Dynamotor DM-28-(*) is: Input, 1.3 amperes at 27.9 volts; Output, 70
milliamperes at 224 volts; Regulation, 12 per cent. Average performance data on
Dynamotor DM28-(*) is as shown in Table B. (Dynamotor and filter disconnected
from receiver and negative high voltage connection made to case of
unit.)
30. REMOVAL OF FRONT
PANEL.
a. The adjustment of the dial
and mask and the servicing of certain parts requires the removal of the front
panel.
Remove the chassis from the cabinet and place it on the repair bench
with the front facing the repairman. Loosen thumbscrews 253 that hold window
frame assembly 252 in place, and remove the window frame. Unsolder the white and
black tracer lead running to the left panel light socket 171 and the white lead
at the antenna binding post 174-1. Hold the thumbscrew rod stop-springs against
the sides of the end plates with I screwdriver, and withdraw thumbscrew rods 299
from the receiver.
Remove knobs 290, 291, 292, 293 and 294, and retaining
nuts from all controls except the DIAL LIGHTS control, the handles 295, the felt
washer on the tuning shaft and the friction spring 242 under the BEAT FREQ.
control. Also remove the retaining nuts and covers 300 on the TEL. jacks and the
four bolts and eight screws securing the chassis, dial assembly, and cable clamp
to the panel. The panel may now be lifted from the chassis after placing the
chassis on its back.
b. The process of replacing the panel on the chassis is
the reverse of its removal. However, when the BLAT FREQ. control is replaced,
follow the instructions given in Paragraph 34e(9) for the adjustment of the
control knob on the shaft.
31. DIAL AND MASK ASSEMBLY.
The dial and mask assembly is fastened to both the front panel
and receiver chassis. For service, the entire assembly consisting of dial scale,
dial mask, band switch shaft, detent wheel and the drive gears may be removed
from the chassis as a unit.
At the front of cast aluminum housing 255, (see
Figure 9) on which the parts are mounted, is the dial scale calibrated in six
frequency bands and' the dial; mask with the cutout for each band. Both these
and the index plate are visible through the glass dial window on the front
panel. The index plate is positioned between the dial and mask where it
indicates the frequency to which the receiver is tuned.
The dial is turned
with the tuning knob by means of a large split gear and pinion assembly 266 that
meshes with the large gear on the back of the dial. On the rear of the housing
below cross shaft 269 is a metal mounting plate fastened with two screws. This
plate mounts the split gear and pinion assembly 266 and after loosening the two
mounting screws, the gears may be adjusted to a position that minimizes
backlash.
Mounted at the bottom front of the housing is stop arm assembly
260. The right (from front of chassis) end of the arm ends in a hook. The left
end has a roller that is held firmly against the outer edge of the dial by a
spring. A portion of the outer edge of the dial is cut away so that as the ends
of the dial scales are reached, and as the roller on the stop arm follows the
dial, the hook on the other end 9f the dial drops and engages the rotating stop
263 mounted on the tuning shaft in front of the pinion gear. This action
provides a positive stop at the ends of the tuning ranges on all bands.
The
tuning shaft is geared to the tuning capacitor "through worm gear 265, small
split gear 267, pinion gear and bushing assembly 268 and the split gear on the
tuning capacitor shaft. Pinion gear and bushing assembly 268 is fastened to the
cross shaft by means of two set screws. When these screws are loosened, the
tuning capacitor is disengaged from the dial and mask assembly, and the
relationship between the dial and the tuning capacitor may be
adjusted.
Attached near the top of the housing is guide arm assembly 271,
which, with the spring 274, positions detent wheel 273. The detent wheel is the
six-pointed wheel mounted near the end of the band switch shaft and serves to
position the band switches. The slotted coupling on the end of the band switch
shaft couples the band switch shaft to bracket and gear assembly
277.
32. SERVICING THE DIAL
AND MASK ASSEMBLY.
a.
REMOVAL.
Before the dial and mask assembly may be removed for servicing, the
front panel must be removed as directed in Paragraph 30a. When this has been
done, the dial and mask assembly is held in place by only two hex head mounting
screws found on the underside of the chassis. One of these screws is under. the
i-f shield plate 231, (see Figure 18) and is accessible only after this plate is
removed. After the two mounting screws are removed, the assembly may be lifted
from the chassis and serviced.
b. DISASSEMBLY.
To remove dial mask 256,
dial 257, dial index plate 258, or the band switch shaft, the taper pin
fastening the hub of the mask to the shaft must be removed. While this is being
done, be certain to support the shaft so excess strain will not be placed on the
center bushing and shaft. After the taper pin has been driven out, the parts may
be readily removed.
c. DIAL REPLACEMENT.
Before replacing the dial, be
certain that spring washer 289 between the dial hub and the housing is mounted
with its convex side towards the dial hub.
The dial is slipped on over the
center bushing and the teeth in the dial gear engaged with the teeth of
the
small pinion gear. This operation may result in an incorrect stop
position. If so, the dial gear will have to be adjusted a tooth at a time until
the correct position is reached. The hook on the roller arm should engage the
stop on the tuning shaft as the index mark on the low frequency end of the
13.5-18.0 mc scale lines up with the frequency indicator of the index plate.
However, the stop arm hook must not start to ,descend until the rotating stop
has passed under it on the last revolution of the tuning shaft.
d. STOP ARM
REPLACEMENT.
RepIacement of the stop arm may also require a readjustment to
obtain the correct stop position. This is done by lengthening or shortening the
roller end of the stop arm after loosening the two nuts on the arm.

Figure 9 – Radio Receiver BC-348-J, Dial and Mask
Assembly
e. REPLACEMENT OF DIAL AND MASK ASSEMBLY.
When the dial
and mask assembly is replaced, the position of coupling 275 on the bracket and
gear assembly and coupling 275 on the dial and mask assembly must be such that
the position of the dial mask will correspond with the band switch
position.
The correct relative positions are obtained when the mask is set to
the 200-500 kc position and when coupling link 164 (see Figure 16), connecting
the band switch drive mechanism to the band switch, is in a nearly vertical
position, even with the front of the output transformer and filter choke unit
155.
f. DIAL CALIBRATION.
Correct dial calibration may be obtained after
replacing a dial and mask assembly by adjusting the relation between the dial
and tuning capacitor. To do this, loosen the two set screws in pinion gear and
bushing assembly 268 until the gear will rotate freely on the shaft. Turn the
tuning condenser until it is completely closed (rotor plates meshed with stator
plates). Set the band switch for the 13.5-18.0 mc band. Turn the tuning knob
until the isolated index mark at the low frequency end of the dial scale is
aligned with the frequency indicator on the index plate. Tighten the two set
screws in the pinion gear and bushing assembly 268 carefully in order to avoid
changing the position of the tuning capacitor, and apply glyptal to the heads of
the set screws.
33. REMOVAL
OF ANTENNA, R-F, DETECTOR AND OSCILLATOR UNITS
a. When the removal of the top or bottom cover plate does not
give sufficient access to the antenna, r-f, or detector units, or when the
oscillator unit is serviced, the entire assembly must be removed. This may be
accomplished by repeating the following steps in the order given:
(1)
Unsolder the leads at the front of the unit.
(2) With the rear of the
receiver towards the repairman, remove top covers 225 on the unit to be serviced
and the adjacent unit at the left.
(3) Set the band switch control to the
3.5-6.0 mc band.
(4) One end of each retaining spring 165 is hooked over the
band switch arm nearest the front of the chassis. Use long nose pliers and lift
the ends of the two springs off the arms (for the antenna unit only one spring
must be removed). Also lift coupling links 164 over the ends of the arms.
(5)
Remove the tie strips on the top of the cans, front 229 and rear 227.
(6)
Remove the screws fastening the bottom tie strips 227 and 228 to the unit to be
removed.
(7) Remove the mounting screws at the front and rear of the
unit.
(8) Carefully lift the unit from the chassis.
b. To replace a unit,
reverse the procedure given above. Do not tighten the screws fastening the unit
until after the band switch sections have been reconnected and the band switch
operated a few times. This will allow the unit to reposition
itself.
34. TROUBLE LOCATION
AND REMEDY.
a. QUICK CHECK.
Most
service men, given a faulty receiver to repair, will seek a clue which will
result in a rapid location of the trouble. If the user can be questioned, a
helpful answer is often obtained.
A careful visual and mechanical inspection
of the chassis and connections is generally one of the first steps. Pulling at
the various parts, including resistors, capacitors, wires and solder
connections, will often locate a faulty connection.
Inspect parts and wiring
for grounds or shorted connections and open circuits. Inspect resistors and
coils for charred surfaces or discolorations that indicate an excessive current
condition. The odor of overheated insulation often tells the story of overload
carried by the conductor.
If nothing is disclosed by the visual and
mechanical inspection of the chassis and connections, the tubes may next be
checked, since they are often the cause of the faulty operation. This may be
done with a tube checker or by replacement with known good tubes.

NOTE
All tubes of a given type supplied with the
equipment shall be consumed prior to employment of tubes from general
stock.
If the tubes are good, connect the power to the chassis
(with latter out of the case) and again move and tap the various parts and wires
as a further means of locating opens, shorts and grounds. Hum, squeals, howls,
distortion and other audible indications, dead voltage points, etc., may
disclose the source of difficulty.
Failure of the quick check to disclose the
source of the trouble should be followed by the reading of voltages at socket
terminals and other points, continuity resistance measurements, realignment,
signal tracing to isolate the faulty stages and replacement of major units as
explained in succeeding paragraphs.
b. SENSITIVITY.
The normal sensitivity
(number of microvolts input to produce 10 milliwatts output into a 4,000 ohm
resistance load) of the receiver is less than 3 microvolts (except on range 1,
which is 5 microvolts) when measured as follows:
(1) With the AVC-OFF-MVC
switch in the MVC position, the c-w oscillator OFF, the crystal filter OUT and a
4,000 ohm non-inductive resistance as the output load, feed a modulated signal
from a signal generator into the receiver through a 100 mmf. dummy antenna.
Adjust the output of the signal generator until the receiver output is 10
milliwatts. Turn off the signal generator modulation and adjust the receiver
volume control until the noise output level is 2.5 milliwatts. Turn on the
modulation and raise the signal generator output until the 10 milliwatt output
is again indicated on the output meter.
(2) 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 retrim or realign the equipment unless
the sensitivity is found to be worse than 7 microvolts with new, average
tubes.
The receiver 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. All others must refrain from changing any of the adjustments of
the radio frequency circuits.

Figure 10 – Trouble Location and Correction Chart
c.
TROUBLE LOCATION AND CORRECTION PROCEDURE.
The following is a generalized
trouble shooting procedure which may be used if no clue to the trouble source
has been found. It has been divided into the following:
Equipment
required--See paragraph 34d
Weak or no signals on all bands;
Modulated
reception--See paragraph 34e
Weak or no signals on any one band;

Modulated reception--See paragraph 34f
Weak or no signals on all
bands;
C-W reception (modulated reception normal)-See paragraph 34g
d.
EQUIPMENT REQUIRED.
Few instruments other than those found in a standard set
analyzer (Test Set 1-56-A) are required in locating the most probable troubles
in this receiver. The individual instruments required are as follows:
(1) A
modulated test oscillator (standard signal generator) with a frequency range
from 200 to 18,000 kc with provision for calibration accuracy better than 0.1%
at aligning frequencies.
(2) *Voltmeter -- 1,000 ohms per volt, ranges: 0-10;
0-100; 0-250; 0-500 volts.
(3) *Continuity tester.
(4) *Out, put meter
rectifier type, 0-15 volt, 4,000 ohms.
(5) Microammeter, 0-200 Microamperes.

(6) Audio frequency oscillator.
(7) Headphones.
(8) Adapter FT-211
consisting of an 8-prong octal plug, and an 8-prong octal socket connected
together by a short length of 8-conductor cable, to permit use of the Test Set
1-56-A Analyzer on octal tubes.
* Part of Test Set 1-56-A.
e. 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 axe heard even
when known to be present, check the dynamotor voltages at the dynamotor terminal
strip (Figure 12, Socket Voltages). The voltages should approximate the values
shown. Conditions of measurements are:
Input voltage 28V; AVC.OFF-MVC switch
MVC; Crystal control OUT; C-W Osc. OFF; Volume Control maximum; Load 4,000 ohms
non-inductive resistance. If these voltage readings do not approximate the
values shown, the fuse should be checked, as well as the dynamotor and filter
circuits, wiring and components.
(2) TUBE CHECK.--If the voltages at the
dynamotor terminal strip approximate the values given, proceed to check all
tubes with the Model 685 tube checker for emission and characteristics or
replace all tubes with those of known average characteristics, if this has not
been done in the "Quick Check".
(3) CHECK OF SOCKET VOLTAGES.-- If tubes
check satisfactorily, or if after replacing with tubes known to be good the
sensitivity is still low, proceed to check all tube socket voltages as outlined
under Paragraph ~b(1) with Test Set 1-56-A. The average socket voltages for
Radio Receiver BC-548-(*) are given in Table C Socket Voltages.
(4) CHECK
CIRCUIT WIRING AND COMPONENTS.--If the tube socket voltages do not approximate
the values shown in Table C Socket Voltages, the associated circuits and
components should be checked for grounds, shorts and similar defects using Test
Set 1-56-A, the wiring diagram Figure 30, Table D Resistance Between Socket
Terminals and Ground, mad Table E.
(5) TEST OF AUDIO FREQUENCY
AMPLIFIER.--After checking socket voltages, circuit wiring and components,
proceed to the test of the audio frequency amplifier. This can be checked by
capacitively coupling a 400 cycle voltage of approximately 2 volts R.M.S. from
the detector signal diode to ground a capacitor of .5 inf. As an alternative, a
modulated 915 kc signal of 2 volts may be coupled through a .l mf capacitor from
the plate of the 3rd i-f Tube vt-ll6 to ground.
Proper functioning of the
audio amplifier will be indicated by an output of 50 milliwatts for the 2 volt
audio signal or 1 milliwatt output for the 915 kc signal input. Circuits, wiring
and components should be checked if this order of response is not
obtained.
(6) TEST OF INTERMEDIATE FREQUENCY AMPLIFIER.--Following a
satisfactory test of the audio amplifier, check the intermediate frequency
amplifier by capacitively coupling the modulated signal generator to the control
grid of the 1st detector tube and ground, through a 0.1 mf. capacitor, the
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 signal generator. (Approximately 35 microvolts input for 10
milliwatts output.) See Table G Alignment Chart, 1st i-f column, for connections
and detailed information.
(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 to do this, couple the signal generator to the
3rd i-f tube control grid through a 0.1 mf capacitor. Set the controls and make
the connections as described in Table G, Alignment Chart, 3rd i-f column. If the
receiver is in proper order from the output to this stage, a signal of 60,000
microvolts from the signal generator will provide a 10 milliwatt output level.
Then couple the signal generator to the 2nd i-f and 1st detector tubes as
explained in the next two columns in the alignment chart, and check the input
signal required for standard output. A faulty stage should be carefully checked
for shorts, grounds and faulty components using the wiring diagrams in Section V
and Resistance Tables D and E.
(8) ALIGNMENT OF I-F AMPLIFIER.—When all
stages have been tested, the i-f amplifier alignment may be checked and
realigning done if necessary. This is done by following the procedure as given
in the 1st, 2nd and 3rd i-f columns in Table G Alignment Chart.
(9) CHECK AND
ALIGNMENT OF C-W OSCILLATOR.-- The c-w oscillator is checked and adjusted after
setting the receiver controls as instructed for i-f alignment in Table G
Alignment Chart. A 915 kc signal of about 30 microvolts is fed into the control
grid of the 1st detector. Remove the modulation from the signal generator. Turn
the c-w oscillator to the ON position. Rotate the beat frequency knob to zero
beat position. The arrow on this knob should be vertical and pointing upward. If
it is not, loosen the set screws and set this knob properly. If no beat note is
heard, check the c-w oscillator circuit for grounds, shorts and defective
components using Test Set 1-56-A and the readings given in Table C Socket
Voltages, and Resistance Tables D and E.

Figure 11 – Crystal Filter Coil
(10) NEUTRALIZING THE
CRYSTAL CIRCUIT.--AII crystal circuits are properly neutralized at the factory
before being shipped. Ordinarily, reneutralizing is not required unless the
neutralizing coil and capacitor (Ref. 150) or crystal and switch assembly (Ref.
160 and 167) have been replaced, or unless the crystal circuit appears to be
excessively broad. If neutralizing is necessary it may be done as
follows:
Disconnect the output meter and put an 0-200 microammeter in the
line from the volume control (rear section) to the second detector cathode. This
is most easily done by unsoldering the lead at the volume control lug.
Set
all receiver and signal generator controls as for i-f alignment. Turn the
crystal control to the IN position. Rotate the signal generator tuning knob
slightly until the crystal peak is located, observing the microarnmeter. Tune
carefully to the exact peak. Note the frequency of the generator. This will be
the frequency of the crystal, which should not be more than 1.3 kc above or
below 915 kc.
Detune the signal generator 10 kc below the crystal frequency.
For example, if the crystal frequency is 916.2 kc, tune the signal generator to
906.2 kc.
Turn modulation of signal generator off and increase the output to
about 1 volt.
Extending from the neutralizing coil are two lugs A and B and
two corresponding wires C and D. See Figure 11.
Extending from lug B is a
wire E passing around the edge of the coil form. Increase and decrease the
capacity between wires C and D by bending them closer to each other and farther
apart until the micrometer reading is at a minimum. In cases where low capacity
is required, cut off the wires C and D. If extreme low capacity is required, cut
off lugs A and B. Also move wire E away from lug A.
(11) CHECK OF HETERODYNE
OSCILLATOR. Having checked the functioning of the i-f and audio amplifiers, if
signals are not heard on any band, the heterodyne oscillator should be checked
for oscillation.
This can be done by coupling a 915 kc signal through a .l
mf. capacitor to the control grid of the 1st detector. Short the stator of the
oscillator section of the tuning capacitor (Section I-D) to ground. The output
should go up. This indicates that the oscillator was functioning satisfactorily.
Do this on all bands.
(12) R-F AMPLIFIER AND HETERODYNE OSCILLATOR. -- Having
completed the test and alignment of the audio amplifier, i-f amplifier and
heterodyne oscillator, test the r-f amplifier as follows:
Turn the band
switch to band No. 1. Set the receiver and generator controls as described in
Table G Alignment Chart, No. 1 band column, but use a 0.1 mf. dummy antenna and
make the antenna connection to the control grid of the 1st detector tube. Turn
the tuning control until the test signal is picked up and carefully tuned in.
The receiver dial should indicate the same frequency as the signal generator. If
there is more than .75 per cent variation, the oscillator must be realigned as
described below.
Proceed to check the r-f amplifier, stage-by-stage, working
towards the antenna. Couple the signal generator through the 0.1 mf. dummy
antenna to the control grids of the 2nd r-f tube and the 1st r-f tube. Then,
using a dummy antenna of 100 mmf, couple the signal generator to the antenna
binding post. At each stage, the receiver output should increase substantially
indicating the stage gain. If there is no gain, the stage should be checked for
defective components, shorts and grounds, using Tables C, D and E.
Do this on
all bands.
(13) R-F AMPLIFIER AND HETERODYNE OSCILLATOR ALIGNMENT.--
Instructions are given in Table G Alignment Chart for completely aligning the
r-f amplifier and heterodyne oscillator circuits. Start with number 1 band
column and continue with all columns to the right.
f. 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 the 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 34e(11) to 34e(13) should be followed for the
defective band or bands.
g. 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 34e(9) and
34e(10).
h. MEASUREMENTS WITH TEST SET 1-56-A.
The tables that follow
show voltage and resistance measurements made from the chassis ground to the
tube socket terminals and other important points. These measurements were made
with Model 665 Analyzer and other equipment contained in Test Set 1-56-A. They
are typical readings that will be approximated in Signal Corps Repair Shops when
using this equipment on receivers in good operating condition. When faulty
operation of Radio Receiver BC-348-(*) is encountered, see that all plugs are
firmly seated and check carefully the cables and plugs with Model 564
Voltohmmeter as instructed on page 11 of the instruction book for Test Set
1-56-A under "Detailed Tests on Radio Sets." If the trouble is not located in
the cables, remove the chassis and check the tubes with Model 685, Tube Tester
as instructed on page 11 of the test set instructions. After this is done,
proceed with the voltage and continuity measurements described below:
(1)
VOLTAGE AND CURRENT MEASUREMENTS. -- Instructions are given on page 12 of the
test set instruction book for making voltage and current measurements with Model
665 Analyzer and Model 666 Socket Selector. Latest instructions which include
the use of the octal socket adapter are necessary. These instructions should be
followed for measurements on all except the second r-f tube.

NOTE
When making measurements at the second r-f tube, the
flexible lead on the orange adapter plug must be removed from the pin jack
marked GND and inserted in the outer No. 3 jack, as the shield is not
grounded.
Great care must be exercised when inserting the
analyzer plug into the sockets on the r-f tube shelf in order to avoid damage to
the prongs, as the shelf is mounted at an angle that just permits the insertion
of the plug.
PROCEDURE
1. With the chassis removed from the cabinet and
power supplied to the receiver through the connector plug, place the controls in
the positions designated on the voltage table
2. Set up the 665 Analyzer and
the 666 Socket Selector for making readings on metal tubes as instructed on page
12 of the test set instructions. (See note above for second r-f tube
setup.)
3. Throw the analyzer AC-DC switch to DC and the VOLTS-MA-OHMS switch
to VOLTS-MA.
4. Use the voltmeter ranges indicated on the voltage table and
proceed to make the desired readings between the chassis ground and the
terminals given on the voltage chart.
When using Model 666 Socket Selector to
read voltages, the capacity between the leads in the cable may cause
oscillations which will affect voltages and currents. In the case of a voltage
that is found to read considerably different from that shown on the table,
replace the tube in the receiver and read the voltage at the socket terminal
with test prods before attempting to locate the source of the incorrect
voltage.
(2) RESISTANCE AND CONTINUITY MEASUREMENTS. -- Resistance and
continuity measurements are to be made with Model 665 Analyzer as instructed on
page 12 of test set instructions.
The tables that follow show resistance
measurements made from the chassis ground to the tube socket terminals and other
important points that are easily accessible. These measurements may be made with
the test prods at the terminals, or in the case of tube sockets, Model 666
Socket Selector may be used as instructed in paragraph 34h(1).
Whenever
possible, select an ohmmeter range that will allow the readings to be made on
the 0 to 60 section of the ohmmeter scale, as greater accuracy is obtained over
this portion of the scale.
Always set the ohmmeter to zero ohms each time a
different scale is used, or before making the initial measurements. To do this,
short together the two test prods and turn the "battery adjustment" knob until
the meter reads exactly zero ohms. If the meter cannot be adjusted to zero, the
battery in the analyzer will have to be changed. For instructions, see the
section on battery replacement under "Maintenance" on page 42 of the test set
instruction book.
PROCEDURE
1. With the chassis removed from the cabinet
and no power supplied to the receiver, place the controls in the positions
designated on the resistance and continuity chart.
2. Set up the Model 665
Analyzer and the Model 666 Socket Selector, if used, as instructed on page 12 of
the test set instructions.
3. Throw the analyzer AC-DC switch to DC and
VOLTS-MA-OHMS switch to OHMS.
4. Proceed to make the desired readings between
chassis ground and terminals as indicated on the resistance and continuity
tables.
i. FAILURE OF DIAL LIGHTS.
The two dial lamps are connected in
series; hence the lack of dial illumination does not indicate failure of both
lamps. Removal of the dial light housing gives ready access to the lamps.

TABLE C -- TUBE SOCKET VOLTAGE CHART
THE VOLTAGES ARE
READ BETWEEN THE SOCKET TERMINAL AND GROUND UNDER THE
FOLLOWING CONDITIONS
UNLESS OTHERWISE STATED:
Headphones, Antenna and Ground disconnected.
AVC-OFF-MVC Switch in MVC position. Band Switch set for Band No. 1, 200-500 kc.
All readings taken with 1000 ohm per volt meter. Tuning Control set at 200 kc.
Plate and Screen voltages read on 500 volt scale. Other voltages read on
appropriate scales. Crystal Control OUT position. C-W Oscillator OFF position.
Volume Control maximum. Dial Light Control maximum. See last paragraph in 34b(1)
regarding possibilities of fictitious voltages when using socket
analyzer.

Socket, terminal or Block No.

6SK7 VT-117 1st R-F

6SK7 VT-117 2st R-F

6SA7 VT-150 1st Det.

6SK7 VT-117 1st I-F

6SK7 VT-117 2nd I-F

6SJ7 VT-116 3rd I-F

6SR7 VT-233 2nd Det. C-W
Osc.

6K6 VT-152 Audio

1.

Shell

0 V.

Shell

8 V.

Note A

Shell

0 V.

Shell

0 V.

Shell

0 V.

Shell

0 V.

Shell

0 V.

0 V.

2.

Heater

18.9 V

Heater 12.6 V.

Heater

0 V.

Heater

25.2 V.

Heater

12.6 V.

Heater

6.3 V.

Grid

6 V.

Note D

Heater

25.3 V.

3.

Sup

0 V.

Sup.

3.8 V.

Plate

210 V.

2 Ma.

Sup.

5.8 V.

Sup.

5.8 V.

Sup.

16.5 V.

Cathode

16.5 V

Plate

200 V.

18 Ma.

4.

Grid

0 V.

Grid

0 V.

Screen

100 V.

Note C

6.2 Ma.

Grid

0 V.

Grid

0 V.

Grid

14 V.

Diode

2 V.

Screen

210 V.

3.4 Ma.

5.

Cathode

3.8 V.

Cathode

3.8 V.

Osc Grid

-.4 V

Cathode

5.8 V.

Cathode

5.8 V.

Cathode

16.5 V.

Diode

0 V.

Grid

-8 V.

Note E

6.

Screen

100 V.

2 Ma.

Screen

83 V.

1.6 Ma.

Cathode

0 V.

Screen

105 V.

1.6 Ma.

Screen

105 V.

1.6 Ma.

Screen

100 V.

.85 Ma.

Plate

55 V.

Note B

(C-W Osc. ON)

.95 Ma

Tie Terminal

.2 V.

7.

Heater

12.6 V

Heater

6.3 V

Heater

6.3 V

Heater

18.9 V

Heater

18.9 V

Heater

0 V

Heater

6.3 V

Heater

18.9 V

8.

Plate

100 V.

9 Ma.

Plate

170 V.

6 Ma.

Grid

-1.8 V.

Plate

185 V.

5.4 Ma.

Plate

210 V.

5.8 Ma.

Plate

210 V.

3.5 Ma.

NOTE F

Heater

12.6 V.

Cathode

0 V.

Note A--The shell (No. I Terminal) of the 2nd r-f
tube is not at ground potential but is connected to the cathode. For that reason
the flexible Lead from the Octal Socket Adapter is not inserted into the ground
pin jack but is inserted into the Adapter No. 3 outer pin jack.
Note B--55
volts read at socket terminal. 45 volts read with adapter plug--see last
paragraph of 34h(1).
Note C---60 volts read with analyzer. See Note
B.
Note D---As read on 50-volt scale.
Note E--As read on 500-volt
scale.
Note F---450 volts read with analyzer. See Note B.

TABLE D -- RESISTANCES BETWEEN SOCKET TERMINALS AND
GROUND
RESISTANCE READINGS ARE MADE UNDER THE FOLLOWING
CONDITIONS:
Power Plug, Headphones, Antenna and Ground Disconnected. C-W Osc.
OFF position. Volume Control maximum. Tubes cold and left in socket to complete
series heater connections. Dial Light Control maximum.AVC-OFF-M-VC Switch MVC
position (except for values preceded by AVC which are read in that position).
Band Switch set for Band No. 1, 200-500 kc. Tuning Control set at 200 kc.
Crystal Control OUT position. Unless otherwise specified, all resistances shown
in ohms.

Socket, terminal or Block No.

6SK7 VT-117 1st R-F

6SK7 VT-117 2st R-F

6SA7 VT-150 1st Det.

6SK7 VT-117 1st I-F

6SK7 VT-117 2nd I-F

6SJ7 VT-116 3rd I-F

6SR7 VT-233 2nd Det. C-W
Osc.

6K6

VT-152 Audio

1.

Shell

0.

Shell

250

Note A

Shell

0

Shell

0

Shell

0

Shell

0

Shell

0

0

2.

Heater

6.5

Heater 5.5.

Heater

0

Heater

6.5.

Heater

5.5.

Heater

3.

Grid

100,000

Heater

6.5

3.

Sup

0

Sup.

250

Plate

520

Sup.

420

Sup.

420

Sup.

4000

Cathode

4000

Plate

1200

4.

Grid

1 Meg AVC 3.3 Meg

Grid

90,000 AVC 2.4 Meg

Screen

20,000

Grid

15,000 AVC 2.3 Meg

Grid

15,000 AVC 2.3 Meg.

Grid

3500

Diode

400,000

Screen

900.

5.

Cathode

250

Cathode

250

Osc 80,000

Cathode

420

Cathode

420

Cathode

4000

Diode

260,000 AVC 2 Meg

Grid

1.1 Meg

6.

Screen

15,000

Screen

500,000

Cathode

1.2

Screen

500,000

Screen

500,000

Screen

120,000.

Plate

500,000 5 Meg Osc ON

Tie Terminal

.5

7.

Heater

5.5

Heater

4

Heater

4

Heater

6.5

Heater

6.5

Heater

0

Heater

3

Heater

6.5

8.

Plate

15,000

Plate

10,000

Grid

1 Meg

Plate

5500

Plate

500

Plate

1500

Heater

5.5

Cathode

0

Note A--The shell (No. l Terminal) of the 2nd r-f
tube is not at ground potential but is connected to the cathode. For that reason
the flexible Lead from the Octal Socket Adapter is not inserted into the ground
pin jack but is inserted into the Adapter No. 3 outer pin jack.

TABLE E -- ADDITIONAL POINTS TO MAKE CONTINUITY

RESISTANCE READINGS TO GROUND
RESISTANCE READINGS MADE
UNDER THE SAME CONDITIONS
AS FOR TUBE SOCKET READINGS

POWER CONNECTOR

(175)

OUTPUT TRANSFORMER

And CHOKE (155-A and B)

DYNAMOTOR

TERMINAL STRIP

Terminal

Numbers

On Connector

Resistance

Terminal

Numbers

On Unit

Resistance

Readings from low

Voltage to High

Voltage end

1.

150 ohms

1.

16 ohms

0

2.

500 ohms

2.

150 ohms

1.5 ohms Note A

3.

1.5 ohms Note A

3.

500 ohms

1.5 ohms Note A

4.

1.5 ohms Note A

4.

1200 ohms

270 ohms

5.

0

5.

25 ohms

500 ohms

6.

500,000 ohms

6.

270 ohms

7.

0

8.

0

Note A-This reading may be found to vary and may
be as high as 11 ohms. This condition is normal and jarring the dynamotor will
often return the reading to approximately the value shown.

Figure 12 – Socket Voltages

TABLE F -- COIL CHARACTERISTICS
Inductance Readings Given
in Microhenries. All D.C. Resistance Readings Given in Ohms. Inductance
Measurements Made at 1,000 Cycles. Resistance Readings Within +/-10%. With Coil
Removed from Shield. "Q" Readings Made With Coil Removed From Shield. Primary
Inductance Readings Within +/-5%. "Q" Readings Within +/-20%. Secondary
Inductance Readings Within +/-2%..

Band

No.

Coil

Ref. No.

Inductance

D.C. Resistance

Q

Frequency for

"Q" Measurement

Pri.

Sec.

Pri.

Sec.

Pri.

Sec.

ANTENNA UNIT

1

120

1348.7

1864.7

125

16.5

88

500 KC(5)

2

121

298.7

27.8

13.8

0.75

145

1.5 MC(5)

3

122

39.0

4.6

4.5

0.18

122

6.0 MC(5)

4

123

14.5

1.1

3.0

0.11

157

9.5 MC(5)

5

124

10.2

0.6

2.4

0.03

139

13.5 MC(5)

6

125

3.0

0.1

1.4

0.02

173

18.0 MC(5)

Band

No.

Coil

Ref. No.

Inductance

D.C. Resistance

Q

Frequency for

"Q" Measurement

Pri.

Sec.

Pri.

Sec.

Pri.

Sec.

R-F UNIT

1

126

(7)

2439.7

61.5

74

500 KC(5)

2

127

(7)

41.6

0.49

3.1

99

1.5 MC(5)

3

128

(7)

7.0

0.53

0.2

161

6.0 MC(5)

4

129

(7)

2.0

0.52

0.1

162

9.5 MC(5)

5

130

(7)

1.2

0.52

0.04

217

13.5 MC(5)

6

131

(7)

0.3

0.75

0.03

220

18.0 MC(5)

Band

No.

Coil

Ref. No.

Inductance

D.C. Resistance

Q

Frequency for

"Q" Measurement

Pri.

Sec.

Pri.

Sec.

Pri.

Sec.

DETECTOR UNIT

1

132

(7)

2433.7

61.5

74

500 KC(5)

2

133

(7)

41.6

0.49

3.1

99

1.5 MC(5)

3

134

(7)

7.0

0.53

0.2

161

6.0 MC(5)

4

135

(7)

2.0

0.52

0.1

162

9.5 MC(5)

5

136

(7)

1.2

0.52

0.04

217

13.5 MC(5)

6

137

(7)

0.3

0.75

0.03

220

18.0 MC(5)

Band

No.

Coil

Ref. No.

Inductance

D.C. Resistance

Q

Frequency for

"Q" Measurement

Pri.

Sec.

Pri.

Sec.

Pri.

Sec.

OSCILLATOR UNIT

1

138

(7)

346.9

1.2

6.3

96

1.0 MC

2

139

(7)

25.8

0.7

1.1

152

12.5 MC

3

140

(7)

5.3

0.54

0.24

114

7.0 MC

4

141

(7)

1.6

0.9

0.07

156

10.5 MC

5

142

(7)

1.5

0.4

0.06

160

12.5 MC

6

143

(7)

0.4

0.76

0.03

179

17.0 MC

Band

No.

Coil

Ref. No.

Inductance

D.C. Resistance

Q

Frequency for

"Q" Measurement

Pri.

Sec.

Pri.

Sec.

Pri.

Sec.

I-F COILS

1st I-F

147

120(1)

120(1)

1.5(2)

1.5(2)

129

128

915 KC(3)

2nd I-F

148

120(1)

120(1)

1.5(2)

1.5(2)

129

128

915 KC(3)

3rd I-F

149

120(1)

242(1)

1.5(2)

5.0(2)

128

112

915 KC(3)

Band

No.

Coil

Ref. No.

Inductance

D.C. Resistance

Q

Frequency for

"Q" Measurement

Pri.

Sec.

Pri.

Sec.

Pri.

Sec.

MISCELLANEOUS COILS

Xtal Neut

150

2.03

52.75

56(6)

400 KC

C-W Osc.

151

1.9(1)

125.5(1)

0.21(2)

1.74(2)

77

915 KC(3)

I-F Trap

152

60.9

2.3

74

915 KC

Choke

420

466.2(4)

12.0

Choke

421

79.1(4)

0.15

Coil in can and resonated to 915 KC. Readings
made at 1,000 cycles.
+/-5%.
Reading made with coil in can, tuning
capacitor removed and replaced with Q Meter capacity of equal value and iron
cores adjusted until circuit resonates at 915 KC.
+/-10%.
Primary
shorted.
Measured across total inductance.
Coils 126, 132 and 138 have no
primaries. Other primary inductances too small for practical measurement.

TABLE G -- ALIGNMENT CHART

Follow alignment in order
from left to right. Start with 3rd I-F adjustment in first column, then
2nd I-F, etc. Connect signal generator to ground post of receiver
using a short heavy lead. Allow receiver and signal generator to heat up for
several minutes.

RECEIVER CONTROLS

3rd I.F.

2nd I.F.

1st I.F.

No. 1 Band

C.W. Osc.

OFF

OFF

OFF

OFF

Crystal

OUT

OUT

OUT

OUT

AVC-OFF-MVC

MVC

MVC

MVC

MVC

Volume

Maximum

Maximum

Maximum

Maximum

Band Sw. Setting

No. 2 Band

1.5-3.5 Mc

No. 2 Band

1.5-3.5 Mc

No. 2 Band

1.5-3.5 Mc

No. 1 Band

200-500 kc

Tuning Control

Position

1.5 Mc

1.5 Mc

1.5 Mc

470 kc

SIGNAL GENERATOR

Antenna Connections

3rd I.F. Tube

VT-116 Control Grid Socket Terminal No. 4

2nd I.F. Tube

VT-117 Control

Grid Socket

Terminal No. 4

1st Det. Tube

VT-150 Control

Grid External

Terminal

Antenna Post

Dummy Ant.

0.1 MFD

0.1 MFD

0.1 MFD

100 MMF

Frequency Setting

915 kc. +/-1000

Cycles or less

915 kc. +/-1000

Cycles or less

915 kc. +/-1000

Cycles or less

470 kc

PROCEDURE

Sensitivity

60,000 Microvolts

2,100 Microvolts

35 Microvolts

9 Microvolts

See Note A

Trimmers Adjusted

3rd I.F.

Tuning Cores

See Fig. 13

2nd I.F.

Tuning Cores

See Fig. 13

1st I.F.

Tuning Cores

See Fig. 13

No. 1 Trimmers on Osc. Det. R.F. and Antenna Units See
Fig. 13

Procedure

Increase Sig. Gen to about 1 volt. Adjust 3'rd I.F.
Tuning Core screws, first top screw and then bottom. Adjust top and bottom
screws again and then a third time to maximum output.

Decrease input from Sig. Gen. to 10 milliwatt output
level. Adjust 2nd I.F. Tuning Core screws, top screw first, then bottom.
Adjust top and bottom screws again, and then a third time to maximum
output.

Decrease input from Sig. Gen. to 10 milliwatt output
level. Adjust 1st I.F. Tuning Core screws, top screw first, and then
bottom. Adjust top and bottom screws again, and then a third time to
maximum output.

Adjust No. 1 Trimmer on Osc. Unit to max. output. Then
adjust Trimmers on Det., R.F., and Antenna Units, in that order to maximum
output, and decrease signal as required to maintain 10 milliwatts output.
Repeat above.

NEUTRALIZING THE CRYSTAL CIRCUIT
Neutralizing
of the crystal circuit is not required in ordinary alignment procedure. It
should not be attempted unless one of the neutralizing circuit elements as
mentioned in Paragraph 34e(10) have been replaced or unless the tuning band of
the crystal circuit appears to be excessively wide.
C-W OSCILLATOR
CHECK
The C-W Oscillator adjustment is likewise not required in ordinary
alignment procedure. It may be done as explained in Paragraph 34e(9).

215 kc.

Osc. Padder

No. 2 Band

No. 3 Band

No. 4 Band

No. 5 Band

No. 6 Band

OFF

OFF

OFF

OFF

OFF

OFF

OUT

OUT

OUT

OUT

OUT

OUT

MVC

MVC

MVC

MVC

MVC

MVC

Maximum

Maximum

Maximum

Maximum

Maximum

Maximum

No. 1 Band

200-500 kc.

No. 2 Band

1.5-3.5 Mc

No. 3 Band

3.5-6 Mc

No. 4 Band

6-9.5 Mc

No. 5 Band

9.5-13.5 Mc

No. 6 Band

13.5-18 Mc

215 kc.

3.3 Mc

5.7 Mc

9.0 Mc

13.0 Mc

17.3 Mc

Antenna Post

Antenna Post

Antenna Post

Antenna Post

Antenna Post

Antenna Post

100 MMF

100 MMF

100 MMF

100 MMF

100 MMF

100 MMF

Shut Off Signal

Generator See Note B

3.3 Mc

5.7 Mc

9.0 Mc

13.0 Mc

17.3 Mc

9 Microvolts

See Note A

9 Microvolts

See Note A

9 Microvolts

See Note A

9 Microvolts

See Note A

9 Microvolts

See Note A

215 kc. Oscillator Padder

See Fig. 13

No. 2 Trimmers on Osc. Det. R.F. and Antenna

See Fig. 13

No. 3 Trimmers on Osc. Det. R.F. and Antenna

See Fig. 13

No. 4 Trimmers on Osc. Det. R.F. and Antenna

See Fig. 13

No. 5 Trimmers on Osc. Det. R.F. and Antenna

See Fig. 13

No. 6 Trimmers on Osc. Det. R.F. and Antenna

See Fig. 13

Adjust 215 kc. Osc. Padder to max. output. Repeat No. 1
Band adjustment & 215 kc. Osc. Padder adjustments 2 or 3 times to
insure correct tracking and dial calibration.

Adjust No. 2 Trimmer on Osc. Unit to max. output. Then
adjust trimmers on Det., R.F., & Ant. Units in that order to max.
output and decrease signal as required to maintain 10 milliwatt output.
Repeat above.

Adjust No. 3 Trimmer on Osc. Unit to max. output. Then
adjust trimmers on Det., R.F., & Ant. Units in that order to max.
output and decrease signal as required to maintain 10 milliwatt output.
Repeat above.

Adjust No. 4 Trimmer on Osc. Unit to max. output. Then
adjust trimmers on Det., R.F., & Ant. Units in that order to max.
output and decrease signal as required to maintain 10 milliwatt output.
Repeat above.

Adjust No. 5 Trimmer on Osc. Unit to max. output. Then
adjust trimmers on Det., R.F., & Ant. Units in that order to max.
output and decrease signal as required to maintain 10 milliwatt output.
Repeat above.

Adjust No. 6 Trimmer on Osc. Unit to max. output. Then
adjust trimmers on Det., R.F., & Ant. Units in that order to max.
output and decrease signal as required to maintain 10 milliwatt output.
Repeat above.

NOTE A – SENSITIVITY
Adjust signal
generator until receiver output is 10 milliwatts. Turn off modulation of signal
generator. Turn down receiver volume control until 0.3 milliwatts of noise
output is reached. Turn on generator modulation again, and raise generator
output to obtain 10 milliwatts on output meter.
NOTE B
This adjustment
uses the noise developed in the receiver as the only signal.
VI. SUPPLEMENTARY DATA
35. GENERAL.
a. The
table of replaceable parts that follows omits standard types of hardware such as
screws, rivets, washers and other miscellaneous parts used in Radio Receiver
BC-348-(*).
Those parts having identical reference numbers followed by a
hyphen and letter are constructed as common assemblies. Reference numbers
followed by a hyphen and number indicate a part used two or more times.
The
figure numbers of the illustrations in which the parts are shown are given in
parenthesis directly below the reference numbers in the first column. Likewise,
the quantity used of each item, if more than one and not indicated in the
reference number column by a hyphen and number, is shown in parenthesis in the
description column below the description.
In some positions, two capacitors
or resistors have been used connected in parallel in place of the individual
unit shown in the schematic, illustrations and parts Iht. If the replacement of
any of these units is ever found to be necessary, the correct individual part
listed in the parts list should be used.
b. The table that follows the Table
of Replaceable Parts in Section VII serves as a guide to the alphabetical code
used in the drawing number column to indicate the manufacturer or source of
supply for replacement parts.

Figure 13 – Location of Trimmer Capacitors
36. CERAMIC CAPACITORS.
In many receivers ceramic capacitors are used in place of the
mica capacitors shown in various illustrations. Either type may be used for
replacement purposes. The values of certain ceramic capacitors used in Radio
Receiver BC-348-Q have been changed during production, The parts affected are
shown in the following listing as well as original and new values. For
replacement use, capacitors of the new and old values are directly
interchangeable.

Reference Number

Old Description

New Description

32

Ceramic capacitor

138 mmf. +/- 1%

Ceramic capacitor

134 mmf. +/- 1%

33

Ceramic capacitor

110 mmf. +0 -5%

Ceramic capacitor

106 mmf. +0 -5%

40

Ceramic capacitor

74 mmf. +/- 2.5%

Ceramic capacitor

72 mmf. +/- 2.5%

43

Ceramic capacitor

62 mmf. +/- 3%

Ceramic capacitor

60 mmf. +/- 3%

48

Ceramic capacitor

22 mmf. +/- 5%

Ceramic capacitor

20 mmf. +/- 5%

49

Ceramic capacitor

20 mmf. +/- 5%

Ceramic capacitor

18 mmf. +/- 5%

50

Ceramic capacitor

20 mmf. +/- 1.5%

Ceramic capacitor

1.5%

37. HEATER SHUNT
RESISTOR.
The heater shunt resistor,
(refer to ref. 82) 190 ohms, 3 watts, connected between ground and pin 2 of the
first r-f tube socket, has been replaced with two 390 ohm, 2 watt resistors
(refer to ref. nos. 114-1 and 114-2) connected in parallel. The 190 ohm, 3 watt
resistors are supplied in spare parts groups and may be used for replacement in
all equipments.
38. STATIC
DRAIN RESISTOR.
Receivers stamped "Ml"
on the front panel have a one megohm resistor (refer to ref. no. 87-6) connected
between the antenna post and ground. This resistor has been added to serve as a
means of discharging static charges.
39. OUTPUT CONNECTIONS.
The output transformer (refer to ref. no. 155-A) is provided with both
high and low impedance output terminals. Receivers with all output circuits
connected for low impedance output are identified by a decalcomania on the front
panel reading as follows:

NOTICE

Connected

for Iow

impedance

output

As shown on the schematic diagram, terminal 1 on
the output transformer (refer to ref. no. 155-A) is the low impedance connection
and terminal 2 is the high impedance connection. The output impedance may be
easily changed on any receiver by reconnecting the wire from terminal 2 to
terminal 1 if low impedance is desired, or from terminal I to terminal 2 if high
impedance is desired.
40.
MOISTURE-FUNGUS RESISTANCE.
The Radio
Receivers BC-348-Q that are stamped "MFP" on the chassis are constructed of
components which have been moisture-fungus proofed in accordance with the
general requirements of Specification 712202-A.

Figure 14 -- Radio Receiver BC-348-J, Front View

Figure 15 -- Radio Receiver BC-348-J, Rear View of
Chassis

Figure 16 -- Radio Receiver BC-348-J, Top View of
Chassis

Figure 17 -- Radio Receiver BC-348-J, Bottom View of
Chassis

Figure 18 -- Radio Receiver BC-348-J, Partial Bottom
View

Figure 19 -- Mountings FT-154-J, FT-154-Q and FT-154-AA with
Plug Assemblies, Rear View

Figure 20 -- Radio Receiver BC-348-J, Front View of
Cabinet

Figure 21 -- Antenna Unit

Figure 22 -- R.F. Unit

Figure 23 -- Detector Unit

Figure 24 -- Oscillator Unit

Figure 25 -- I-F C-W Oscillator and Crystal Coil
Assemblies

Figure 26-- Radio Receiver BC-348-J, Schematic Diagram (part
1)

Figure 26-- Radio Receiver BC-348-J, Schematic Diagram (part
2)

Figure 27-- I-F Transformers, Wiring Diagram

Figure 31-- Antenna, R.F., Detector and Oscillator Units, Wiring
Diagram

Figure 33-- Radio Receiver BC-348-Q, Schematic Diagram (part
1)

Figure 33-- Radio Receiver BC-348-Q, Schematic Diagram (part
2)

Figure 34-- Radio Receiver BC-348-Q, Wiring Diagram of
Chassis