Ge Bc-375-e User manual

PRELIMINARY INSTRUCTIONS

FOR

RADIO TRANSMITTER BC-375-E

AND

ASSOCIATED EQUIPMENT

Manufactured by

GENERAL ELECTRIC COMPANY

U.S.A.

PUBLISHED BY AUTHORITY

THE CHIEF SIGNAL OFFICER

Order No. 16-WFSCPD-42

TABLE OF CONTENTS

Par.

SECTION I

DESCRIPTION OF COMPLETE EQUIPMENT

Page

General



SECTION II

INSTALLATION

General



Radio Transmitter BC-375-E (Includes Mounting FT-151-A)



Dynamotor Unit PE-73-C



Cords



Antennas



Antenna Tuning Unit BC-306-A



Reel RL-30-B and Connector Clamp MC-163



Interconnection with the Radio Receiver in Aircraft



SECTION III

PREPARATION FOR USE

10.

General



11.

Radio Transmitter BC-375-E .

12.

Operation at High Altitude ....



13.

Dynamotor Unit PE-73-C



SECTION IV

OPERATION

14.

Radio Transmitter Adjustment



15.

Antenna Adjustments



16.

Dynamotor Unit PE-73-C



17.

Reel RL-30-B



18.

Operating Routine...



19.

Summary Covering Normal Operation



SECTION V

MAINTENANCE AND REPAIR

20.

Inspection



21.

Lubrication



22.

Service Notes

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iii

LIST OF ILLUSTRATIONS

Fig. 1. Radio Transmitter BC-375-E, Schematic Diagram

Fig. 2. Radio Transmitter BC-375-E, Connection Diagram

Fig. 3. Radio Transmitter BC-375-E, Dimensions and Weights

Fig. 4. Transmitter Tuning Units, TU-5-B, TU-6-B, TU-7-B, TU-8-B,

TU-9-B, TU-10-B, and TU-26-B ; Schematic Diagrams

Fig. 5. Transmitter Tuning Unit TU-5-B, Connection Diagram

Fig. 6. Transmitter Tuning Unit TU-6-B, Connection Diagram

Fig. 7. Transmitter Tuning Unit TU-7-B, Connection Diagram

Fig. 8. Transmitter Tuning Unit TU-8-B, Connection Diagram

Fig. 9. Transmitter Tuning Unit TU-9-B, Connection Diagram

Fig. 10. Transmitter Tuning Unit TU-10-B, Connection Diagram

Fig. 11. Transmitter Tuning Unit TU-26-B, Connection Diagram

Fig. 12. Case CS-48, Dimensions and Weight

Fig. 13. Dynamotor Unit PE-73-C, Connection Diagram

Fig. 14. Dynamotor Unit PE-73-C, Dimensions and Weights

Fig. 15. Antenna Tuning Unit BC-306-A, Connection Diagram

Fig. 16. Antenna Tuning Unit BC-306-A, Dimensions and Weights

Fig. 17. • Mounting FT-115-B, Dimensions and Weight

SECTION I

DESCRIPTION OF COMPLETE EQUIPMENT

1. GENERAL

Radio Transmitter BC-375-E with its associated

equipment is designed for aircraft and for applica-

tions requiring a medium power equipment with the

characteristics of strength, light weight, flexibility

and portability. The equipment is designed to provide

communication by voice, tone, or continuous-wave

telegraph over the frequency range of 150 to 12,500

kilocycles. However, only the frequencies covered by

the transmitter tuning units listed in next column

are available for use with this equipment.

Suitable tuning equipment is provided in the radio

transmitter to permit operation into a wide range of

antennas over most of the frequency range. Antenna

Tuning Unit BC-306-A may be used to extend the

range of antenna tuning for frequencies between 150

and 800 kilocycles.

TU-5-B

1500-3000 kc

5. TU-9-B

7700-10,000 kc

2. TU-6-B

3000-4500 kc

6. TU-10-B

10,000-12,500 kc

3. TU-7-B

4500-6200 kc

7. TU-26-B

200-500 kc

4. TU-8-B

6200-7700 kc

Dynamotor Unit PE-73-C furnishes the power

required by the radio transmitter and operates from a

24/28 volt storage battery or other direct-current

source used in aircraft.

SECTION II

INSTALLATION

GENERAL

Radio Transmitter BC-375-E and associated equip-

ment are part of aircraft radio sets. Installation con-

ditions vary widely, and no detailed single set of

installation instructions can be given in this book.

However, separate instructions are furnished for each

radio set and only such general instructions as can be

followed in all installations are given in the following

paragraphs.

The requirements for shielding and bonding of

cords in aircraft installations are very rigid, so that

difficulties with extraneous pick-up from the radio

receiver side tone and radio frequency from the

power supply will be minimized.

RADIO TRANSMITTER BC-3 7 5-E

(Includes Mounting FT-1 5 1-A)

The radio transmitter outline dimensions and

weights are shown in 1'ig. 3.

The radio transmitter should be located in relation

to its associated equipment so that dynamotor and

antenna leads will be of minimum length. Two types

of shock-absorber mountings are available for use with

the radio transmitter: Mounting FT-151-A is a tray-

type and is part of the Radio Transmitter BC-375-E.

Mounting FT-115-B is a disposable-type. The tray-

type mounting, to which the radio transmitter is

attached by four snap-slide catches, is normally

used. The disposal mounting is sometimes added

near the top of the radio transmitter to prevent

swaying. Mounting FT-115-B consists of a rubber

button in a steel shell which may be attached to the

top, bottom, or sides of the radio transmitter. Certain

of the radio transmitter-cover fastening screws have

been placed so as to allow attachment of this mount-

ing.

The radio transmitter should not be mounted closer

than two inches from a wall, and in aircraft and

vehicular installations it is desirable that Mounting

FT-115-B be added at the top of the radio trans-

mitter to prevent excessive swaying. The power

cords should not be fastened nearer than 18 inches

from the radio transmitter. This precaution prevents

vibration from being transmitted to the unit through

the cords. It is important that sufficient space be

allowed in the front of the radio transmitter to

facilitate replacement of tubes, removal of trans-

mitter tuning units, and the making of tuning adjust-

ments. After installation has been completed and

tested, the snap-slide catches should be securely

safety-wired at the places provided.

DYNAMOTOR UNIT PE-73-C

The dynamotor unit should be mounted in an

accessible position to allow access to the relay-fuse

box, keeping in mind that the length of the leads from

the battery to the dynamotor unit must be as short as

practicable. This unit when operated in conjunction

with Radio Transmitter BC-375-E draws from 20 to

30 amperes. No. 6 Awg or larger conductor should

therefore be used for these leads in order to keep

the voltage drop at a minimum. Mounting FT-107

should be securely attached so that the dynamotor

unit cannot be torn loose by acceleration, vibration

or shock.

After the dynamotor unit is clamped securely to

Mounting FT-107 by means of the four snap-slide

catches, Plugs PL-59 n,nd PL-61 of the applicable

cords should be plugged in Sockets SO-39 and SO-41,

respectively on the dynamotor unit. The primary leads

from the battery should then be clamped securely to

the proper binding posts on the dynamotor unit ter-

minal board.

5. CORDS

The cords to be used with the various radio sets

are listed in the parts lists for same. Schematic

diagram (Fig. 1) shows the necessary cording for the

Radio Transmitter BC-375-E. Plug PL-64 and Socket

SO-44 contain the necessary interconnections for the

radio receiver and transmitter control.

6. ANTENNAS

Because of the wide variety of antennas in the

radio sets of which Radio Transmitter BC-375-E is

a part, no specific details will be given for antenna

installations. However, the following general pre-

cautions should be observed:

The antenna and ground leads should be made

as short as possible, except where the length of lead-in

is definitely determined by the placement of radio

set and antenna.

Antenna leads should be insulated to withstand

radio-frequency potentials as high as 5000 volts.

Antenna leads should be so placed that operat-

ing personnel will not come in contact with them

during normal operation of the equipment. Points

accessible to personnel should be covered with

Isolantite beads.

4.. Ground connections should have as low resist-

ance as possible. In aircraft installations the ground

path should be well bonded. The GND post should

always be connected to the ground system by a

short lead, allowing, of course, for flexure due to

vibration and shock. A link will be required between

posts LOAD A and LOAD B when Antenna Tuning

Unit BC-306-A is not used.

5. Antenna leads between the antenna and radio

transmitter and between the radio transmitter and

the radio receiver should be located as far as possible

from any primary wiring of the airplane. This applies

especially to the wiring to the dynamotor unit and is

necessary in order to decrease the pick-up of electrical

interference by the antenna circuit wiring.

7. ANTENNA TUNING UNIT BC-306-A

This tuning unit is provided with Mounting FT-142.

It should be mounted so that the tuning controls will

be accessible during operation. It is desirable that the

length of leads between the.radio transmitter and the

antenna tuning unit be kept to a minimum and that

the leads be well insulated.

' Terminals A-TRANS and B-ANT on the antenna

tuning unit are connected to LOADING TERMI-

NALS A and B respectively on the radio trans-

mitter when operation is with the low-frequency

transmitter tuning unit (Transmitter Tuning Unit

TU-26-B). However, at high altitudes (above 15,000

ft) and frequencies below approximately 400 kc,

better operation may be obtained by connecting

terminal A-TRANS on the antenna tuning unit to

the antenna post on the radio transmitter; placing a

jumper between LOADING TERMINALS A and B

on the radio transmitter; and connecting the antenna

to terminal B-ANT on the antenna tuning unit. This

alternate connection should be used only if it is

necessary to reduce the r-f voltages at the radio

transmitter because of high altitude flashovers. The

sensitivity of the radio receiver may be reduced

slightly. The post marked GND on the antenna

tuning unit should always be connected to the radio

transmitter ground.

8. REEL RL-30-B AND CONNECTOR

CLAMP MC-163

The reel should be mounted in such a position that

the crank is readily accessible and the counter is

visible. In order to prevent excessive "piling" when

the wire is being reeled in, the reel should be mounted

at least six inches above the fairlead and so placed

that the point at which the antenna wire leaves the

spool is in line with the axis of the fairlead; that is,

for best operation there should be no bends in the

antenna wire between the point where it leaves the

reel and the far end of the fairlead. The four mounting

bolts can be inserted in the reel base through the hole

provided in the web of the spool. Unless the mount-

ing screws themselves are grounded, a heavy ground-

ing wire or strap could be clamped under the nut of

one of the mounting screws for the purpose of thor-

oughly grounding all metal parts of the reel.

To fasten the antenna wire to the bottom of the

spool channel it is first necessary to remove the

spool cover by withdrawing it forward over the spool.

This can best be done by placing the fingers of both

hands on opposite sides of the cover and pressing

on the spool with the thumbs. The end of the wire

should be looped under the anchor pin in the spool hub

and then twisted around the wire proper a few times.

The twisted loop should then be placed in the groove

and all of the wire wound on the spool by turning the

crank in a clockwise direction. At this point the cover

should be replaced by passing the wire through the

slot opposite the wire opening and pushing the cover

over the spool until it rests against the flange on the

back plate. The cover should now be rotated so that

the wire from the fairlead to the spool will not rub the

edges of the wire opening in the cover either when

the spool is full or nearly empty. The counter should

now be set to zero. This should be done each time the

reel is used in order to reduce the error in the reading

caused by irregularities in winding.

Electrical connection to the radio transmitter is

made by means of Connector Clamp MC-163, the

installation of which is as follows :

Remove the brass bushing from Connector

Clamp MC-163 by loosening screw in top of cover

and fasten the antenna lead from the radio transmitter

to this bushing by means of the screw provided.

Replace the bushing in cover and place the con-

nector over the top of the fairlead, clamping it into

place by means of the screw in the side of the cover.

Wind the desired amount of Wire W-106 or

W-106-A on the spool and feed the free end through

the connector clamp and fairlead until it touches the

ground below the airplane.

Connect the free end of the wire to the cable

loop on the Weight WT-7-A or WT-7-B. Considerable

care should be taken in making the connection

between the antenna wire and the weight cable in

order that the joint will run smoothly through the

connector clamp when the antenna is being let out or

reeled in. The loop formed in the antenna wire should

be as small as practicable and the section where the

antenna wire is twisted back upon itself should be

tapered and made as short as a safe joint will permit.

Wind up the antenna wire in the reel until the

Weight WT-7-A or WT-7-B rests securely in the

fairlead socket. Let out and reel in approximately

ten feet of the antenna several times (have someone

pull lightly on the weight to keep the antenna wire

tight) to insure that the connector clamp is working

properly and that the joint between the antenna

wire and weight cable runs smoothly through, the

clamp.

Fairlead Extension F-9 is used when the standard

Fairlead F-8 is not long enough. It consists of Cou-

pling MC-161 and a 36-inch length of standard

phenolic tubing of the same cross section as the tube

of Fairlead F-8. The extension is installed by slipping

Coupling MC-161 over the top of the fairlead tube so

that it covers approximately 3 inches of the tube

and then securing it in place by means of one of the

clamps provided. The tubing may then be clamped

into the coupling by means of the remaining clamp.

This tubing may be cut to fit a particular installation

or more than one extension may be used if required.

9. INTERCONNECTION WITH THE RADIO RE-

CEIVER IN AIRCRAFT

It is necessary, in order to minimize receiver noise,

that the cord shields be well grounded and so sup-

ported that they cannot rub on other cords or metal

surfaces. In order that the radio receiver used be co-

ordinated with the radio transmitter, the receiver

control wires should be connected to terminals 21

and 22 of Plug PL-64. These points are normally

closed but are opened when the key is closed and the

radio transmitter starts functioning.

SECTION III

PREPARATION FOR USE

10.

GENERAL

After installation and before the equipment is put

into operation, the circuits of all cords and ex t erna

wiring should be checked to insure compliance with

the circuit diagram and the applicable intercon-

nection diagram. This circuit check should be made

with all plugs removed from the sockets in the radio

transmitter and power unit to determine that all cord

and junction box connections are made as indicated

on the diagrams. A voltmeter and battery may be

used for this purpose. The plugs should be replaced

in their respective sockets after this check.

11.

RADIO TRANSMITTER BC-375-E

The following procedure is recommended for pre-

paring the radio transmitter for use after installation.

Remove Plug PL-59 from the power unit or

dynamotor unit and cover its socket with a Socket

Cap M-164-A.

Remove the tube compartment shielding cover

and insert the tubes. From the front of the radio

transmitter, progressing from left to right, the tubes

are as follows:

Tube VT-25—Speech Amplifier

Tube VT-4-C—Master Oscillator

Tube VT-4-C—Power Amplifier

Tube VT-4-C—Modulator

D-c Filament Supply:

(Note: This equipment

is not designed for alternating-current operation.)

(1).

Remove all links from the filament resistor

connection board.

(2).

Assuming that the primary source voltage

is 28 V (the terminal voltage of a 24-volt storage

battery under charge), place the 24 V/28 V switch

in the tube compartment in the 28-volt position.

(3).

Set the transmitter signal switch on CW

and the filament voltmeter switch on CW FIL.

(4).

Place the OFF-ON switch in the ON

position. This should start the dynamotor unit and

apply filament supply potential to the radio trans-

mitter, but not to the tubes. Connect link between

adjacent studs on the 28 V and CW FILAMENT

rows. The CW filaments will light and the fila-

ment voltmeter read. Repeat this until the two

studs are found that when connected will cause

the filament voltmeter to read 10 volts. Then place

the OFF-ON switch in the OFF position and

connect a link between the two studs just selected.

(5).

Set the radio transmitter signal switch on

VOICE and the filament voltmeter switch on

MOD. FIL. Repeat the operation of (4) for the

28 V and MOD. FILAMENT rows of studs.

(6).

Again place the OFF-ON switch in the ON

position. Set the filament voltmeter switch on

CW FIL. (signal switch on VOICE). The CW

filament voltage will now be less than 10 volts.

In the same manner, connect between the adjacent

studs of the COMP and CW FILAMENT rows

until the filament voltmeter again reads slightly

over 10 volts. The OFF-ON switch may then be

placed in the OFF position and this link fastened

in the proper place.

(7).

Cut off the charging generator, place the

24 V/28 V switch in the 24-volt position and in a

similar manner to the preceding select the proper

studs on the 24 V and CW FILAMENT rows and

the 24 V and MOD. FILAMENT rows so that the

filament voltmeter will indicate 10 volts in either

position. As now adjusted the filament voltages

will not require further adjustment unless the

cable lengths or charging rates are changed. Ordi-

narily, in aircraft installations, a battery-charging

generator is used at all times. The 24 V/28 V switch

should accordingly be placed in the 28-volt position.

If it is desired to check the operation of the radio

equipment when the charging generator is below

operating speed, this switch should be changed to

the 24-volt position since operation at low voltage

will reduce the life of the tubes.

Replace Plug PL-59 in the dynamotor unit or

power unit.

NOTE : It will be noted that the filament volt-

meter will show from 0.3 volt to 0.5 volt rise when the

transmitter key is closed and plate current is being

drawn.

This is a normal condition of the radio transmitter

circuits and should be ignored, providing the above

filament adjustment has been made correctly.

12.

OPERATION AT HIGH ALTITUDE

Radio Transmitter BC-375-E and associated equip-

ment may be expected to give satisfactory service on

CW at all altitudes up to 27,000 feet. On TONE and

VOICE, however, insulation breakdown may be

experienced with Transmitter Tuning Unit TU-8-B

(6200-7700 KC) above 25,000 feet and with Tuning

Unit TU-9-B (7700-10,000 KC) above 19,000 feet.

These altitude limitations may be exceeded slightly

by care in tuning and by carefully guarding against

accumulation of dust and other foreign matter in the

equipment. Complete assurance of effective operation

between 6200 and 10,000 kilocycles at altitudes

between 19,000 and 27,000 feet may be had on CW

alone. Transmitter Tuning Unit TU-26-B may be

expected to give satisfactory service at all altitudes

up to 15,000 feet.

13.

DYNAMOTOR UNIT PE-73-C

Before being used for the first time, it is recom-

mended that the end bells of the dynamotor unit be

removed to make certain that the armature rotates

freely in its bearings, and a check should be made to

determine how much time has elapsed since the

dynamotor unit was last serviced. If this time exceeds

five months, follow lubrication instructions appearing

in this book. Do not lubricate more often or add more

lubricant than is specified in those instructions.

The voltage at the input terminals of the dyna-

motor unit must be 28 volts in order to obtain rated

power output from the radio transmitting equipment.

SECTION IV

OPERATION

14. RADIO TRANSMITTER ADJUSTMENT

WARNING: Operation of this equipment in-

volves the use of high voltages which are dangerous

to life. Operating personnel must at all times observe

all safety regulations. Do not change tubes or make

adjustments inside equipment with high voltage

supply on. Do not complete or maintain any con-

nection between radio transmitter and dynamotor

unit unless all shields on the transmitter are in

place.

Assuming that the installation and preliminary

adjustments have been made in accordance with

SECTIONS II and III. the following operating pro-

cedure is recommended. Some of these adjustments

need be made only at the time of installation.

Under no conditions should any of the switches on

the radio transmitter or tuning unit be changed with

the key or microphone button depressed. Insure that

all switches position correctly; do not attempt to leave

switches between points. Failure to observe this pro-

cedure causes undue arcing of the switch and severe

strain on the vacuum tubes.

a. CW Operation:

Select the transmitter tuning

unit for the desired frequency. CW operation may

then be obtained as follows:

(1).

Place the signal switch on CW.

(2).

From the calibration chart on the front of

the transmitter tuning unit, set the band change

switch A, the M.O. TUNING Control, and the

P.A. TUNING Control for the desired frequency,

and set the ANT. COUPLING SWITCH D on

Point 1. (It should be remembered that the band

change switch does not appear on Transmitter

Tuning Units TU-7-B to TU-10-B inclusive.)

(3).

Place the OFF-ON switch on the ON

position. The dynamotor should now start and the

m-o and p-a filaments will light. In case repeated

failures of Fuse FU-22 are encountered and if such

failures persist after compliance with instructions

pertaining to Dynamotor Unit PE-73-C, it is

recommended that Fuse FU-22 be removed from

the relay-fuse box and one extra Fuse Link M-168

inserted in the fuse cartridge. Fuse FU-22 will then

have two fuse links in it and should be replaced

in the relay-fuse box of the dynamotor unit. In no

case should the use of more than two fuse links be

attempted.

(4).

Press the test key or the transmitting key.

The vacuum tubes will now draw plate current as

indicated by the TOTAL PLATE CURRENT

METER. The p-a tuning should be checked

immediately for resonance by varying the control

slightly until a minimum total plate current is

indicated. When the p-a circuit



properly reso-

nated, the total plate current will be from 80 to

110 milliamperes.

(5).

The antenna should next be tuned to

resonance. Because a wide variety of antennas is

possible, no specific instructions for antenna

tuning are given other than instructions to effect

antenna resonance by means of the controls pro-

vided on the transmitter, and on the antenna tuning

unit whenever the latter is in use. The subject of

ANTENNA ADJUSTMENTS (paragraph 15) is

considered more in detail elsewhere in this instruc-

tion book, and should be referred to in preparation

for operation of the equipment.

Antenna resonance is indicated by a reading of

current on the ANT. CURRENT meter and by an

increase in total plate current. The increase in

plate current is the more sensitive indication of

approaching antenna resonance. When the antenna

is tuned to resonance, the total plate current

reading will be somewhat higher than the off

resonance value. If the plate current is below 200

to 220 milliamperes, the coupling to the antenna

should be increased by placing control D on a

higher point and the antenna circuit retuned.

When finally adjusted, the total plate current

should read from 200 to 220 milliamperes. If

necessary, detune the antenna circuit slightly,

keeping the p-a dial C in resonance to keep within

the plate current limits.

(6).

The equipment is now delivering rated

power output, and the transmission may be carried

on by operating the transmitting key. In order to

shut down the equipment, it is necessary only to

place the OFF-ON switch in the OFF position.

Voice Operation:

Assuming that the equip-

ment has been placed in operation on CW, the

following procedure is recommended for obtaining

voice operation. Note the value of total plate current

for CW operation. Then place the signal switch in

the VOICE position and, by means of the MOD.

BIAS adjustment in the tube compartment, adjust

until the total plate current with the microphone

switch depressed is approximately 20 to 35 milli-

amperes higher than for CW. The modulator tubes

are now biased nearly to cutoff for proper Class B

operation, and radiophone communication may be

carried on by speaking into the microphone. It will be

noted that the total plate current increases when the

microphone is spoken into. This increase is due to

current drawn by the modulator tubes. With sustained

normal level of speech impressed on the microphone,

the plate current should rise to an average of 300

milliamperes. If this value is not obtained, the

INPUT LEVEL control in the tube compartment

can be adjusted until the proper amount of modula-

tion, as indicated by the correct plate current, is

obtained.

Speech-amplifier Bias:

The correct speech-

amplifier bias will usually be found betWeen 6.0 and

7.5 on the bias adjustment dial. Settings in this range

will normally give the proper speech-amplifier plate

current and optimum side-tone frequency on all

tuning units.

Tone Operation:

After the equipment has

been adjusted for VOICE operation, it is necessary

only to place the signal switch on TONE for proper

tone telegraph operation. The total plate current on

TONE will usually be between 300 and 350 milli-

amperes.

Side Tone in Aircraft Set:

If the radio receiver

circuits have been properly co-ordinated with the

radio transmitter, the receiver control' circuit will

operate each time the transmitting key or microphone

switch is closed, and the transmitter side tone will be

supplied to the headset. When the key or the switch

is opened the receiver again becomes operative, thus

allowing break-in operation. The SIDE TONE level

control located in the tube compartment, is used

to adjust side tone to a suitable value for any par-

ticular installation. It will be noted that a higher level

is desirable on voice position than on the CW and

TONE positions.

The side-tone control is a four-position switch, two

positions of which are used for high-level and low-level

side-tone signals for a 2000-ohm load, and the other

two positions are used for the high-level and low-level

side-tone signals for a 150-ohm load. With the side-

tone control set in the 2000-ohm high-level position

(4), the output voltage will be between 17% and 30

volts. In the 2000-ohm low-level position (3), the

output is approximately 10 volts. The output in the

150-ohm high-level position (2) will be between 4.5

and 7.5 volts. While the 150-ohm low-level position

(1) will give approximately 2.5 volts, the operator

should determine the headset resistance and select

the correct side-tone switch position at the time of

installation.

f .



Calibration Reset:

Due to necessary manu-

facturing tolerances, the inter-electrode capacity of a

vacuum tube varies between limits fixed for each

particular type of tube. Since the tube capacity is an

appreciable part of any master-oscillator circuit, it is

impossible to make up a calibration chart which

will be exactly accurate for all tubes whose capacities

are within the allowable limits. For this reason

Radio Transmitter BC-375-E is provided with a

calibration reset capacitor which enables the operator

to reset the radio transmitter frequency to correspond

with the calibration chart when the set is first placed

in operation and thereafter whenever the master-

oscillator tube is changed. A heterodyne frequency

meter or other standard of frequency is required.

The procedure is as follows:

(1).

Allow radio transmitter to warm. The

operator will obtain the most accurate results by

allowing the radio transmitter to "warm up" on

key locked CW for a period of at least 20 to 30

minutes before set Ging the calibration reset capac-

itor or checking the calibrated transmitter

frequency.

(2).

With the transmitter tuning unit for the

highest working frequency placed in the radio

transmitter, tune the transmitter for CW operation

on one of the calibrated frequencies at the high

frequency end of the band. (Approach the cali-

brated point by proceeding from a lower dial

reading to a higher one.)

(3).

Place the frequency meter in operation and

adjust it to the frequency indicated on the trans-

mitter calibration in accordance with the operating

instructions and calibration chart furnished with

the frequency meter. The calibration accuracy of

this frequency meter should be 0.01 per cent, or

better.

(4).

Open the calibration reset port, located on

the front panel, between the TEST KEY and

TONE-CW-VOICE switch, insert a screwdriver,

and rotate the calibration reset capacitor until the

transmitter frequency coincides with that of the fre-

quency meter or standard.

(5).

Close the calibration reset 'port.

The transmitter calibration is now reset for any

tuning unit of the same order number and serial

number as the radio transmitter and the accuracy

of calibration will be within 0.05 per cent plus the

accuracy of the standard. The calibration must be

checked in this manner each time the m-o tube is

changed.

15. ANTENNA ADJUSTMENTS

The antenna is, in general, a system of conductors

which, when excited by a radio-frequency voltage,

sets up an electromagnetic and electrostatic field

causing a component of this field to travel away

from the antenna with the velocity of light.

The radiation characteristics of antennas, direc-

tivity, and radiation efficiencies are determined by

their physical shape and location with respect to other

bodies. In aircraft, since there is no wide latitude in

physical design of fixed antennas, the problem be-

comes mainly one of determining the best methods of

operating available designs.

It is necessary first to determine how the antenna

impedances vary at the operating frequencies in order

that the loading facilities may be properly used.

The reactance of an antenna may be approximated

from the theory of transmission lines in which uni-

formly distributed inductance and capacity are

assumed, resistance and insulator leakage being

neglected.

Selecting a certain length of antenna and plotting

its reactance variation with frequency, we obtain

repeating cotangent curves. It may be seen that at

certain frequencies the reactance becomes zero.

Under these conditions, the antenna is "resonant,"

analogous to a simple-series circuit turned to reso-

nance.

It may be noted that the "series" resonant points

occur at all odd multiples of the first resonant fre-

quency, which is called the fundamental frequency.

The fundamental frequency, the third harmonic,

and the fifth harmonic, all points of zero reactance,

correspond to a voltage distribution along a simple

vertical wire of "quarter-wave," "three-quarters

wave," and "five-quarters wave." Antennas of this

type, operated at zero or low reactances, are com-

monly called "current fed"; that is, they require low

driving voltages for their operation.

At even multiples of the fundamental frequency, it

may be seen that the antenna reactance is very high.

Operation at the second and fourth harmonics corre-

sponds to "half-wave" and "full-wave" operation.

Under these condicions, antennas are "voltage fed" ;

that is, they require high driving voltages.

The resistance component of the antenna imped-

ance is made of two parts: radiation resistance which

represents the radiation of power or waves away

from the antenna and which is productive of a useful

result; and loss resistance which is a combination of

losses due to conductor and ground resistance, and to

dielectric hysteresis. Loss resistance performs no

useful function and every effort should be made to

maintain it very small in comparison with the radia-

tion resistance. The efficiency of an antenna with

respect to radiation of power may be expressed as the

ratio of radiation resistance to total antenna resist-

ance.

Antenna resistance varies over wide limits with

frequency. The resistance values approach very high

values at the even harmonics, and minimum values

at odd harmonics. It is thus seen that the operation

of a radio transmitter over a wide frequency band

requires that the radio transmitter be capable of

providing a considerable range of output voltages.

This is accomplished in each transmitter tuning unit

by a six-point ANT. COUPLING SWITCH, D. In

order that the voltage range required be kept a mini-

mum, antennas are usually "resonated" ; that is, they

are series tuned by either inductance or capacity as

may be necessary so that the required output volt-

age range depends only on the antenna resistance

variation over the desired frequency range.

When operating antennas at frequencies lower than

their fundamental or odd harmonics, they appear

to the antenna tuning equipment as a capacity in

series with a resistance. In order to balance out this

capacitive reactance, it is necessary to load the

antenna by means of an inductance, the reactance of

which is made equal to that of the apparent antenna

capacity. For operation of antennas at frequencies

higher than their fundamental and odd harmonics,

the converse is true, a capacity in series with the

antenna being necessary in order to balance the

apparent antenna inductive reactance.

In most cases, except where 3/4- and 5/4-wave

trailing wires are used for high frequencies, antenna

operation will be around the fundamental frequency.

In general, it may be said that if antennas are

operated so that their effective length is an appreciable

percentage of the operating wavelength, they will

have marked directive properties. This corresponds to

operation near to and higher than the fundamental

frequency. When antennas are operated at frequencies

much lower than the fundamental, the ratio of

radiation to total resistance is less favorable, but the

directive properties are not nearly so evident. The

fundamental frequency of an antenna depends mainly

on its effective length including the ground lead.

Fundamental frequencies in the range of 2500-7000

kilocycles will be the most common with the usual

types of fixed antennas, while fundamentals as low as

1500 kilocycles will be obtained with 200-foot trailing-

wire antennas.

The antenna tuning equipment in the radio trans-

mitter is designed to feed antennas at any frequency

from 800 to 12,500 kilocycles. Over this band of fre-

quencies it is necessary to feed antennas at, above and

below their fundamental frequencies. Therefore, the

circuit and circuit constants are so selected as to per-

mit of both current and voltage feed. A rotating

inductor and a tapped inductance provide the induc-

tive reactance variation. A variable capacitor pro-

vides the variation in capacitive reactance and also

serves as a coupling impedance for voltage feed.

Controls governing the variation in inductive or

capacitive reactance are so arranged that inductance

or capacitance is increased with increase in dial

reading.

With the ANT. CIRCUIT SWITCH, N, bn

position 1, the radio transmitter works into a simple

series resonant circuit, in which the high voltage

built up across the antenna tuning capacitor is used to

"voltage feed" the antenna. This circuit is generally

used for high frequencies and fairly long fixed antennas.

The antenna feed circuit is maintained at resonance

by means of ANT. IND. TUNING, M, while the

voltage fed to the antenna is varied by means of

ANT. CAP. TUNING, 0, and ANT. COUPLING

SWITCH, D. Feed circuit resonance is indicated

by the ANT. CURRENT meter. The current in the

feed circuit is adjusted by control D and should not

exceed 6 amperes. The step-by-step tuning procedure

is as follows:

(1).

Set control 0 at some arbitrary scale read-

ing.

(2).

Resonate circuit by means of control M for a

maximum reading on ANT. CURRENT meter.

(3).

Adjust control D so that antenna ammeter

reads below 6 amperes.

(4).

Re-resonate circuit as in 2.

Repeat the above procedure until the proper loading

(200-220 ma on CW) is indicated on the plate am-

meter. The lower the dial reading on control M and

the lower the reading on the ANT. CURRENT

meter that it is possible to obtain at a given frequency,

the more power will actually be delivered to a given

antenna. The lower the reading on control M and the

lower the ANT. CURRENT readings, the less the

tuning circuit losses and thus the more useful power is

delivered to the antenna for radiation purposes.

With ANT. CIRCUIT SWITCH, N, on position 2,

the radio transmitter works into a series resonant cir-

cuit where the antenna is "current fed." The antenna

circuit is resonated by means of ANT. IND. TUN-

ING, M, and ANT. CAP. TUNING, 0, as indicated

by the ANT. CURRENT meter. This circuit is used

generally for operation near the fundamental fre-

quency of the antenna.

With control N on position 3, the radio transmitter

works into a series resonant circuit providing "current

feed" and inductive loading. The antenna circuit is

resonated by means of a continuously variable

rotating inductor, control M. This circuit is used for

operation below the fundamental frequency of the

antenna.

With control N on position 4, the antenna circuit

is identical with that obtained on position 3 except

that an additional tapped inductance, controlled by

ANT. IND. SWITCH, P, is added in series with the

rotating inductor. This circuit is used when the

operating frequency is relatively far below the funda-

mental frequency of the antenna.

It is recommended that the operator check the

possible resonance of coil, item 1170 of Fig. 1, at

operating frequencies above 4500 kilocycles. Although

this coil is not connected in the antenna circuit, the

inherent capacity coupling may cause absorption of

useful radio-frequency power. Absorption may be

easily determined by placing control switch P at sev-

eral points while watching the antenna current. Con-

trol P must not be allowed to remain at a point which

indicates a decrease in antenna current. In general,

it will be found that no difficulty will occur if the

following points are used.

Transmitting Tuning Unit

P Switch Position

TU-7-B

TU-8-B

TU-9-B

TU-10-B

The antenna resonance is then made in the usual

manner using controls M, N, and possibly 0.

For operation in the frequency range of 150 to 800

kilocycles, the externally connected Antenna Tuning

Unit BC-306-A is used. This unit contains the neces-

sary inductive reactance to resonate specified antennas

at frequencies well below their fundamental. The

variation in inductive reactance is provided by the

ANTENNA VARIOMETER SWITCH, control E,

and the ANTENNA VARIOMETER, control F.

The proper connections are described in SECTION

II,

INSTALLATION. In general, it will be necessary

to use Antenna Tuning Unit BC-306-A with Trans-

mitter Tuning Unit TU-26-B. The antenna loading

equipment contained in the radio transmitter will

resonate a 400 micromicrofarad antenna to approx-

imately 650 kilocycles. Therefore when the minimum

inductance is reached on the Antenna Tuning Unit

BC-306-A (E = 2, F = 0), place control E in position 1

which disconnects this unit. Proceed to resonate the

antenna circuit with control N on 4. Control M is

the continuously variable inductance between induc-

tance steps on control P.

Under some conditions it may be found that one

coupling tap will not give sufficient loading while

the next higher tap will give overloading of the trans-

mitter. In this case use the higher coupling tap by

slightly detuning the antenna circuit and keeping

the p-a dial C tuned to a minimum plate current

until the normal transmitter loading of 210 to 220

milliamperes total plate current is obtained.

•

16. DYNAMOTOR UNIT PE-73-C

Starting and stopping of the dynamotor unit is

controlled remotely by the OFF-ON switch at the

Radio Transmitter BC-375-E. No adjustments are

required or provided on the dynamotor unit.

When operated at full-rated load continuously for

M hour the temperature rise of the dynamotor will not

exceed 55 degrees Centigrade. However, if the dyna-

motor is operated for a greater length of time, even at

lighter loads, without being allowed to cool off, its

temperature will continue to increase and will ulti-

mately reach values which are injurious to the

insulation and may even burn out the windings.

This also applies to the starting relay. If it is desired

to operate the dynamotor unit continuously at full

load for a period greater than IA hour (such as for

testing purposes in the laboratory), the end bells of

the dynamotor and the cover of the relay-fuse box

should first be removed. The dynamotor unit can then

be operated at rated load continuously for any period

of time without injurious heating. Care should be

exercised to place guards around the dynamotor unit

when operated in this manner so that the operators

cannot come in contact with exposed high voltages.

When operated in conjunction with the transmitter,

the dynamotor operates at practically zero load unless

the key or microphone switch is closed. Continuous

operation under this condition will not result in

overheating although the transmitter should be turned

off whenever possible between the periods of trans-

mission.

17. REEL RL-30-B

Braking, locking and winding operations are

controlled from the crank on the reel. Normally the

reel is in the locked position such that the wire will not

unwind from the spool. Reeling-in is accomplished by

rotating the crank in a clockwise direction, as indi-

cated by the direction arrow Wind on the nameplate

at the center of the reel spool. A ratchet mechanism

on the spool prevents the wire from unreeling when

the crank is released. By rotating the crank in a

counterclockwise direction the braking mechanism

is released, thereby permitting the wire to be reeled

out. Braking force decreases gradually as the handle

is rotated through approximately the first 40 degrees

of its motion. Beyond this point, and up to the

extreme limit of its motion the brake is completely

released and the spool is free to spin. The speed of

unwinding can be readily controlled by regulating

the braking effect with the crank. A spring return on

the crank automatically resets the brake to the normal

or locked position when it is released.

Tuning the antenna by adjusting its length should

be accomplished by allowing slightly more wire than

is necessary to run from the reel and then reeling

in slowly to obtain the proper length by observing

the resonant condition. During the reeling-out

operation, care should be taken that all of the wire

is not unreeled. The speed with which the reeling

takes place would be sufficient to snap the wire if it

reached its ultimate length, causing a loss of both the

weight and wire.

When unreeling wire, never allow the crank to

snap into the locking position when the spool is

rotating rapidly. The sudden stop which would

result may break the antenna wire and place undue

stress on the reel mechanism.

The following table gives the approximate antenna

length for various frequencies as well as the approxi-

mate counter reading when 250 feet of Wire W-106

or W-106-A is wound on the spool.

ICC

1/4 Wave

3/4 Wave

Length

(Ft)

Counter

Reading

Length

(Ft)

Counter

Reading

2000

123

108

3000

4000

5000

147

130

6000

123

108

7000

105

8000

9000

,72

10000

18. OPERATING ROUTINE

The operating routine and type of transmission to

be used will be governed by tactical requirements. The

following recommendations are given, however, to

assist in routine operation of the equipments :

Complete equipment operation should be

checked before the start of any mission.

Make certain that spare fuseholders are filled

with good fuses, and that spare tubes (if carried) are in

good condition.

The transmission range of the equipment on CW

is considerably greater than on VOICE. If distance,

atmospheric noise, etc., make VOICE communication

difficult, changing to TONE or CW will probably

improve communication.

In case of failure of one or two Tubes VT-4-C

with no spares on hand, CW communication can be

carried on by inserting the good tubes in the m-o and

p-a sockets. CW operation can also be maintained

when the Tube VT-25 is removed, although no side

tone will be supplied in this event. Note that the radio-

frequency and audio-frequency tubes are connected in

series parallel. If one tube is removed, another filament

will go out.

19. SUMMARY COVERING NORMAL

OPERATION

Although the normal operation of this equipment is

extremely simple after the correct installation has

been made, it is well to study the proper sequence of

various equipment procedures as summarized in the

following paragraphs. It is assumed that the installa-

tion has been tested and all circuits are normal.

Do not change any switches with the key or

,microphone button depressed.

To Start the Equipment:

Place the OFF-ON

switch at ON. The dynamotor unit will start and the

filaments will light. Unless the TONE and VOICE

emissions have been adjusted, start up initially on

CW and adjust for the desired type of emission as

given below. To control carrier depress the key.

To Stop the Equipment:

Open the key; then

place the OFF-ON switch at OFF.

To Change Frequencies:

Select the required

transmitter tuning unit and place in the radio trans-

mitter. Place the M.O. TUNING control, BAND

CHANGE SWITCH (if any) and the P.A. TUNING

control on the desired calibrated frequency as given

on the calibration chart. Place the emission switch

on CW, start up the radio transmitter and immediately

check the P.A. TUNING dial for minimum plate cur-

rent as indicated on the TOTAL PLATE CURRENT

meter. Resonate the antenna circuit and load

to a total plate current of 210 to 220 milliamperes

with the p-a dial resonated, i.e., always tuned to a

minimum plate current. Proceed to key the carrier

on CW or select the other types of emission as given

below.

To Select Any Available Type of Emission:

Start up the radio transmitter and tune for CW

transmission first as given in paragraph 19c., then,

open the key and place the TONE-CW-VOICE

switch on VOICE. Remove the tube shield and adjust

the MOD. BIAS control after key is depressed,

until the total plate current reads 20 to 35 milli-

amperes above the CW value. Proceed to control

carrier by microphone button and modulate by voice,

adjust the side-tone signal to the desired level by

means of the SIDE-TONE control located in the

tube compartment. Adjust the voice level by the

INPUT LEVEL control so the total plate current

reaches 300 to

milliamperes on peaks of sustained

voice.

For tone transmission the operator must always

adjust transmitter for VOICE, first. Then open the

key, place the selector switch on TONE and proceed

to key carrier as in CW.

e. To Increase Power Output:

Go to a higher

number on the ANT. COUPLING SWITCH D at

the same time keeping the P.A. TUNING dial C

and the antenna circuit in resonance. Power may be

increased until a CW plate current of 220 ma at 28

volts input is reached.

To Decrease Power Output.:

Go to a lower number

on ANT. COUPLING SWITCH D, at the same time

keeping the P.A. TUNING dial C and the antenna

circuit in resonance.

It should be noted that the CW power output may

be increased or decreased in the above manner from

the value of plate current with the antenna circuit

open to the full load rating of 220 milliamperes.

However, the TONE and VOICE carrier powers

SHOULD NOT BE INCREASED OR DE-

CREASED. Unless the TONE and VOICE carriers

are adjusted as given in paragraph 19d., distortion

will result. Also, the misadjustment may cause arc-

overs which would damage the equipment.

f. To Reset to the Calibration Chart,

after changing

of the m-o tube, etc.: Start the radio transmitter on

the highest frequency tuning unit available. Warm up

the radio transmitter on CW key locked for 10 to 20

minutes. The tube shield must be in place. Listen to

the calibrated frequency on a suitable accurate hetero-

odyne frequency meter and adjust the radio trans-

mitter to zero beat by means of a screw driver inserted

into the reset port on the.left front radio transmitter

panel. This should bring the radio transmitter back to

calibration and all other frequencies should be within

0.05 per cent. For greater accuracies reset as above to

the exact frequency desired.

SECTION V

MAINTENANCE AND REPAIR

20. INSPECTION

A thorough periodical inspection of the complete

installation at least once every 50 hours of operation

will materially aid in the maintenance of uninter-

rupted performance. The following inspection is

recommended:

Determine that the storage battery is at the

proper gravity and that the charging generator and

regulator are adjusted so as to keep the battery

fully charged.

Clean accumulated dust and dirt from all

units. Use an air hose or bellows, paying particular

attention to the loading coil, item 1170, in the antenna

compartment of the radio transmitter. At this time

inspect the rotating coil in this compartment. The

winding should be thoroughly cleaned using a clean

cloth moistened with naphtha or petroleum spirits.

Make certain that all safety wiring is in place,

that all mounting brackets and supports are rigidly

fastened and that all nuts and machine screws are

supplied with lockwashers and are tight.

Determine that no cords have broken shielding

and that all grounding and bonding is in place.

Inspect plugs for proper fit and plug sockets

for compressed pin springs. Compressed pin springs,

which have taken a permanent set, can be restored by

a light hammer blow on the end of the pin.

Make certain that all fuses are held tightly

in their clips. A loosely held fuse should be removed

and the clips bent with the fingers until they grip the

fuse tightly. The clips and fuse ends should be kept

clean and the contact surfaces bright.

Antennas should be inspected for broken or

frayed leads, and insulators should be wiped clean.

Be sure that antenna leads have not been bent close

to metal frameworks where high antenna voltages

might cause sparkovers.

Inspect keys and microphones for broken cords

and deformed plugs.

Inspect dynamotor brushes for length and

broken pigtail leads. Detailed instructions on care of

brushes are given herein.

Inspect both dynamotor commutators for

cleanliness and excessive wear. Information on care

of commutators is given in these instructions.

Rotate the dynamotor armature by hand

to make certain it turns freely and is not rubbing

against the field poles or leads. When all four brushes

are removed, the armature should spin freely on its

bearings. Refer to detailed information on dynamotor

bearings.

21. LUBRICATION

a. Materials:

The lubrication of the equipment

involves the use of three lubricants and several

cleaning solvents. These are:

(1).

Light Oil: A high grade, low pour test, mineral

oil with viscosity rating SAE 10.

(2).

Light Oil: A high grade, low pour test, mineral

oil with viscosity rating SAE 20.

(3).

Grease: Use only high-temperature ball-bear-

ing grease such as Grade 295, Air Corps Specification

No. 3560. "Lubrico M-21" as supplied by the Master

Lubricants Co., Philadelphia, Pa., or N. Y. & N.

Lubricant "No. 1572 Special" are recommended for

use where the temperature does not drop lower than

—31°F. Do not use unauthorized greases as they may

melt out at high temperatures or oxidize rapidly and

become gummy when packed in the bearings.

(4). Cleaning Solvents: Naphtha or petroleum

spirits are strongly recommended. Carbon tetrachloride

is used for dynamotor unit bearings and commutators.

b. Periodic Lubrication Routine

Dynamotor Unit PE-73-C:

Under normal operat-

ing conditions, the dynamotor bearings should be

lubricated in accordance with the following schedule :

(1).

After every 1000 hours of operation or at

intervals of six months, add 3 drops of SAE 20 oil

into the small oil hole which is provided in the top of

each bearing housing and is closed by a screw plug.

(2).

After every 5000 hours of operation or at in-

tervals of one year, add approximately 0.05 cu. in.

(/-in. cube) of ball bearing grease to each bearing

through the oil hole mentioned above.

NOTE: Do not add more oil or grease then speci-

fied above, since the excess lubricant will tend to

work out of the bearing housing onto the commu-

tators and brushes where it will cause trouble if not

wiped off. Too much grease in the bearing will have a

tendency to churn and may cause the bearing to

overheat. Always make certain that the lubricants

used are clean and that no dirt, moisture or foreign

matter enters the oil hole when adding the lubricant.

Tighten screw plugs securely into the oil holes as

soon as lubricant is added.

(3) After every 10,000 hours of operation or at

least every two years, remove the armature from

the dynamotor, thoroughly clean the bearings and

bearing housings with carbon tetrachloride and

repack each ball bearing level full of approved ball

bearing grease. See procedure for disassembly and

cleaning of bearings (paragraph 22d. (4).)

c. Reel RL-30-B:

The construction of this unit is

such that it will require very little attention. The

following lubrication routine is recommended to

obtain the best operating results:

(1).

After every 20 hours of flying service apply

six drops of SAE 10 oil at the oil hole indicated on the

hub nameplate.

(2).

After every 1000 hours of flying time or every

six months, whichever comes first, do as follows:

(a)

Remove the three fillister head screws which

hold the crank handle lever to the hub and remove

the crank.

(b)

Remove the small cotter pin and castle nut

on the end of the reel shaft.

(c)

Remove reel spool and hub from the shaft.

(d)

Clean the accumulated dirt from reel

shaft and the parts in the mounting base. Examine

the exterior parts of the reel hub and clean off all dirt.

(e)

Lubricate these points using SAE 10 oil:

Counter gear shaft bearing-1 drop.

Main shaft-several drops along the bearing

surfaces.

Rear ball bearing on hub adjacent to ratchet

teeth-5 drops.

Front thrust bearing around the edge of the

retaining nut at the front end of the hub-5 drops.

Front ball bearing around the gap between the

outside of the hub and the edge of the piece into

which the three screws from the crank handle

lever are threaded-5 drops.

(f)

Reassemble the reel as follows: Place the

reel spool and hub on the shaft. Give the spool a

slight spin clockwise until the ratchet engages the

pawls.

Replace the castle nut with the fingers, run it

down until snug and then back it off until the

hub runs free (at least IA of a turn). Replace the

cotter pin. Assemble the crank handle and tighten

the three fillister head screws.

This completes the operation of servicing the

reel.

d. Miscellaneous Lubrication

The following miscellaneous lubrication instruc-

tions should be followed in connection with the

maintenance of Radio Transmitter BC-375-E. Places

to oil and grease are listed. Do this after every 500

hours of service or at least after every 1000 hours.

Do it oftener if dirt accumulation is excessive. Clean

parts as required.

Oil SAE 20—Switch shafts in radio transmitter,

transmitter tuning units, and antenna tuning unit:

Vernier mechanisms in radio transmitter, transmitter

tuning unit, and antenna tuning unit. Variable capac-

itor shafts in radio transmitter and transmitter tuning

units.

Grease—Contacts of switches, and switch position-

ing devices in radio transmitter, transmitter tuning

unit and antenna tuning unit.

NEVER lubricate the contact roller or the contact

roller shaft of the rotating antenna inductor. These

parts should run DRY and should always be kept

spotlessly clean. Naphtha or petroleum spirits should

be used to clean these parts.

22. SERVICE NOTES

In the servicing and locating of faults in the radio

transmitter and associated equipment, it is necessary

to remove various shields and covers to make the

circuits accessible. Great care must be taken in

testing with shields removed, because a great many

points of high voltage are thus exposed. Whenever

such testing is necessary, the proper procedure is to

remove all power from the equipment, make the

necessary circuit changes or meter connections and

then apply power, keeping clear of all meters which

are connected in the "high" sides of circuits where

there may exist an appreciable voltage to ground.

Always remember _that when the dynamotor is

running and the transmitting key is open, high

voltage is present in the radio transmitter even

though there is no plate ammeter reading.

In checking low voltage and filament circuits, the

single conductor cord with the Plug PL-59 should be

disconnected from the transmitter, thus removing

the high-voltage supply. It will be noted from

observation of the schematic diagram (Fig. 1) that

two tube filaments are connected in series. Thus, if one

fails, two filaments will go out. The defective tube can

be determined by use of a continuity meter.

The following information is supplied to aid in

servicing the equipment :

Voltage Readings

(A high-resistance voltmeter should be used—at

least 1000 ohms per volt.)

(1).

Low-voltage input of 24 to 28volts, depending

on the power-supply voltage, should be obtained from

terminal 45 of Socket SO-41 to ground.

(2).

Speech-amplifier plate voltage of approxi-

mately 425 volts should be obtained at the plate

connection of the speech-amplifier tube when the

transmitter is on VOICE. It is recommended that a

tube socket adapter be used which will allow not only

readings of voltages but also all currents for the Tube

VT-25.

(3).

Modulator bias voltage of 72 to 75 should be

obtained across capacitor 1160, the positive side

being at ground potential. Selector switch should be

on VOICE.

(4).

Speech-amplifier bias voltage of 35 to 40

should be obtained across capacitor 1144, the positive

side being at ground potential. Radio transmitter

switch should be on VOICE.

(5).

Plate voltage of 1000 to 1100 volts should be

obtained between terminal 61 of Socket SO-39 and

ground.

(6).

Keying voltage of approximately 200 volts

will be obtained across resistor 1115 when keying

relay 1189 is open.

(7).

Microphone-supply voltage of 4.5 to 5.3 volts

d-c should be obtained across resistor 1145.

For this test, the microphone should be in the

circuit.

b. Current Readings

(1).

Speech-amplifier plate current should be from

19 to 21 milliamperes. Adjustment of its bias voltage

to obtain this value is accomplished by potentiom-

eter 1114 which is accessible in the tube compart-

ment. The normal setting of this potentiometer will

usually be between dial positions 6 and 7.5.

(2).

Modulator plate current may be determined

by observing the increase in total plate current reading

when changing from CW to VOICE and impressing

nbrmal modulation. The modulator plate current

should average 100 to 160 milliamperes for sustained

tones. A greater or smaller value than this indicates

that a readjustment of the input level should be made.

(3).

Master-oscillator plate current can be de-

termined on the total-plate current-meter by remov-

ing the p-a and speech-amplifier tubes and placing the

transmitter on CW. The current indicated should

be from 30 to 75 milliamperes, depending on the

transmitter tuning unit in use.

(4).

Power-amplifier plate current may be deter-

mined by subtracting from the total plate current on

CW the currents drawn by the master oscillator and

speech amplifier. Its value should be from 100 to 150

milliamperes at full load.

(5).

Master-oscillator grid current can be deter-

mined by connecting a d-c milliammeter in series

with resistor 1109. Correct value should be from 30 to

50 milliamperes.

(6).

Power-amplifier grid current can be deter-

mined by inserting a d-c milliammeter in series with

resistor 1112. Correct value is from 15 to 25

milliamperes.

Various other circuits may be checked by referring

to the schematic diagram. Then by referring to the

various unit connection diagrams, the location of the

various circuit elements may be ascertained.

c. Neutralization of Power Amplifier

Neutralization of the power amplifier is not ordi-

narily necessary, since this is done when the units are

tested at the manufacturer's plant. However, if the

setting is disturbed for any reason, the following pro-

cedure may be used to restore the adjustment.

The radio transmitter should first be set up and

tuned to see that all power supplies are correct and

the transmitter is functioning properly. Then remove

the power supply cords and the back of the radio

transmitter. From Fig. 2 locate wire 43 (red) on

filter capacitor 1163. Remove this wire with a solder-

ing iron and tape clear of terminal on capacitor

1197A. This removes the power-amplifier d-c plate

voltage and leaves the r-f circuits intact. Connect

the vertical plates of a cathode-ray oscillograph to

terminals LOAD A and GND on the radio trans-

mitter. Place the ANT. IND. TUNING, M at dial

zero and the ANT. CIRCUIT SWITCH, N on

position 3. Tune the p.a. to resonance as shown by

a maximum amplitude of r-f carrier on the oscillo-

graph. Proceed by turning the neutralizing control,

behind the tuning chart, for a minimum amplitude

on the oscillograph, at the same time maintaining

the p.a. in tune. When the best minimum is reached,

with the p.a. in tune, the tuning unit is neutralized.

In the lower frequency tuning units, the oscillograph

pickup will be practically zero at neutralization, while

on the higher frequencies, considerable amplitude

will be noticed from stray ground currents. A check

on the neutralization may be made by noticing the

low reaction on the total plate meter, at neutraliza-

tion, as the p.a. is tuned through resonance. The

operator must remove all power plugs, while the

transmitter back cover is removed and lead (43)

is resoldered.

d. Dynamotor Unit PE-73-C

(1).

BRUSHES: The brushes can be removed

by unscrewing the slotted ,brush cap on each side of

the bearing bracket. It is recommended that each

brush be suitably marked to indicate which brush

holder it came .from and its relative position in that

brush holder in order that the brushes may later be

replaced in their original positions. THIS IS IM-

PORTANT. Blow out dust and clean all foreign

matter from each brush holder and brush and make

certain that the brushes slide freely in their brush

holders, dressing the brushes with fine sandpaper, or

a file, if necessary. Any brush which has worn down

to a length of less than

A in. (measured from contact

surface to near end of spring) should be replaced

with new one. In installing new brushes it may be

necessary to "sand-in " the brush in order to make

its contact surface fit the contour of the commu-

tator. The brush must slide freely in its holder.

"Sanding-in" may be accomplished by using a

strip of No. 00 sandpaper about 5 in. by 1 in. for

L.V. brushes and S in. by

in. for H.V. brushes.

Wrap the strip of sandpaper around the commutator

with the sand surface out. Insert the brushes in the

brush holders and replace the brush caps so that

the brushes are pressed tightly against the sandpaper.

Holding the ends of the sandpaper so as to stretch

it tightly against the commutator, rotate the armature

back and forth until the full width of the brush face

is making contact against the sandpaper as indicated

by the sanding marks or scratches on the contact

surface of the brush when it is withdrawn. Sand the

sides of the brush, if necessary, for a free fit in the

holder.

No sanding should be necessary to secure a good

brush fit with the original brushes if they are replaced

correctly. Never apply oil, grease or any other lubri-

cant to the brush, commutator or brush holder.

Under normal conditions of operation, it is estimated

that the useful life of brushes is 2000 hours for low-

voltage and 10,000 hours for high-voltage brushes.

(2).

COMMUTATORS: Both commutators should

be wiped with a clean, Tintless cloth. Any scum appear-

ing on the low-voltage commutator should be removed

by moistening the cloth in carbon tetrachloride.

The normal black or dark brown polished surface

on the high-voltage commutator should not be

removed. The type of brush material used in the

low-voltage brushes of the dynamotor unit may,

under certain conditions of installation, cause a

discoloration of the low-voltage commutator. Under

these circumstances the commutator will have a

mottled appearance which is caused by the formation

of a very thin oxide film on the surface of the commu-

tator bars. This film is normal and is not injurious

to the commutator or brushes. Do not try to remove

the oxide film described above. A rough or pitted

commutator should be smoothed with No. 00 or finer

sandpaper. Never use emery cloth or a file. Under

normal conditions of operation the low-voltage and

high-voltage commutators should not require turning

down before 2000 hours and 10,000 hours of service

respectively. However, if the commutator bars have

worn down flush with the mica, the armature should

be removed from the dynamotor to turn down the

commutator face and undercut the mica between bars.

(3). BEARINGS: If the armature does not spin

freely when rotated by hand with the brushes re-

moved the following may be the cause:

(a)

Dirt or other foreign matter in a bearing.

(b)

Defective ball bearing (cracked race, chipped

or flattened ball) .

(c)

Grease in bearing has become hard and

gummy due to oxidation during long periods

without actual use.

In any case, the armature should be removed from

the frame as described in paragraph 22d (4) and

the bearings thoroughly cleaned. If, after cleaning,

the outer race will not spin smoothly, it is probably

due to a cracked race or chipped or flattened ball,

and the defective bearing should be removed and

replaced with a new one. Always use a bearing puller

to remove a defective bearing and never hammer

or pry the bearing off since this may bend the shaft

and injure the commutator. A new bearing should be

pressed on the shaft until the inner race of the bearing

rests against the shoulder on the shaft. For this

operation always use an arbor press and a metal

cylinder or collar which bears only against the inner

race of the bearing. Care must be taken that no force

or stress is placed on the outer race of either bearing

at any time since this will usually result in a damaged

bearing.

(4). DISASSEMBLY: The following procedure

is recommended for disassembling the dynamotor

unit for cleaning and repacking the bearings:

(a)

Remove the three "safety-wired " screws

on each end of the dynamotor unit and take off the

two end bells.

(b)

Unscrew the slotted brush cap in each brush

holder and remove all four brushes. Take special

care to mark the position of the brushes in their

brush holders such that they may later be replaced

in their exact original positions.

(c)

Unscrew the four slotted screws located

around the rim of the low-voltage bearing bracket

and pry the bracket loose from the frame. The

low-voltage end of the dynamotor unit can be.

identified by the long, small diameter commutator

and the large copper-graphite brushes.

(d)

Detach the two field leads from the terminal

clamps on the low-voltage brush holders by un-

screwing the screw in each terminal clamp. It is

not necessary or advisable to remove the terminal

clamp from the brush holders.

(e)

The bearing bracket can now be taken out

of the way and the armature withdrawn from the

frame. Take care not to injure the armature wind-

ings or commutator by rubbing against the field

poles. Do not lose any shim washers which may be

in the bearing housings, or which may stick to the

bearing. If any shims are present they must be

reassembled in the same housing.

(f)

End Play Adjustment : A spanner screw is

provided on one bearing bracket for adjusting end

•

play in the dynamotor.. This is properly adjusted

at the factory and ordinarily will not need to be

changed unless the armature is replaced. Before

making any adjustments with this screw, it is

necessary that two setscrews on either side of the

bearing be loosened. Then the spanner screw

should be turned out (counter clockwise) one turn.

Operate the dynamotor until it is hot (about 30

minutes.) The end play should then be adjusted by

turning the spanner screw as required until the

end play as shown on an indicator reading to

1/1000 inch, is between approximately 4/1000 to

8/1000 inch.

In case an indicator is not available an alternative

method of adjusting the end play is as follows:

Screw the spanner screw in until the bearing

starts to growl, then back it off immediately

turn (45 degrees). This latter method must be

used with extreme care since if- the bearings are

set up too tight or allowed to remain tight for any

period of time they may be damaged. The set-

screws should then be tightened to hold the spanner

nut in position.

(g)

In cleaning the ball bearings it is not neces-

sary to remove them from the armature shaft.

Simply immerse the bearing in a shallow pan of

clean carbon tetrachloride and wash all of the grease

from the bearing. The use of a small camel's-hair

brush will greatly aid the thorough cleaning of the

bearing. Change the cleaning solvent in the pan as

soon as it becomes dirty. Always use clean solvent

for the final rinsing of the bearings.

Do not allow the cleaning solvent to come in

contact with the commutators or windings.

In case

the cleaning fluid is accidentally splashed on these

parts, wipe it off immediately. Make certain the

bearing is thoroughly clean and dry before repack-

ing with new grease. If the bearing is not to be

repacked with grease within a few minutes after

cleaning, flush it with a clean, light mineral oil to

protect the polished balls and races from rusting.

(h)

In reassembling the dynamotor unit, follow

in reverse order the procedure for disassembly.

Make certain that the two field leads are securely

attached to the proper terminal clamps on the low-

voltage brush holders and that the "slack " or

"loop" in these leads is "tucked" back of the

field coils. Do not loosen or remove the brush

holders from the bearing brackets. If a brush holder

is loosened or removed for any reason, replace it

securely in exactly its original position and check

to make certain the contact surface of the brush

fits the commutator perfectly. If necessary, sand-in

the brush to secure a good fit against the commu-

tator.

(5). TYPICAL PERFORMANCE: The dyna-

motor unit alone without the transmitter should

operate approximately as follows on a load test :

Input

Output

Volts

Amperes

Volts

Milliamperes

24.0

5— 7.5

1140-1160

24.0

14-16.5

1070-1090

220

24.0

20-22.5

1025-1050

350

Note : The input current above includes that

taken by the starting relay.

TONE \

i• VOICE

•

SWITCH 1141

807

801



Bog

1803



80 5

0501)

804

821

82 2

814

1109

1199

1110

1113

1117

1101

l•

1160

1109

1197A

1164 a

—•

1109

1114

162

1107

1122

1119

MOD

4-G

1115

1108

1187

ANTENNA

SIMPLIFIED



CIRCUIT



SWITCH

SCHEMATIC

Ill



POSITION



"N"

LE:'0

„

POSITION



"N"

POSITION



"r "

...)

POSITION *4



1183

1184

1179

T-4-C

1112

P.A.

M.0

1161

MOD

•

--/

61 is e

L-

1167

ANTENNA INDUCTAFNCEI

TUNING "M

1121

1116

1134

1133

1135

MOD

MICROPHONE CONNECTIONS

cr,

KEY CONNECTIONS

114 9

.--•

la%

114 5

147

1193

114 6

ANTENNA

CIRCUIT

SWITCH

ANT

CAPACITOR

TUNING

1170

—

1129

TONE



ToV

1142



1197C

168

SW2

CD-

1169

+1156

--1156

1156

11191

115 0

1192

1148

RADIO

TRANSMITTER BC

375

ANTENNA TUNING UNIT BC

306

DYNAMOTOR

UNIT PE

BROWN

YELLOW

GREEN

BLUE

RED

WHITE

ORANGE

BLACK

KEY



/1194



NIC. INPUT 011128--MOWN



MC.SIIMTCH OR KEY(T1P)---WMTE



SIDE TONE



BLUE



KEY RELAY ARM



RED



KEY RELAY BACK—YELLOW

•



11010,811C.,OR KEY (SLEEVE)—GREEN



•



03NDONIC..OR KEY (SLEEVE)—BLACK



(ON

OK7 SWITCH (DYN. RELAY)

OPT

014

255.

119 0



1126



1172

TEST KEY

1131

1127

1189

(IBS y

1128

SO-39

186

1198

ANT. INDUCTANCE

SWITCH "

0 ANT

LOAD "8 '



LOAD '

11"



0 NEC

INTERLOCK OPERATED

BY TRANS. TUNING UNIT

TO AIRPLANE

JUNCTION BOX

TYPICAL TRANSMITTER TUNING UNIT TU



TO PRIMARY POWER SUPPLY

28 VOLTS DC

Fig. 1. Radio Transmitter BC-375-E, Schematic Diagram

Fig. 1

CONNECTION

SIZE



OF CONDUCTOR

•

102

DIA COPPER WIPE TINNED

OW DIA COMM SIRE TINNED

00,01N



COPPER WIRE TINNED

011

DIA



COPPER WIRE TINNED

COLORED

N i•ANS STRANDS ES/ MEV

RIMER MLR-ATM.

E SPECS



N-/97E345

UNMANNED

COLORED

IS ISAIAS STRANDS



MeV I/o.•

EARNER INSULATION

G E SPECS



IC-71M345

UNIT

PIECE OF



it 10126 W012630005 V.; BUNSEN INSULATION

•

E SPECS F-767234!



-11

CAO TN~S POSES

NES

LE TOUCHES EDGE 146

C•SE

SPOT SURFACE OF WOE.) AT TINS FOIST

SECURE • 8000 GROUND CONNECTION

(IBS

1166

TINS

.".",e'sr.?,2 '17.'i."3"

TOR 30

LOAD

ANT

1--

L—



—



CS11

16011_



Fu (

3•11 so

R 0/SLAGS

—

44 4

(

73 65 u i3



•

ORARGE/SLU



9 0

REO

-.•

ROWS

10.

MEN

',GREEN

ES NI

fns

1100

SLE

REAR VIEW

RISS

11 0 S



•



1i9



1192

r1105



F )

$.00



1136

ISOLAN

SUPPORT

TITE

START

1170

SCREEN

ND CO L

FINISH —0

ONANGE



SRO

REAR VIEW OF RESISTOR BOARD



(RESISTOR

1196)

CLEAT MS C•GLE

TO TOP SmPELO

RANGE



USE CLEAT N-7453041

61r.

1747

1.16

TEMMIx•

I A

TO 110 TERM/SAL Et 2

AO St

)

NOTE

WIREMAN SHOULD PLACE WIRE 0 ON !TEM 1163

SUCN TNAT IT MAY SE REMOVED wiTROUT

SEMOYiNG OINER ROMS ON

SAME

TERRIMAL

RAREL

T°

8-0



•

K-k.



00.6E



~14 5

'eW

7‘)

97"

36 TS 31.



3 013



76 30 91

—

trr

"`"4



-11

ORANGE

1154

GREEN

SR=

{BEEN-4



ORANGE

) V=

TS 111 17

8R0

CRANT

E__L



SNEER

SLACK

ORANGE

„ — —

116

MIDOL

RED



183

ACE

112E

_))

ONE SIDE OF 1103 6401110f 0



CONNECTION

GREEN

.49

•



1PLUE

PARALLEL

SEC

1135

PRIM •

FIL

WIRES 60 068

TOM PLACED

RETWEEN

TPA

•

SASSIER

MOWN

SOS

ORANGE /SLUE

SLAGS

GREEN

•



•

E 10



40 90 30 s TO



•



• 0



4100 ORANGE 41 62



GREEN,.



•

li7.1171

f..

o•: RI

SLAGS -

RED



NOTE

- ;ASTER WIRES SECURELY TO TERMINALS NEGNANICALLY REPOSE

SOLDERING USE 110501 CORE SOLDER 82003C

TN E HEAVY LINES



WAYS •RE

GUIDE



ASSIST IN TRACINGHIGH

CONNECTIONS

ON THE

DIAGRAM

AND DO

NOT NECESSARILY MOIL ATE CAKES

CON DUC O



112

9EpUl NYENIENT

N 11/704t•

•

POINTS MA,RNED

•

USE TERMINAL 1,7872 50•

AT POINTS MANSE



F USE TERMINAL 18-7S 7020

AT POINTS 210 NED •2 USE TERMINAL N 78723.6

REAR

VIEW

Ni POSTS M ***** WUSE



COTTER PIN

AT POINTS MARNES

USE CONNECTOR lc 7•7200

AT POGITS

MARRED

use

NOT TN/ PIPPED WANNER 1

417453

Fig. 2. Radio Transmitter BC-375-E, Connection Diagram

Fig.

•

LLOW



TO OPERATE

SNAP-SLIDES

a Low •- TO REMOVE PLUGS

•

OPERATE SIM, SLIDES

WEIGHTS IN POUNDS

Radio Transmitter BC-375-E,

without Mounting, Transmitter

Transmitter Tuning Unit

TU-5-B



...

TU-6-B

14.437

12.437

Tuning Unit, and Tubes



49.0

Transmitter Tuning Unit

TU-7-B

11.687

Mounting FT-151-A



3.75

Transmitter Tuning Unit

TU-8-B

11.437

One Set of Tubes, Consisting

Transmitter Tuning Unit

TU-9-B



...

11.437

of Four Tubes VT-4-C and

Transmitter Tuning Unit

10-B



...

11.437

One Tube VT-25



2.0

Transmitter Tuning Unit

TU-26-B

12.750

Fig. 3. Radio Transmitter BC-375-E, Dimensions and Weights

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