RCA BTA-250M User manual

.L
.
Printed in U. S. A.
BTA-250M.
BROADCAST
TRANSMlTTiR
INSTRUCTIONS
Manufactured by
RADIO CORPORATION OF AMERICA
ENGINEERING PRODUCTS DEPARTMENT
Camden, New Jersey, U. S. A.
16-30220-l

TABLE OF CONTENTS
Page
TECHNICAL SUMMARY ..........................................................
6
Electrical Specifications .............................................................
6
Mechanical Specifications ...........................................................
6
Tube Complement ................................................................. b
EQUIPMENT ....................................................................
7
RECOMMENDED TEST EQUIPMENT ................................................
7
DESCRIPTION ...................................................................
9
INSTALLATION ..................................................................
10
General .........................................................................
IO
Installation Precautions .............................................................
10
Antenna Current Readings .......................................................
10
Atmospheric Static Accumulations .................................................
11
Electrical Storms ..............................................................
11
Antenna Current Meters .........................................................
11
Wiring .........................................................................
11
Preliminary Adjustments ............................................................
12
Output Circuit ....................................................................
14
OPERATION ....................................................................
22
MAINTENANCE ..................................................................
22
PARTS LIST .....................................................................
25
Figure Title
LllST OF ILLLJSTRATlONS ,
Page
1
2
3
4
5
6
7
8
9
10
11
12
BTA-250MTransmitter ........................................................
Simplified Schematic Diagram, Typical Transmitter Output and Tower Lighting Circuit
(A-8842553) .............................................................
CoilTurnsvsFrequency,LlO5 (8881438-I) ........................................
Inductance of L106, Antenna Resistance Above 70 Ohms (8881438-2) ..................
Inductance of L106, Antenna Resistance Below 70 Ohms (8881438-3) .................
Coil Turns vs Inductance, L106 and L107 (8881438-4) ............................
Capacity of Cl20, Cl21, Cl22 Combination (8881438-5) ...........................
Transmitter, Front View-Door Open .............................................
Transmitter, Rear View-Door Open .............................................
Outline, Transmitter (B-453333) ................................................
Connection Diagram, Transmitter (EE-3 11102) ......................................
Schematic Diagram, Transmitter (E-3 I 1343) .......................................
8
10
17
18
I9
20
21
23
24
28
29
31
LIST OF TABLES
Table Title Page
I External Connections to BTA-25 OM Unit .......................................... 11
2 Oscillator Plate Coil Taps ......................................................
12
3 Typical Meter Readings ........................................................ 13
4 PA Capacitors C117, Cl23 ..................................................... 13
5 Connections, Capacitors Cl20, C121, Cl22 (8842552) .............................. 16
6 Current Rating, Cl20, CI21, Cl22 ............................................... 17
7 Typical Tube Socket Voltages ................................................... 22
3

WARNING!
Operation of electronic equipment involves 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 the equipment with voltage
supply on. Under certain conditions dangerous
potentials may exist in circuits with power con-
trols in the off position due to charges retained
by capacitors, etc. To avoid casualties, always
discharge and ground circuits prior to touching
them.
ABOUT FIRST AID
Personnel engaged in the installation, operation
and maintenance of this equipment or similar equip-
ment are urged to become familiar with the follow-
ing rules both in theory and in the practical appli-
cation thereof. It is the duty of every radioman to
be prepared to give adequate First Aid and there-
by prevent avoidable loss of life.
1.
2.
3.
4.
5.
6.
7.
PRONE-PRESSURE METHOD OF
RESUSCITATION
PROTECT YOURSELF with dry insulating material.
BREAK THE CIRCUIT by opening the power
switch or by pulling the victim free of the live
conductor.
DON’T TOUCH VICTIM WITH YOUR BARE
HANDS UNTIL THE CIRCUIT IS BROKEN.
FIRST AID
(A) w
LAY PATIENT ON STOMACH, one arm extended, the 8.
other arm bent at elbow. Turn face outward resting on
hand or forearm. 9.
REMOVE FALSE TEETH, TOBACCO OR GUM from patient’s
mouth. 10.
KNEEL STRADDLING PATIENTS THIGHS. See (A).
PLACE PALMS OF YOUR HANDS ON PATIENT’S BACK
with little fingers just touching the lowest ribs.
WITH ARMS STRAIGHT, SWING FORWARD graduallv 11.
bringing the weight of your body to bear upon the
patient. See (B). 12.
SWING BACKWARD IMMEDIATELY to relieve the pres-
sure. See (C).
AFTER TWO SECONDS, SWING FORWARD AGAIN.
Repeat twelve to fifteen times per minute.
WHILE ARTIFICIAL RESPIRATION IS CONTINUED, HAVE
SOMEONE ELSE:
(a) Loosen patient’s clothing.
(b) Send for doctor.
(c) Keep patient warm.
IF PATIENT STOPS BREATHING, CONTINUE ARTIFICIAL
RESPIRATION. Four hours or more may be required.
DO NOT GIVE LIQUIDS UNTIL PATIENT IS CONSCIOUS.
E
E
E
B
E
E
e
e
I
0
I!
e
I
I
e
e
E
%
I
1
6
6
6
6
E
E
E
E
E
E
E
E
E
E
E
6
6
6
6
6
6
6

REPLACEMENT PARTS
AND
ENGINEERING SERVICE
When ordering replacement parts, please give symbol, description, and stock number of each
item ordered.
The part which will be supplied against an order for a replacement item may not be an exact
duplicate of the original part. However, it will be a satisfactory replacement, differing only in
minor mechanical or electrical characteristics. Such differences will in no way impair the operation
of the equipment.
Service parts may be ordered through the local Broadcast representative, his office, or directly
from the Service Parts Order Service Bldg. 60, 19th and Federal Streets, Camden, N. J. Emer-
gency orders may be phoned, telegraphed, or teletyped to RCA Emergency Service, Bldg. 60,
Camden, N. J. (Telephone: Woodlawn 3-8000.)
ELECTRON TUBES
Replacement tubes should be ordered from local distributors or the nearest RCA tube ware-
house.
RCA tube warehouses are located at the following addresses: .
34 Exchange Place
Jersey City 2, New Jersey
589 E. Illinois Street
Chicago I 1, Illinois
420 S. San Pedro Street
Los Angeles 13, California
If, for any reason, it is desired to return tubes, please return them to the place of purchase.
If this is not convenient, please notify your RCA serving warehouse so that Return Authoriza-
tion may be forwarded to you.
PLEASE DO NOT RETURN TUBES DIRECTLY TO RCA WITHOUT AUTHORIZATION
AND SHIPPING INSTRUCTIONS.
It is important that complete information regarding each tube (including type, serial number,
hours of service, and reason for its return) be given.
When tubes are returned, they should be shipped to the address specified on the Return AU-
thorization form. A copy of the Return Authorization and also a Service Report for each tube
should be packed with the tubes.
ENGINEERING SERVICE
RCA field engineering service is available at current rates. Request for field engineering service
may be addressed to the local Broadcast Sales Engineer or the RCA Service Company, Inc., Com-
munications Service Division, Camden, N. J. Telephone: Gloucester 3-4560 during working
hours; emergency service is provided through Woodlawn 3-8000.

TECHNICAL SUMMARY
ELECTRICAL SPECIFICATIONS
Frequency Range ..................................................
535- 1620 kilocycles
Frequency Stability (assigned frequency) ...................................... 2 5 cycles
Power Output ............................................................ 250 watts
Carrier Shift .......................................................... 2 s maximum
Output Impedance ...................................................... 20-250 ohms
Type of Modulation ........................................... High level, Class “B,” AM
AF Input Level (100% modulation) ...................................... $10 t-2 dbm
AF Input Impedance ...................................................
150/600 ohms
AF Response (reference 1,000 cycles) :
50-7,500 cycles ......................................................... -tl
db
30-l 0,000 cycles ......................................................
k1.5 db
AF Distortion (95 % modulation, 50-l 0,000 cycles) .......................... 2 % maximum
Noise Level (below 100 70 modulation) .......................................... -60 db
Input Power Line Requirements ...................... I lo- 125 volts, 50/60 cycles, single phase
Permissible Combined Line Voltage and Regulation ................................. 1-5 $%
Power Consumption :
Zero Modulation ............................................. 1,000 watts, approx.
Average Program Modulation ................................... 1,15 0 watts, approx.
100 % Modulation ............................................ 1,400 watts, approx.
Crystal Heater Input Power ............................. 1 17 volts, 50/60 cycles, single phase
Crystal Heater Power Consumption .............................................
30 watts
Monitoring Outputs:
Frequency ................................................
10 volts RMS, 75 ohms
Modulation ...............................................
10 volts RMS, 75 ohms
MECHANICAL SPECIFICATIONS
Cabinet Height ......................................................... 847/8 inches
Overall Height .......................................................... 865/8 inches
Width.. ............................................................... 28 inches
Depth (less door handles) ................................................. 201/z inches
Weight, net ..................................................... 600 pounds, approx.
TUBE COMPLEMENT
Crystal Oscillator ............................................. I-RCA Type 807
Buffer ...................................................... I -RCA Type 807
Power Amplifier .............................................. 2-RCA Type 8 13
First Audio .................................................. 2-RCA Type 807
Modulator .................................................. 2-RCA Type 6 I 3
High-Voltage Rectifier ......................................... 2-RCA Type 866A

EQUIPMENT
The BTA-250M Broadcast T ransmitter is identified by RCA reference ES-28937 and includes the follow-
ing major items:
RCA
Reference
No.
I BTA-250M Transmitter, including UL-4392 crystal oscillator, but less
tubes and crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MI-28053
1 Set of Tubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MI-28049
2 Side Panels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MI-3054l-G84
I Touch-Up Finish Kit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MI-7443
1 Nameplate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . MI-28 180- 1
I Crystal Unit, TMV-129B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MI-7467
2 Instruction Books . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IB-30220- I
- Choice of: 1 Ammeter, for Remote Pick-Up,. . . . . . . . . . . . . . . . . . . . . . . . . . . . . MI-28037-B
or I Ammeter, R.F. Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MI-28048
Available Accessories:
Antenna Tuner, Type BPA- I I A, including MI-28027A Remote Pick-Up
Assembly and MI-7 174 Antenna Ammeter .............................. ES 28906-A
Remote Metering Kit, less meter ......................................... MI-2802 7-A
Spare Crystal Unit, TMV-129B .......................................... MI-7467
Kit of Tubes (Spares) ................................................ MI-28049
Kit of F. C. C. Spare Tubes ............................................ MI-28084
Electrical Side Shields, per side-
Two .......................................................... MI-30546-G28
One .......................................................... MI-30546~G2 1
Audio Rack, Type BR-84D, less front door and side panels .................. MI-3095 l-D84
Modulation Monitor, Type BW-66E ..................................... MI-30066-A
Frequency Monitor, Type BW-1 1A
....................................... MI-300 11 -A
RECOMMENDED TEST EQUIPMENT
RCA
Reference
No.
Type WA-28A Low Distortion Audio Oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MI-30028-A
Type WM-7 1A Distortion and Noise Meter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MI-3007 1-A
Type WV-97A VoltOhmyst
Type WO-56A Cathode Ray Oscilloscope

Figure I-BTA-250M Transmitter
8

,
DESCRIPTION
The Type BTA-250M Broadcast Transmitter is
a complete, self-contained unit that provides a
power output of 250 watts at any frequency in the
535 to 1620 kilocycle range. Mounted in a single
RCA Type BR-84 steel cabinet, as shown in Figure
1, the unit utilizes a temperature-controlled crystal
for close regulation of the operating frequency.
All operating controls are accessible from the
front of the panel through the access openings pro-
vided; meters are mounted above the hinged front
door. A power source of 1 17 volts, 50/60 cycles,
single phase capable of supplying approximately
1,450 watts is required for the transmitter.
A crystal oscillator, buffer-driver, and power-
amplifier comprise the r-f portion of the transmitter.
Referring to the schematic diagram, Figure 12,
capacitor Cl is used for adjusting the oscillator fre-
quency within narrow limits, and is the only variable
tuning element in this stage. Pilot lamp I1 0 1 serves
to indicate when the crystal heater is energized,
while an extra socket, Xl 10, provides means for
storing a spare crystal. The spare crystal heater is
energized at all times, permitting instant substitution
of the spare for the operating crystal.
An RCA type 807 tube is used in the buffer-
driver stage, V 10 1. The plate circuit is shunt fed
through coil L 10 1. Taps on L 10 1 enable the buffer-
driver stage to be adjusted for optimum output over
a wide frequency range.
The power amplifier is a plate-modulated Class
“C” amplifier which uses two RCA type 8 13 tubes
connected in parallel. The low-pass “T” filter used
as output in this stage serves to match the high plate
impedance to the low antenna impedance as well
as acting to reduce harmonic radiation. Since screen
grid tubes are employed, neutralization of the PA
stage is not required.
Two push-pull stages comprise the transmitter
audio system. The two type 807 tubes in the first
stage operate Class “A” whi1.e the modulator uses
two type 8 13 tubes as a Class “AB,” amplifier. To
reduce distortion, approximately 14 db of negative
feedback is used on the input stage. The tapped
secondary of modulation transformer T102 makes
it possible to modulate the screen of the buffer-
driver, Vi0 1, as well as the screens and plates of
PA tubes VI02 and VlO3.
Potentiometers R155, R 158, and R 159 provide
means for maintaining the overall hum at minimum
level.
D-c supply for the transmitter is furnished by
two rectifiers. High-voltage for the modulator
plates and the PA screens and plates is supplied
by two type 866A mercury vapor tubes, V108
and V109, functioning as a single-phase full-wave
rectifier.. Selenium rectifiers SRI 02 to SRI 07 also
are connected in a full-wave rectifying circuit to
furnish approximately 750 volts for the type 80 7
tubes and a negative 200 volts for modulator bias.
Power to the transmitter is fed through the main
circuit breaker Sl 0 1, the trip coils of which are con-
nected in the filament circuit. Thus a filament over-
load would remove all power from the transmitter.
Control of filament voltage is provided by rheostat
R 149, in conjunction with meter rectifier SR 10 1 and
meter M 104. Pilot light I1 02 serves to indicate
the status of the filament circuit.
When filament power is applied by the closing of
S 10 1, time delay relay K 10 1 is energized. Relay
K 10 1 closes in ‘approximately 30 seconds de-ener-
gizing the trip coil of plate breaker S104. Plate
power may then be applied manually by closing
S 104, energizing the rectifier plate circuits.
Circuit breakers S 102 and S 103 are connected in
the cathode circuits of the modulator and power
amplifier, respectively, protecting these stages in
case of overload.
Milliammeter M 104 and selector switch S 105 pro-
vides facilities for checking all stages in the trans-
mitter. Switch positions along with the various
circuits involved are listed in Table 3, under IN-
STALLATION.
For frequency monitoring, receptacle J 10 1, con-
nected to the cathode of buffer-amplifier V 10 1, pro-
vides means for sampling the voltage at this point.
Output, approximately ten volts at approximately
75 ohms impedance, may be varied by potenti-
ometer Rl 12.
R-f output for modulation monitoring is obtained
from transformer T103 which is connected to J102.
Output of this circuit is designed to supply approxi-
mately ten volts at approximately 75 ohms imped-
ance. Taps on the secondary of T103 provide
means for adjusting the secondary voltage.

INSTALLATION
GENERAL
Upon receipt the unit should be unpacked and in-
spected for evidence of any damage that may have
been incurred during transit. The tubes and crystals,
which are packed separately, should not be installed
at this time but left in their cartons until the unit
is mounted in place and all external wiring com-
pleted.
Figure 10 shows the transmitter outline dimen-
sions and the base slot location for making external
connections. Ample working space should be pro-
vided at the front and rear of the transmitter.
this situation exists it is possible to have two ground
return paths for the 60-cycle lighting current-one
through the antenna coupling equipment and trans-
mitter output circuit, the other in the ac lighting
circuit through the tower lighting chokes to ground
where one side of the ac is grounded. A simplified
schematic diagram of a typical circuit illustrating
this possibility is shown in Figure 2. To prevent
the meter fluctuations it is necessary for the 60-cycle
tower lighting load to be returned via a path other
than the r-f circuits feeding the tower.
The transmitter frame should be grounded with
adequate, short connections. A two- or three-inch
wide copper strap, approximately 0.032 inch thick,
is recommended for this purpose.
If a toroidal tower lighting transformer is used,
no antenna current meter fluctuations will occur.
Where lighting chokes are utilized, the circuit should
be checked for the existence of a second ground
path as previously described.
INSTALLATION PRECAUTIONS
Prior to installation of the transmitter, certain
installation factors should be considered in the
light of possible future operating trouble and parts
failure. Thus, some tower lighting circuits may
cause fluctuations in the antenna current meter read-
ings when the tower lights are operating. Also, in
certain localities, provisions must be made for pre-
venting lightning damage and dissipating static
charges which may accumulate on the tower. Pre-
cautions taken during installation will prevent these
operating difficulties and resultant equipment dam-
age.
Antenna Current Readings
Under certain circumstances, when the tower
lights are on, the 60-cycle tower lighting current
may cause inaccuracies or fluctuation in the antenna
current meter reading. This condition is created
when the tower itself serves as one side of the light-
ing circuit and, hence, provides a common path for
the tower lighting current and the r-f current. Where
Elimination of the 60-cycle return path through
the coupling equipment or transmitter output circuit
is achieved by inserting a blocking capacitor in
the antenna feed line. The capacitor may be con-
nected in either of two places, just ahead of the
antenna current meter or between the transmitter
output and transmission line. The location depends
upon the type of coupling circuit used in the line
terminating unit. Value of the capacitor, shown
dotted on Figure 2, should be approximately O.O%l
mf with a current rating equal to two or three times
the normal antenna current. For low-frequency,
low-impedance transmission lines, it may be neces-
sary to use higher capacitance, so that the reactance
does not exceed one-quarter of the transmission line
impedance.
To determine whether antenna current meter
variations are caused by the condition just described,
turn on the tower lights while the transmitter is OFF.
The presence of any current reading on the antenna
current meter at this time indicates the need for cor-
rective measures.
SUGGESTED LOCATIONS
OF A BLOCKING CAPACITOR
(ONE REQUIRED) TO
A-8842553
REMOTE4
ANTENNA AMMETER
Figure Z-Simplified Schematic Diagram, Typical Transmitter Output and Tower Lighting Circuit (A-8842553)
10

-.
Atmospheric Static Accumulations
In certain localities atmospheric conditions build
up high static potentials on the transmitting towers,
making it imperative to provide a drain path to
ground for these accumulations. If no direct path is
provided, the charge builds up potential until flash-
over occurs either across the tower base arc gap or
across one of the capacitors in the coupling system.
Where tower lighting chokes are used and one
side of the ac supply line is grounded, the lighting
choke will act as a satisfactory discharge path.
When neither side of the ac line is grounded, or
when a toroidal tower lighting transformer is utilized,
a drain path must be provided. Such a path
already exists in the BTA-250M transmitter through
L105, L 106, and L 107. Should it be necessary to
break this path to ground, by the use of a series
capacitor, for example, it may be necessary to pro-
vide an alternate path. To check for the exist-
ence of a suitable dc path to ground, turn off
the transmitter and tower lighting circuit. Connect
an ohmmeter between the tower and ground. Any
resistance up to approximately 250,000 ohms will
provide a satisfactory return circuit. When no dis-
charge path is indicated, one may be supplied by
the installation of an r-f choke or a 100,000 to
200,000-ohm Clobar resistor. Connect either the
choke or the resistor from the antenna feed line to
ground. The line terminating unit will generdlly
serve to house the component used.
If antenna current meter fluctuations result from
the addition of the choke or resistor, it will be
necessary to add the blocking capacitor discussed
under the preceding heading “Antenna Current
Readings.”
Electrical Storms
Areas subject to lightning storms require a direct
electrical path from the tower to ground, to avoid
capacitor and antenna current meter burn-out by
lightning striking the tower. This requirement is
generally met by arc gaps installed across the base
insulators. These arc gaps should be carefully ad-
justed during bad weather-which could be either
dusty or humid conditions. Spacing of the arc gaps
should be set so that flashover occurs just beyond
the point of 100% sine wave modulation.
If the gaps are properly spaced, the gaps at time
of discharge present a low impedance path to
ground, and will carry directly to ground any cur-
rent caused by the lightning striking the tower.
Although there is a second path to ground, through
the tuning equipment or transmitter output, the
higher impedance of this second path usually pre-
vents excessive discharge under normal conditions.
In instances where the tuning house is located under
the tower or directly adjacent to it, the ratio of these
two impedances may not be sufficiently high to pre-
vent appreciable discharge current through the tun-
ing equipment to ground, with consequent destruc-
tion of coupling equipment. To increase this ratio,
a one- or two-turn loop should be installed in the
antenna lead from the tower to the tuning house.
No such loop is required where the tuning house is
several feet from the tower. The longer lead in the
latter instance provides the necessary higher im-
pedance.
Antenna Current Meters
Where thermocouple-type instruments are used
for antenna current metering, the meter circuits
should be equipped with make-before-break knife
switches such as are incorporated in RCA line tun-
ing units. This type switch enables one side of the
meter circuit to be opened when readings are not
being taken.
WIRING
Wiring to and from the transmitter should be car-
ried in conduit or a trench terminating below the
unit. The base plan indicates where this wiring
should enter the unit. Table 1 lists the external con-
nections that should be made to the transmitter.
TABLE 1
EXTERNAL CONNECTIONS TO BTA-250M UNIT
POINT OF
CONNECTION EXTERNAL CIRCUIT
TBl Ground
TB2 1
TB3 \ AF input
TB4 -
TB5 -
TB6 -
TB7 R e m o t e antenna ammeter
TB8 (when used)
TB9 / I 1 7 volts, 50/60 cycles for
TBlO 1 crystal heater
TBll )
TB12 1
I 17 volts, 50/60 cycles power
input
Frequency monitor
Modulation monitor
JlOl
5102
11

A separate 1 17-volt source should be used for the
crystal heater power supply, since this power must
be supplied continuously for maintaining the crystals
at the proper operating temperature.
When a six-wire, 230-ohm transmission line is
used, it should be terminated at the lead-in insulators
mounted on top of the cabinet.
Connections from these insulators to the bush-
ings on the wall of the transmitter house are gen-
erally of copper tubing. When a coaxial transmis-
sion line is used, remove the lead-in insulators and
bring in the transmission line through one of the
holes. Fasten the outer conductor of the line
securely to the equipment chassis, and scrape the
paint from the chassis so that a good electrical
ground is obtained.
If the MI-28048 ammeter is to be installed for
reading the transmitter r-f output, the meter should
be mounted behind the dummy meter case on the
panel using the brackets and hardware supplied.
Connect the meter as shown on the schematic dia-
gram, Figure 12.
When the RCA BPA-1 1A Antenna Tuner and
MI-28037-B Remote Antenna Ammeter are utilized,
remove the dummy meter case, and mount the
MI-28037-B Remote Antenna Meter directly on the
meter panel. Wire the meter as indicated in the
upper right-hand corner of the schematic diagram,
Figure 12.
On reactor L 1 10, adjust the arc gap to 0.02 inch.
After all wiring is completed install the tubes and
plug the crystal holder into socket X2. Plug the
spare crystal into socket XI 10.
PRELIMINARY ADJUSTMENTS
SAFETY WARNING
VOLTAGES USED IN THIS EQUIP-
MENT ARE DANGEROUS, AND CON-
TACT WITH LIVE PARTS MAY
PROVE FATAL. OBSERVE ALL
SAFETY PRECAUTIONS WHEN
WORKING ON THE EQUIPMENT.
(SEE FIRST AID NOTICE IN FRONT
PART OF THIS BOOK.)
Before applying any power to the transmitter
make sure all circuit breakers are in the OFF
position. Close the FILAMENT circuit breaker,
SIOI. The filaments of all tubes should light.
When the METER switch, SlO5, is in position IO,
meter MI 04 should indicate the voltage applied
to the primaries of the filament transformers, T106
and T107. Therefore, if the measured voltage is
other than 120 volts, open the FILAMENT breaker
and reconnect the power leads on T 106 and T107 to
the taps next lowest to the measured voltage. Then,
using an external meter of known accuracy, measure
all filament voltages at the tube sockets. Adjust
the FILAMENT rheostat so that the measured volt-
ages are within two per cent of the rated value of
the tubes. Record the reading on meter M 104, in
position IO of SlO5, for future reference.
Primary taps of the plate transformers, T104
and T105, should now be adjusted by measuring
the line voltage at the transformer terminals and
adjusting the power leads to the taps most nearly
corresponding to the measured voltage.
Note:
Before proceeding further it is important that
the transmitter be permitted to operate for at least
30 minutes with filament power only. This warm-
up period is required to properly distribute the
mercury within the type 866A tubes. The pro-
cedure need be repeated during subsequent op-
eration only if mercury is splashed on the tube ele-
ments during handling or when replacing tubes.
It is suggested that all new 866A tubes be
“seasoned” as described and then stored in an up-
right position. In case of tube failure the trans-
mitter may thus be restored to operation with a
minimum of delay.
Now, before applying any plate power, open the
FILAMENT breaker and adjust the oscillator plate
choke, L I, to the proper tap as indicated in Table 2.
The flexibIe lead, or jumper, on Ll should be at
the setting which provides the smallest number of
active turns on Ll , or the lowest inductance. The
crystals are factory-calibrated for tuning on the
lower inductive side of resonance, hence, for proper
starting and stability, the minimum number of turns
should be used. Final adjustment should always
be on the high-frequency side of minimum oscil-
lator plate current.
TABLE 2
OSCILLATOR PLATE COIL TAPS
BAND COVERAGE JUMPER CONNECTION ON Ll
535-700 kc Remove (Use full coil)
700-1000 kc Tap 2
1000-1300 kc Tap 3
1300-1620 kc Tap 4
1.2

TABLE 3
TYPICAL METER READINGS
METER Ml04
CIRCUIT METER METER
Ml02 Ml03
CURRENTS
POSITION
SWITCH CORRESPONDING
s105 TO 100%
METER READING
Oscillator cathode, VI
1 30
ma
Buffer-driver grid, V I 0 I
2 3.5
ma
Buffer-driver cathode, V 10 1
3 65
ma
PA grid, V102, Vi03
4 25
ma
PA cathode, V102
5 150
ma
PA cathode, V103
6 150
ma
I st AF cathode, V104, V105
7 8.5
ma
Modulator cathode, VI 06 ( 100 s mod.)
8 150
ma
Modulator cathode, V I 07 ( I 00 R mod. )
9 150
ma
Primary, filament transformers
10 100 volts
PA plate current
250 ma
PA plate voltage
1,450 volts
Connect the jumper on coil L 103 as follows:
535-640 kc-Use full coil
640-790 kc-Tap I
790-980 kc-Tap 2
980-l 200 kc-Tap 3
1200-I 420 kc-Tap 4
1420-1620 kc-Tap 5
Make sure the MOD and PA circuit breakers are
in the OFF position. Remove the plate cap from
one rectifier tube, type 866A. Operate the PLATE
circuit breaker, S104, to the ON position. This
applies plate voltage to all tubes, but plate current
should flow in only the low power tubes.
2,
Rotate the METER switch, S 105, to positions 1,
and 3 and note the current readings on meter
Ml 04. Meter readings should be approximately
100% ; however, readings from 80 “/o to 120 “/o are
normal. If th e b ff
u er grid current is low or the crys-
tal is sluggish in starting, shut off all power and read-
just the oscillator plate choke tap. If the frequency
monitor output jack, JIOI, is not loaded, the buffer
grid current may be excessive.
duced by adjustment of RI 12. Current may be re-
After th e oscillator and buffer stages are func-
tioning satisfactorily,
Connect the PA tank open the PLATE breaker.
capacitors Cl 17 and C 123
and adjust L105 tank coil shorting tap as indicated
in Table 4 and Figure 3. Disconnect the coupling
tap on LlO5 and reconnect it one turn from the
grounded end of the coil.
The harmonic filter and antenna coil shorting taps
and the harmonic filter capacitors should now be
adjusted as described under the succeeding heading,
“Output Circuit.” This subject is treated separately
due to the calculations involved, but adjustment
should be made at this point.
Having made the output circuit calculations and
adjustments, operate the PA circuit breaker, S103,
to the ON position and again close the PLATE
breaker switch. Quickly tune the PA to minimum
plate current by rotation of the PA TUNING knob.
The PA PLATE CURRENT meter should indicate
TABLE 4
PA CAPACITORS C117, Cl23
BAND COVERAGE CAPACITORS CONNECTED
535-600 kc
Cl17,C123

approximately 50 ma. After adjusting the PA plate
circuit, open-the PA and PLATE switches.
Replace the plate cap removed previously from
the 866A tube. Set the POWER OUTPUT control
to mid-position. Since the tap setting on L106 is
approximate only, it is necessary to readjust this
tap for maximum loading.
Shut off power and follow an adjustment pro-
cedure of moving the tap on LlO6 slightly in one
direction and then noting the plate current when
power is reapplied. Repeat this adjustment cycle,
changing the direction if necessary, until maximum
plate current is obtained. This maximum should
be a peak reading which should drop off on either
side of resonance. If resonance can not be ob-
tained, the output coupling is excessive, and the
tap setting on LlO5 must be changed.
After obtaining peak plate current, readjust the
PA TUNING control for minimum plate current.
If appreciable detuning is evident, the output
coupling is excessive and must be reduced by read-
justing the tap on L105.
After resonating the output circuit by readjust-
ment of L106, output may be set at the required
level by readjusting the coupling tap on LlO5.
Slight readjustment of Cl 18 may be required to
keep the circuit at resonance.
After the output circuit has been completely ad-
justed, the efficiency of the power amplifier may be
improved somewhat by a slight detuning of Cl 18
in the direction of increasing output. Final adjust-
ment of the output power may be made by adjust-
ment of the POWER OUTPUT control. Check the
PA cathode current on tubes VI 02 and VI 03 in
positions 5 and 6 of the meter switch.
To adjust the modulator, open the PA and
PLATE switches. Using a screwdriver, adjust the
modulator bias potentiometers R15 1 and RI 52 to
the maximum clockwise position, which will apply
maximum bias to the modulator tubes.
Close the PLATE breaker and MOD breaker
switch S I 02, thus applying plate voltage to the mod-
ulator stages. Observe the modulator cathode cur-
rent on the panel meter, at positions 8 and 9 of the
METER switch. Now adjust RI 51 and RI 52 so
that the cathode currents on V 106 and VI 07, re-
spectively, are 30 per cent. The designation I,;,
refers to VI 06 and I,<, to VI 07.
Open the PLATE switch and connect a l,OOO-
cycle audio signal of suitable level to terminals
TBA-2 and TBA-3. At 100 per cent modulation,
meter Ml 04 should indicate approximately 100 per
cent in positions 8 and 9 of S 105.
To adjust for minimum hum, open the PLATE
switch. Rotate RI 55 to the extreme counterclock-
wise position, and set RI 58 and RI 59 at the center
position, i. e., the shaft screwdriver slot must be
vertical. Reapply power and, using a distortion and
noise meter, measure the hum level. In the
sequence listed, adjust R 158, R159, and then RI 5 5
for minimum hum.
When a noise meter is not available. rotate RI 55
to the counterclockwise position. Adjust RI 58
and RI 59 to the center position, using a scale not
exceeding 10 volts maximum on an a-c voltmeter,
for setting these potentiometers at the electrical
center.
OUTPUT CIRCUIT
The output circuit is designed to match any im-
pedance between 20 ohms and 250 ohms resistance.
Inductor L 107 is provided to cancel the capacitive
component of the antenna impedance when the
antenna is connected directly to the transmitter.
The circuit is normally an “L” type low-pass
filter, for antenna resistances greater than 70 ohms.
When the antenna resistance is less than 70 ohms, a
“T” type low-pass filter circuit is used.
In determining the output circuit components,
first note whether the antenna resistance is greater
or less than 70 ohms. If the antenna resistance
is less than 70 ohms, adjust LlO7 so that its re-
actance equals the capacitive antenna reactance;
or if the antenna resistance is less than 70 ohms,
the reactance of L 107 should equal the antenna
reactance plus 35 ohms. To determine the in-
ductance of LlO7, use the formula:
L =. 159 x,
f
where L is the inductance in microhenries, Xi. is the
required reactance in ohms, and f is the operating
frequency in kilocycles. Then use Figure 6 to find
the number of turns to be shorted.
The adjustment of inductor L106 and the shunt
capacitance may be determined from Figures 4, 5,
and 6. It should be noted that curves for the
inductance of L I 06 appear on both Figures 4 and 5.
Figure 4 is to be used where the antenna resistance
is greater than 70 ohms: if the antenna resistance
is less than 70 ohms, Figure 5 is to be used.
Following is the procedure for tuning the output
network :
1. Determine the required reactance of L 107.
AS previously noted, the reactance should be equal
to the capacitive reactance of the antenna when the
13

antenna resistance is greater than 70 ohms. When
the antenna resistance is less than 70 ohms, the
reactance of L107 should be equal to the capacitive
reactance of the antenna plus 35 ohms.
2. Determine the inductance of L IO 7 using the
formula:
159 x,
L=--fi
3. From Figure 6, determine the number of
turns to be shorted.
4. From Figure 4 or 5, determine the required
inductance of L106, using the known frequency
and antenna resistance.
5. Using Figure 6, determine the turns on L106
to be shorted, to obtain the inductance required
in Step 4.
6. Determine the required shunt capacitance
from the known frequency and antenna resistance,
using Figure 7.
7. Using the capacitance from the previous step,
and referring to Table 5, determine the manner of
connecting Cl 20, C 12 1, and C 122 for the closest
approximation to the required value.
8. Determine the antenna current:
I-E--
9. Determine the voltage across the
capacitance: I--- --__-
E,. =I, \/ 35” + Ra2
when Ra is less than 70 ohms
or E,. = I,R, when R, is greater than 70 ohms
IO. Calculate the current in each capacitor used
and make sure it does not exceed the rating deter-
mined from Table 6.
shunt
As an example of the preceding calculations,
assume that the power output from the transmitter
is 250 watts and that the transmitter is operating
into an antenna whose impedance is 20 -j30 ohms.
Assume the frequency to be 1250 kc.
I. The antenna resistance is less than 70 ohms,
therefore, the reactance of L107 is 30 + 35 = 65
ohms.
2. The inductance of L 107 is
15 9x,, 159 (65)
L = ------= ------. = 8.3 microhenries.
f 1250
3. From Figure 6, 26 turns must be shorted.
4., From Figure 5, the inductance of LlO6
should be 3.8 microhenries.
5. From Figure 6, 3 1 turns must be shorted.
6. From Figure 7, the required shunt capacitance
is 6200 mmf. Note that since the antenna resist-
ance, 20 ohms, is less than 70 ohms, the curve
marked “ANT RES. = LESS THAN 70 OHMS”
on Figure 7 is to be used.
7. From Table 5, 5900 mmf is found to be the
nearest approximation to the required 6200 mmf.
Therefore, Cl 20 and Cl 2 1 should be connected in
parallel.
8. 1, = L/r= \/see= 3.54 amperes
__ -. -
9. E,. = I, 35” + Ra” = 3.54 \/--. ~-
35’ + 20”
II 142 volts
IO. (Cl 20) I,. = 6.28fCE, = 6.28 ( 1,250,OOO)
(2000) (IO) lz (142) = 2.23 amperes
(C121) I;. = 6.28 (1,250,OOO)
(3900) (lo)--” (142) = 4.35 amperes
Reference to Table 6 indicates that the currents
are not excessive.
Check the output frequency on the frequency
monitor connected to J IO I. Any slight frequency
adjustment necessary may be made by rotation of
capacitor Cl in the oscillator stage. Potentiometer
Rl 12 adjusts the level to the monitor.
Modulation may be adjusted by using the I OOO-
cycle input signal connected to terminals TBA-2 and
TBA-3. One hundred per cent modulation may
be checked in any of the usual methods. Cathode
currents in the modulator are available in positions
7, 8, and 9 of the METER switch. Monitor level
may be adjusted by setting the secondary taps on
T 103 as necessary.
When all adjustments are completed, operate all
panel switches to the OFF position.
15

TABLE 5
CONNECTIONS, CAPACITORS C120, C121, Cl22
(8842552)
CAPACITANCE
MMF CONNECTIONS
2,650 --P2'= I,c122
2,820
3,900
Cl21,
I
8,200 , Cl22
12,100
14,100
16

TABLE 6
CURRENT RATING, Cl20, C121, Cl22
CAPACITOR VALUE
Cl20
2,000 mmf
AT 300 KC
2.2 amps
Cl21
Cl22
3,900
mmf 3.4 amps 4.5 amps 5.5 amps
8,200 mmf 4.5 amps 6.0 amps 6.0 amps
FREQUENCY IN KILOCYCLES
Figure 3-Coil Turns vs Frequency, L 105 (888 1438- 1)
17

Figure 4-Inductance of L106, Antenna Resistance Above 70 Ohms (8881438-2)
18

560 700 900 1100 1300 1500 1700
FREQUENCY IN KILOCYCLES
Figure S-Inductance of L106, Antenna Resistance Below 70 Ohms (8881438-3)
19

z
-