Gates BC- 250T Instruction Manual

INSTACTIONS FOR INSTALLING AND OPERATING
THE GATES MODEL BC- 250T
250 W. TRASMITTER
113-875
Gates Radio Company
tlincy, Illinois

REMOTE CONTROL ACCESSORIES
BC- IT, BC- 500T, BC- 250T

ADDENDA SdEET
INSTALLATION INSTRUCTIONS FOR R102E CONTROL
IN
GATES TRjINSMITTERS BC- 1T, BC- 500T, BC- 250T
Refer to the following overall transmitter schematic diagrams:
BC- 1T - C-78130
BC- 500T - E-25569
BC- 250T - E-25582
A steel plate, drilled for mounting the M-4719 Plate Voltage Kit,
M-4720 Plate Current Kit and M-4703 Rheostat Assembly, is available
for the installer's use. Tapped mounting holes for this plate } lave
been provided in the cabinet corner supports, directly beneath the
terminal boards and conactors ( right side of transmitter as viewed
from front). Photographs showing the remote control kits in place
is included witn these instructions.
M-4703 Motor Rheostat Assembly
As viewed from the rheostat end of this assembly, with the three
rheostat terminals to the left, strap the center terminal ( arm) to
the top terminal. Since application of " increase" voltage brings
about clockwise rotation of the arm ( same view), resistance will
decrease. , riti application of . idecrease° voltage, the resistance
will increase.
2he existing plate voltage meters in the BC- 1T, BC- 500T and BC- 250T
Transmitters read plate to cathode voltage, whereas the remote con-
trol voltage sampling kit will read plate to ground voltage. There-
fore, in order for the two meters to track the rheostat must be wired
in series with the high voltage lead feeding the final amplifier.
Proceed as follows:
1. Disconnect from modulation reactor L-3, the high voltage
lead which runs upward to the final amplifier RF choke
L-9. Remove this wire from the cable until sufficient
length is available, thet connect to one of the motor
rheostat terminals.
2. Using Packard cable or a high voltage equivalent, run a
lead from the other rheostat terminal to this same modula-
tion reactor terminal discussed in Step 1. Set the trans-
mitter voltage control R-14, for maximum voltage ( minimum
resistance).
3. The control circuit hook-up between the M-4703 motor and
remote . s,ontrol unit is explained in the Re:lote Control
instruction book.
-1-

Plate Voltage Extension Kit M-4719
1. Using Packard cable or a high voltage equivalent, connect
the M-4719 1 1-1V' terminal to the motor rheostat terminal
furthermost from the poWer supply. Do not connect to the
rheostat terminal which goes to the modulation reactor.
Connect the M-4719 kit termiA.al - G. to a good ground point
within the transJylitter.
2. Refer to the Remote Control Instruction Book for connection
of meter sar.cple voltage to remote control unit.
Plate Current Extension Kit M-4720
1. The plate current kit is to be connected between the lower
end of the P.A. overload relay K-6 and ground.
2. A ground lead runs from a chassis ground terminal to one
of the coil terminals on K-7 modulator overload relay,
then to one coil terminal of K-6 P.A. overload relay.
Clip out the portion between K-6 and K-7. K-7 is to
retain its direct chassis ground since we want only P.A.
current to flow through the plate current kit.
3. Both K-6 and K-7 must retain their shunt resistors directly
across their respective coils.
4. The " G terminal of the two- terminal strip on kit M-4720
connects to a good ground point within the transmitter.
Run a lead from the other terminal, upward to the K-6
te_aainal which was formerly grounded.
5. Refer to the Remote Control Instruction Book for connection
of sampling voltage to Remote Control Unit.
Remote Plate Start- too Circuitry
The BC- 1T, BC- 500T and BC- 250T overall schematic diagrams plainly in-
dicate the necessary connections for filament and plate remote control.
The plate start- stop circuitry consists merely of shunting the trans-
mitter plate- start switch with a set of n)rmally open remote control
contacts. The ) late- stop function is accomplished by connecting a
set of normally closed remote contacts in series with the transmitter
plate- stop switch.
Remote Filament Start- Stop Circuitry
Note that a jumper is to be removed in the filament contactor circuit.
Removal of this jumper disables the holding contacts on the filament
contactor. A set of remote normally - opon contacts shunted across the
transmitter's filament- start switch will then serve as filament start
-2-

and hold, satisfying FCC requirements for a ' fail-safe' circuit. If
the telephone line between studio and transmitter fails, or if the
remote control equipment becomes defective the remote holding con-
tacts open. Tnis, in turn, causes the filament contactor to drop
out, removing the transmitter from the air.
In multiple transmitter installations, the filament- fail safe opera-
tion is accomplished in a slightly different manner, so that indivi-
dual filament control for the various transmitters is possible. Each
transmitter employs its individual slave relay associated with its
respective transmitter filament contactor. The fail-safe relay in
the remote control unit, in turn, holds energized all of the slave
relays. The slave relay contacts are wired in series with their
respective filament cont,actor off circuits, providing a holding
circuit. Thus, separate on- off filament control is possible for
each transmitter. These circuits, along with drawings, are preseni;ed
in detail in the Remote Control Instruction Book.

INDEX
Page
Section I Electrical Description 1
Section II Mechanical Description • 1
Section III Installation 2
Section IV Gates BC- 250T Transmitter Details 3-11
Section V Initial Tune-up of Gates BC- 250T 12-16
Section VI General Operating Procedure 17-18
Section VII Summary 19
Guarantee 1-2
Photographs
Electrical Parts List 1-5
Drawings
A-30957 - Tuning Chart, BC- 250T
A-30958 - Typical Voltage Chart
A-30584 - Typical Curves, Vacuum Ovenless Crystals
A-30585 - aood Base
B-13816 - Schematic, M-5422 Oscillator Unit
B-65286 - Schematic, Driver Unit
C-77711 - Schematic, Audio Input/C.thode Follower
C-77723 - Simplified Schematic, Control System
C-77736 - Outline Diagram
B-65293 - Bias Supply, BC- 250T
E-25582 - Main Overall Schematic
1/16/58
-1- BC- 250T Xmtr.

SECTION I
ELECTRICAL DESCRIPTION
Gates transmitter, type BC- 250T, has been designed as a completely new
250 watt broadcast unit capable of excellent service in the standard
broadcast band. Electrical specifications are as follows:
1. Power output, 250 watts. The Gates BC- 250T can be satis-
factorily operated at 275 watts output, if necessary, to
overcome losses in transmission lines and/or phasing
equipment.
2. Frequency Range - 1600 Kc to 540 Kc,
3. Power Supply - 230 volt, 50/60 cycle, 2 wire.
4. Power Consumption - Approximately 1450 watts with
normal program modulation of 85%.
5. Frequency Stability - + 10 cycles, obtained by use
of vacuum, ovenless quartz crystal assemblies.
6. Type of modulation - High level Class modulation
capability 100%.
7. Audio Input - + 7 DB + 2 DB for 100% modulation at 1000 cycles
8. Frequency Response - -+2 DB, 50 to 10,000 cycles.
9. Audio Input Impedance -- 150/250/600 ohms.
10. Distortion - 3% or less, 50 cycles to 7500 cycles
measured at 95% modulation.
11. Noise - Minus 60 DB below 100% modulatian.
12. Carrier Shift - 3%, or less, 0-100% modulation.
13. Output Impedance - To match 50/70 ohms at all frequencies
540-1600 Kc. Coupling unit available for other impedances.
14. Tubes Used -
One - 12BY7A Oscillator Two - 6BG6G Cathode Follower
One - 12BY7A First IPA Audio Drivers
Two - 6BG6G Second IPA Two - 810 Class uB.' Modulators
(parallel operated) One - 5R4GY Bias Rectifier
Two - 810 Power Amplifier Two - 866A Intermediate Voltage
Two - 6BG6G Input Audio Rectifier ( 600/650)
Two - 8008 High Voltage Rectifier
(1300 V.)
15. Crystals Used - Provision made for two vacuum mounted oven-
less type crystal assemblies, each switchable into oscilla-
tor circuit from front panel.
16. Printed ‘ Firing - The oscillator and first IPA, the radio
frequency driver, the audio input/cathode follower stage
and the feedback ladder assembly are designed around
printed wiring.
SECTION II
HEC - di.IÏICAL DESCRIPTION
1. Overall dimensions - 78" high, 36-1/16 wide and 31-29/32"
deep. Back and right hand side are
of bolt- On construction. Front door
swing is 28.
2. Floor space required - 8 sq. ft.
3. Weight, in operating condition - 750 pounds.
1/16/58
-1- BC- 250T Xmtr.

C
C
SECTION III
INSTALLATION
This instruction book affords valuable information for persons who
are installing and operating the Gates BC- 250T transmitter. The
following mentioned points should be studied so that the unpacking
and setting up procedure will be well in mind when doing the actual
work.
1. Check all packing lists for materials supplied.
2. Study the instruction book before attempting to
set up the equipment.
3. Have the transmitter location clean so that the
various parts can be safely placed out of harms
way when the unit is unpacked.
4. It is well ta have a mounting base, set in place,
upon which the transmitter can be set. This base
can be made from 2° x 4° lumber dimensioned as
shown in Gates Dwg. A-30585. It should be painted,
preferably black. This base should be lagged to
the floor and measures taken to insure that the
top side of the frame is perfectly level. This
will give a good, solid, level base on which the
transmitter can be set. This procedure also
allows the external transmitter wiring to enter
the cabinet from practically any point underneath
and be run to the entry holes provided in the base
of the cabinet.
5. Use heavy primary wire from the building switch
box terminals to the transmitter fuse block.
or M3 copper wire should be suitable for these
two leads.
6. Be sure the power company has installed large
enough service for all the equipment, transmitter,
lights, water pump, etc., which will be used at
the transmitter site.
7. Do a good job of installing the equipment. Time
spent in making the installation as good electric-
ally and mechanically as possible, will pay off
in the future by insuring less, off- the- air time.
1/16/58
-2- BC- 250T Xmtr.

SECTION IV
BC- 250T TRANJMITTER DETAILS
The transmitter has been readied for shipment by having all tubes
removed from their respective sockets, relay contacts have been
blocked and tied and other parts, such as parasitic suppressors,
tube caps and leads, etc., tied down securely to prevent damage
during shipment. Vacuum enclosed time delay relays have been re-
moved from their sockets. The power amplifier RP choke, L9, and
the coupling capacitor, C9, also have been removed. The connector
straps have been adequately tagged for easy, correct re- connection.
These components have been adequately marked and packed in cartons
that are in turn safely secured within the transmitter. All of the
power and modulating equipment has been shipped intact within the
cabinet, thus relieving tile installation engineer of the re- instal-
lation of these components.
Coil L13 located at top rear of cabinet has special mounting straps
provided to insure safe travel during transit. These, along with
all other packing material, string, tape, etc., should be removed
from the components.
All relays should be inspected for free travel of armatures and con-
tacts. The following information concerning the Gates BC- 250T, per-
tains to its general construction and operation. It is highly de-
sirable to study the various sections of the transmitter in order to
completely understand and comprehend its operation.
The complete transmitter is built in a welded steel cabinet with
most of the low power RF and audio components mounted vertically
on a formed aluminum panel and shelf assembly. The high power modu-
lators and power amplifier 810's are located on a tube shelf at the
top of this basic assembly. The power amplifier tuning and loading
components are mounted on a large aluminum panel assembly at the top
of the cabinet. This assembly is perforated to allow the heated air
within the cabinet to pass through it and then out of cabinet.
Located in the base of the transmitter are the heavy power components,
such as power transformer, rectifier filament transformers, filter
-omponents, modulation choke and transformer, and rectifier tubes.
The transmitter is completely dead front; there is a perforated metal
inner shield extending downward from the power amplifier panel to the
air intake panel. The inner shield is joined to the air filter panel
by means of two thumb " quick off" fasteners. 1, fear the right hand
fastener are the interlocks for the 600 volt and 1350 volt power sup-
plies. All controls are available to the operator through a cut out
in this perforated screen panel. If this panel is removed, the 600
volt and 1350 volt power supply interlocks function, removing these
voltages from the transmitter.
This transmitter is crystal controlled by means of the il - 5422 oscil-
lator unit which is located on the panel and shelf assembly. The
crystal change- over switcn, Sl, along with the frequency adjust vari-
able condensers, Cl and C2, are cinveniently located on the front
control panel. The M-5422 frequency control unit uses a 12BY7A tube
1/1 6/58
-3- BC- 250T Xmtr.

connected circuitwise as an electron coupled oscillator, controlled
by a vacuum mounted, ovenless type crystal assembly. Positions for
two crystals is provided in the oscillator unit. The untuned plate
circuit of the oscillator is capacity coupled to the grid of the
first IiA, another 12BY7A physically located in the small oscillator
unit. This stage operates under very c)nservative conditions and
makes a fine isolation buffer between the oscillator and the R.2.
driver.
Due to the conservative operating potentials and the extremely low
crystal cUrrents, the M-5422 oscillator unit has exceptional frequen-
cy stability. The crystals are of a low teifeperature co- efficient
type which do not need a heated oven to maintain frequency well within
FCC limits. Frequency trimmers, Cl and 02, provide a small deree of
frequency adjustment which may be required if the crystals age. The
M-5422 oscillator unit is easily tuned to the operating frequency.
No tuning is required for the oscillator itself, just insert the cry-
stal, or crystals, and set the crystal selector switch accordingly.
The first IPA stage ( 12BY7A, V2 in unit) must be tuned. Provision
is made to supply U.F. voltage from a resistive divider at the output
of the M-5422 oscillator to a frequency monitor such as the Gates
M-2890. This monitor drive is available between terminal # 27 on TB1
and ground. See overall schematic - J'e-25582. For typical voltages
occurring in the oscillator unit see Gates Dwg. A-30958. The total
plate current drawn by the oscillator and first nA tubes in the osc-
illator unit is in the • neighborhood of 20 to 25 ma. as measured on
the multimeter when the multimeter switch is in the position marked
'Plt. Our. Osc/Bufl. The positive plate potential applied to the
oscillator unit is derived from the 600 volt power supply through a
20,000 ohm dropping resistor, R9. Printed wiring is used in this
oscillator unit.
The second IPA stage ases two 6BG6G beam power pentodes operating in
parallel. These tubes nave approximately 600/650 volts applied to
their plates. The cathode current of the two 6BG6G tubes will run
between 150 and 200 ma. depending upon loading and operating fre-
quency. This cathode carrent is indicated on the multimeter when
the selector switch is set to ' RF Driver Cath.. The stage is tuned
from the front panel by control knob designated at Ji3.2 Driver Tuning'.
This stage will tune from 1600 Kc to 1050 ic with no padding conden-
ser required. Jee frequency determining component chart, Gates Dwg.
A-30957. Thee is meter indication of the 112 drive supplied to the
two 6BG6G's by setting the selector switch to position marked . 1i1J'
Driver Grid'. In normal operation the grid current indication of
this stage will be on the order of .3 to . 5 ma. lith normal voltages
applied and adequate grid drive the 6BG6G's in this RF driver stage
will supply between 100 and 120 grid mils to the power amplifier.
This current will vary according to tuning and operating frequency,
but should never run less than 100 ma. This F.A. grid current is
also measured by the multimeter, being indicated on the meter when
the selector switch is placed in the " Power Amp. Grid" position.
The plate dissipation of the two 6BG6G tubes is kept within accept-
able limits by the use of a cathode resistor, 1i2, used for develop-
ing some cathode bias. If the drive to this stage is lost, the cath-
ode developed bias will tend to hold the plate current within reason-
able bounds. Most of the circuitry of this 2nd IPA stage is made
1/16/58
-4- BC- 250T Xtr.

up of printed wiring, only the plate coil, L8, tuning capacitors,
07 and 015, and plate choke, L7, are mounted external to the printed
wiring board.
The plate voltage which energizes the two 6BG6G RF drivers is develop-
ed in a 600/650 volt power supply using two 866A tubes as rectifiers.
Typical voltages appearing within this stage are shown on Gates Dwg.
A-30958 and also on Schematic B-65286.
Neutralization of the power amplifier is accomplished by the " Ricen
method, the out of phase voltage being obtained from the 6BG6G tank
coil, L8. There are several taps brought out adjacent to the elec-
trical center of this coil, these taps affording rough neutraliza-
tion. By means of the variable neutralizing condenser, 010 and these
taps, it is possible to completely neutralize the 810 tubes in the
power amplifier. The neutralizing condenser, C10, can be adjusted
from the front of the transmitter, by use of a screw driver working
through the small aperture in the lower right side of the upper front
panel. The neutralizing condenser, 010, is located directly behind
this front panel, its shaft being insulated by means of a slotted
bakelite shaft bushing. The power amplifier makes use of two 810
tubes. Most of the power amplifi'úr and output circuitry is mounted
on an aluminum panel and chassis, assembly at the top of the cabinet.
The heated air 4)asses through the P.A. assembly through numerous
openings. This P.A. assembly is ,. lade up of PA tank coil, L12, PA
tank condensers, Cil and 012, ( always operating in parallel) outpuL,
coils, L13 and L14, loading condensers, C13 and C14, modulation pick-
up coil, L15, plate blocking condenser, C9, and plate bypass conden-
ser, C8. The amplifier circuit consists of an ° L.' and two .' f° sec-
tions, a circuit proved over the years as one which is flexible and
also very effective in attenuation of undesirable harmonics. The
coil and capacitor values as supplied with the transmitter, are ef-
fective in loading into a 51.5 ohm load. 3ee tuning chart, Dwg.
A-30958.
The output circuit of the BC- 250T includes a pickup coil, L15,
which supplies sufficient RF voltage to operate a modulation monitor
such as the Gates MO- 3629. This voltage is available at a small
terminal board, TB2, located on the front panel, at the left, ins'
This amplifier makes use of no variable, air dielectric condensers
(except the neutralizing condenser, 010).
The P.A. tank circuit is tuned by means cf a rolling contact induc-
tor, L12. This method of tuning is helpful in preventing arcs or
flash-overs that may occur in variable air dielectric condensers,
especially if the transmitter is used in locations where the dust
problem is bad. The P.A. tank condensers are two type G1 Sangamo
mica's, always connected in parallel. The combined total value will
range from . 0004 mfd. at 1600 Kc to . 001 mfd. at 540 Kc. The load-
ing condensers, 013 and 014, also vary according to frequency. For
information concerning these variable frequency determining compen-r 4-
see Gates Dwg. A-30957 included in this instruction book.
1/1 6/59
-5- BC- 250T Xmtr.

The power amplifier plate current is read on M2, a 0-500 MA DC meter.
This current will generally run from 260 ma. to 280 ma. depending
upon the efficiency and the applied plate voltage. The normal plate
voltage, as read on plate voltmeter, 113, will be around 1350 volts,
As mentioned previously, the P.A. grid current, as indicated on the
multimeter, will be 100 to 120 ma. depending upon frequency, tuning,
etc. Two F.A. tuning control knobs are located on the power ampli-
fier panel, the left hand one controls the variable output coil, L14,
and is marked ' Loading. The right hand knob allows rotation of the
variable P.A. tank coil tuning the power aMplifier. This control is
marked ' P.A. Tune. The power amplifier is protected from severe over-
load by a cathode overload relay. This relay, K6, is located on the
aluminum shelf adjacent to the multiple 810 filament transformer, T9.
Its coil is shunted by a semi- variable resistor, R21, this resistor
allows the trip out point to be varied considerably above and below
the normal cathode current drawn by the 810 power amplifier under
normal conditions. This relay shunt resistor is adjusted to allow
the relay, K6, to pull in at about 325 ma. As normal cathode current
will be around 260 to 280 ma. this gives considerable operational
latitude before the transmitter will kick off the air. Of course,
this adjustment can be made to suit the individual who operates the
equi:)ment. Ihen F.A. overload relay, JÇ:6, pulls in, its contacts close,
completing a 230 volt AC circuit through the coil of master overload
relay, K3. K3 energizes and its contacts open, causing the holding
circuit of the plate contactor, K2, to open and drop out the plate
relay. This de- energizes the main power transformer, Tl.
Audio wise, the Gates BC- 250I transmitter is novel in many respects.
The audio input and audio driver portion is made up of printed wir-
ing. A small bakelite printed board located on the panel and shelf
assembly mounts the two 6BG6G audio input tubes, the two 6BG6G cath-
ode follower tubes, along with balance control, condensers and
resistors for these two stages.
The audio input connections are made to TB1-29 and TB1-30, with a
convenient ground termination on TB1-28. Provisions have been made
on audio input transformer, T6, to allow the input impedance to be
either 125, 250 or 600 ohms. The input transformer is connected for
600 ohm operation when the transmitter leaves the factory. This will
take care of most audio iruut requirements. A balance control, . 13,
is located on the printed circuit board near the right hand lower cor-
ner. This control is in the cathode circuit of the two 6BG6G audio
input tubes, V1 and V2.
By use of this control, low frequency audio distortion can be mini-
mizes. This is best accomplished, when test equipment is available
to show the actual distortion present. If no such equipment is
handy, it would be reasonable to adjust this balance control to mid-
value and so operate the transmitter. The second audio stage, a
pair of 6BG6G tubes, V3 and V4, aro operated as cathode followers,
these tubes provide a low impedance driving source for the grids of
the two 810 modulators.
1/16/58
-6- BC- 250T Xmtr.

Plate potential for the audio input and cathode followers is provi-
ded by the 600/650 volt power supply, mentioned previously as sup-
plying voltage to the M-5422 oscillator unit and the RF driver stage.
Normal voltages to be expected at various points in the audio input
circuits are given in Gates Dwg. A-30958 9 typical voltages.
The cathode current of the two audio input tubes, V1 and V2, is
measured by multi- meter, the selector switch being set at " Input
Audio Cathode. This current will normally run from 5 to 10 ma.
The cathode follower tubes, V3 and V4, are biased by voltage con-
trolled by potentiometers, 112 and R1, located on the small aluminum
front panel. These controls indirectly adjust the operating bias on
the modulators by varying the operating constants of the cathode fol-
lowers, this causes a bias voltage change on the modulators by having
a voltage drop occur across the high resistance cathode resistors,
Rll and 1112, of the cathode followers. A very smooth modulator bias
change can be attained in this manner, making it possible to adjust
the modulators for correct operating conditions. There is no meter
plate current indication for the cathode follower tubes, V3 and V4,
it is believed that if proper modulator operation is had, then the
6BG6G cathode followers are operating satisfactorily.
All external connections to the audio printed wiring is made to
numbered terminals, this is shown clearly on small individual draw-
ing C-77711 of this section, and in the overall schematic E-25582.
High level Class " JP modulation is used in the BC- 250T, a pair of
810 tubes providing the means. The modulation transformer, £ 3,
working with these tubes is located in the base of the cabinet, on
the right hand side, toward the rear. The grids of the modulators
are excited by the two 6BG6G cathode followers. The modulators
operate with approximately 30 na, static plate current per tube,
this operating parameter being set up from the front panel by use
of the two ' bias" controls, R1 and 112. To enable the operator to
individually adjust each modulator tube, a meter switching circuit
has been designed. A three position switch, 31, is located on the
aluminum front panel, just below the two bias controls. This switch
has markings as follows; in the left position Modulator 1, 1, ( V8) is
metered, in the right hand positi.rn Modulator /, 4 2 ( V9) is metered and
when the switch is in the center position marked " Total, both tube
currents are combined and read. These currents are indicated on the
modulator front panel meter, 114. The total plate current will be
from 250 to 300 ma. during times of heavy modulation.
Each modulator has its filament energized by a separate filament
winding on multiple filament transformer, T9. The third winding
on this transformer, £ 9, is 10V, 9 ampere capacity and energizes
the filament of the power amplifier tubes, V6 and V7.
The modulators are protected from severe overload by relay, K7.
This relay, K7, is also provided with a coil shunt resistor, 1122,
making it possible to adjust the pull- in point of the relay. For
normal program modulation the modulator plate current should not
exceed perhaps 300 ma. Under sine wave audio conditions this cur-
rent may rise to 400 ma. at the higher audio frequencies. A
1/16/58
-7- BC- 250T Xmtr.

satisfactory setting for the overload relay would be around 450 ma.
This will take care of accidental audio peaks that might go through.
This overload relay, K7, has its normally open contacts connected
in parallel with the normally open contacts of P.A. overload relay,
K6. If a modulator overload does occur, the K7 modulator overload
relay pulls in causing 230 volts a.c. to be applied to the coil of
master overload relay, K3, which in turn energizes and pulls in to
open the holding circuit of plate contactor, K2. This removes the
high voltage from the modulators and the power amplifier.
Feedback from the plates of the modulators back to the audio input
tube grids has been provided. A small bakelite priated wiring
board is located on the panel and shelf assembly, directly above
the modulation transformer, T3. By means of a resistor/capacitor
divider network out of phase voltage is fed bac!: to the audio in-
put. The BC- 250T transmitter makes use of approximately 9 DB of
feedback measured at 1000 cycles and 90c/; modulation. This feedback
helps to reduce the hum and also improves the distortion figures. •
All relays for the operation of the transmitter are mounted on the
panel and shelf a ssembly, the previously mentioned P.A. and the
modulator overload relays, K6 and K7, being on the top shelf ad-
jacent to the multiple filament transformer, . 29. All other relays
are mounted at the bottom of the panel and shelf assembly along
with necessary fuses, time delay relays, etc. The main line fuses,
Fl and F2, are located at the left bottom section of the inside
panel asseilbly, directly over TB- 1. This fuse receptacle is the
terminal location for the two 230 volt primary input wires. Fifteen
ampere cartridge fuses are used. Next in line is the filament con-
tactor, Kl.
Filament contactor, Kl, is energized by depressing the filament
start switch, S5, located on the front column directly beneath the
multimeter. This operation causes the primary input voltage to be
connected directly to the primaries of all filament transformer
(T4, 8008 fil. trans., T5, 866A fil. trans., 1 28, dual 6.3 volt fil.
trans. for audio, RF driver and osc. unit and T9, the 10 volt mul-
tiple fil. trans. for the modulators and power amdlifier). At the
same time the heater for time delay relay, K5, is energized. After
30 seconds the contacts of K5 ( low voltage time delay) close and
this action energizes the primary of T2, the 600/650 volt power
transformer. ( If the interlock switch, 33, is depressed.) Along
with tlis sequence of events, the primary of the bias transformer,
T7, is energized and as the bias rectifier, V5, heats, bias voltage
is developed. This will be approximately neg. 280 volts. Listing
the functions performed when 35, the filament start button, has
been depressed, we have --
1. All filaments energized, filament pilot light on, low volt-
age time delay relay heating.
2. Bias supply energized, neg. 280 volts, developed setting
up bias for cathode follower and modulators.
3. 600/650 power supply energized ( if low voltage door inter-
lock S3 is depressed). This has energized the plate circuits
of the crystal oscillator, first IPA and hF driver and if
they are correctly tuned, there will be grid drive to the
1/16/58
-8- BC- 250T Xmtr.

'power amplifier. Also the 600/650 volts has been applied
to the input audio and cathode follower stage.
4. High voltage time delay relay, K4, heated and its control
contacts closed. ( After primary of 12 is energized.)
Plate contactor, K2, is located next to the filament contactor, near
the lower part of the panel and shelf assembly. If the high voltage
door interlock switch, 34, is closed and high voltage time delay re-
lay, K4, is heated and its contacts closed, it would be possible to
energize this plate contactor, K2, by depressing the plate start but-
ton located just beneath the meter column. When plate contactor, K2,
closes, several operations occur.
1. The high voltage plate transformer, 11 primary, is
energized, developing high voltage ( 1350/1400 approx.)
for and modulator plates.
2. Plate nilot light energizes.
This would place the transmitter on the air.
The Gates BC- 250T transmitter makes use of 3 separate power supplies.
1. Bias Power Supply.
It is made up of a combined filament and plate transformer
T7, working in conjunction with bias rectifier, V5, a 5R4GY,
filter choke L6, filter condenser 06 and associated resis-
tors and potentiometers. The bias potentiometers, R1 and
R2, indirectly vary the modulator bias, by controlling the
cathode follower bias and thus the current flow, through
the cathode follower resistors, Rh l and R12. There is ap-
plied a negative 280 volts between these resistors and
ground. An opposing voltage of approximately 240 volts
is developed by current flow trough Rll and R12, thus
putting the difference ( about 30/40 volts) on the grids
of the modulators.
This supply energizes when the filament start button is depressed.
2. 600/650 Volt Power Supply
This supply makes use of a pair of 866A rectifiers,
the filaments of which are heated by transformer, T5.
The primary of T5 is energied at the time the fila-
ment start switch is operated. Approximately 30 seconds
after the filaments are energized, and low voltage time
delay relay, K5, has heated and closed its contacts,
this causing the primary of T2, the 600/650 volt power
transformer to energize ( if the low voltage door inter-
lock switch 33 is closed) develop approximately 600
volts d.c. after being rectified by V4 and V3, the two
866A rectifiers. This voltage is applied to the M-5422
oscillator unit through a dropping resistor, E9, which
reduces it to approximately 195V. The 600 volts is ap-
plied to the RF driver. Also, this same voltage is ap-
plied to the audio input/cathode follower stages. The
1/16/58
-9- BC- 250 .2 Xmtr.

primary of this power transformer ( T2) is fused by a
small fuse, F4, of 3 ampere size.
High voltage for the modulators and power amplifier
approximately 1350 volts is developed by a power
supply consisting of main power transformer, Tl,
8008 filament transformer, T4, swinging choke, Ll,
smoothing choke, L2, and filter capacitor, C2 and
C3. These components are located in the base of
the transmitter along the left hand side and to-
ward the front. This high voltage supply is inter-
locked with the front panel grill and interlock
switch, S4.
Provisions have been made to operate the Gates BC- 250T transmitter
be remote control. The connections are clearly shown on Gates over-
all schematic E-25582. It is necessary to remove the jumper wire
that normally connects between contact # 1 and # 3 on a filament start
contactor, Kl, and wire normally connected between TB1-10 and TB1-22.
For shipping purposes, some components have either been removed or
mechanically made secure within the cabinet proper. All of the re-
moved items are boxed and th.e carton containing them is shipped with-
in the cabinet. The following items have been removed -
C
e
1. All Vacuum Tubes.
2. Time Delay Relays, K4 and K5.
Contacts of the filament and start relays have been held firm by use
of paper and tape. Tube caps and parasitic suppressors have been
taped down to prevent damage by vibration during shipment. Certain
components have been supported by wooden braces. All of this material
should be removed from the transitter, care snould be taken to see
that all relays are free of foreign material that wJuld prevent them
to operate normally. The connections to the various components that
have been removed are clearly tagged for easy and correct replace-
ment. Remove extra coil bracing from L13 which is located in the
top, at rear. Check all mechanical connections for tightness.
Replace all vacuum tubes, crystals and time delay ralays in their
correct sockets, check locations by reference to overall schematic
E-25582 and the various pictures that are supplied as a part of
this instruction book.
At this time do not place the plate caps on the 866A rectifier, V3,
and V4, and the 808 rectifiers, V1 and V2.
The external connections are few and these are easily made to the
BC- 250T transmitter. The 230 volt, 50/60 cycle AC enters the cabi-
net base at the right hand side, near the rear corner. The two
primary wires connect directly to the fuse block, XF1. The shielded
audio input pair enters the cabinet at the right hand side near the
front. These connections arc made to the terminals # 29 and # 30 on
TB1. A ground is close by on terminal # 28.
1/16/58
-10- BC- 250T Xmtr.

The modulation monitor connects to TB2-1 & 2. This terminal board
is located on the back side of the tuning and loading panel, at the
left front side. This connection can be made with shielded twisted
pair, such as is used in audio wiring.
The frequency monitor connects to TB1-27 and 28, the ground being
terminal # 28. This connection can be made of shielded single con-
ductor wire, or low impedance line such as RG/62U.
A ground stud is provided on the cabinet frame for a transmitter
ground connection. Located near the modulation transformer, T3,
this connection facilitates the grounding of the transmitter to the
overall station ground system. A copper strap can be brought in
through the access hole in the right rear base section of the trans-
mitter. ( The AC primary wires were brought in through this opening
also.) This internal ground follows up the cabinet support and fin-
ally connects to the aluminum PA tuning assembly at the top of the
cabinet.
The RF output termination is a ceramic feedthrough insulator located
close to the output coil, L14. Connection can be either made from the
top of the cabinet through an access opening, or the low impedance
line can be brought in through the base of the transmitter and con-
nected to the out - put circuit.
A word of comment here, concerning the station ground system. It
should be constructed as good as can be, all connections well made
and soldered, or preferably brazed. It is wise to bond all electri-
cal conduit, metal frame work of building, water piping, etc., to
the overall ground system. If these suggestions are followed, there
will be less trouble over the years as the ground system ages.
1/1 6/58
-11- BC- 250T Xmtr.

z
C.:
SECTION V
INITIAL TUNE-UP OF TIE GATES BC- 250T
Before proceeding with the initial tune-up of this transmitter, it
would be well to re- check the necessary things to be done, before
any voltage is supplied. Briefly, check the following list.
1. Proper primary line voltage connected to fuse bloCk,
XFl e two 15 ampere fuses should be in the clips of
this fuse block.
2. Proper locatipn of all tubes in sockets. These tube
locations can be checked by reference to the stencil-
ling in the transmitter and to information furnished
in this instruction book. Also, make sure the crystal,
or crystals, are in the oscillator crystal sockets. If
only one crystal is used, be sure the crystal selector
switch, Sl, is in the correct position to operate with
the one crystal.
36 Check to see that all tie- down twine and other material
used in shipping the transmitter, has been removed from
the various components, especially the relays.
4. de- check all components that were installed. Be sure
they are connected correctly. The parts and the connec-
tors have been tagged to insure correct replacement.
5. Go over the complete transmitter, checking the tightness
of all nuts and bolts, terminal connections, etc.
6. Give all soldered connections a brief lookinc over. The
equipment has passed several rigid inspections during its
course of manufacture, but something may have been over-
looked that cpuld cause trouble in the future.
7. Make certain the transmitter and associated equipment is
well grounded.
8. It is suggested that all audio input wiring be shielded
and placed in conduit or wiring troughs, away from A.C.
wiring.
9. Be sure the plate leads to the 866A and 8008 rectifier
tubes arc not placed on the tubes, and that they are
free from grounding. This will insure the fact that no
high voltage will be present until needed.
10. Make sure the transmitter is connected to a 51/70 ohm
load.
Lets go through the procedure to tune the Gates BC- 250T to 1400 Kc.
The crystals should be for 1400 Kc and so marked. All fuses should
be in their correct mountings and 230 volts 50/60 cycle AC should be
connected into the transmitter. Use extreme care during tune-up,
as there are high voltag,es developed within the transmitter which
could cause death if a person c,ould come in accidental contact
Two door interlock switches are provided on S3, in series with the
600/650 volt power tr - insformer, T2 primary, and the other interlock
switch, S4, being in series with the coil of the plate start contac-
tor of the high voltage circuit.
1/1 6/58
-12- BC- 250T Xmtr.