Harris GATES FM-20H3 User manual

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HARRIS
1 NTE RTYPE
CORPORATION
FM -20H3 20KW FM TRANSMITTER
FOR
994 6746 105 FM -40H3 FM TRANSMITTER
OATES®
A DIVISION OF HARRIS-INTERTYPE

WARRANTY
Seller warrants new equipment manufactured by Gate Radio Company against defects in material or work-
manship at the time for delivery thereof, that develop under normal use within a period of one year (6 mono s
on moving .parts) from the date of shipment, of which Purchaser gives Seller prompt written notice. Other
manufacturers' equipment, if any, including electron tubes, and towers shall carry only such manufacturers'
standard warranty.
Seller's sole responsbility for any breach of the foregoing provision of this contract, with respect to any
equipment or parts not conforming to the warranty or the description herein contained, is at its option, (a) to
repair or replace such equipment or parts upon the return thereof f.o.b. Seller's factory within the period
aforesaid, or (b) to accept the return thereof f.o.b. Purchaser's point of installation, whereupon Seller shai
either (1) issue a credit to Purchaser's account hereunder in an amount equal to an equitable portion of the'
total contract price, without interest, or (2) if the total contract price has been paid, refund to Purchaser aril
equitable portion thereof, without interest.
If the Equipment is described as used, it is sold as, is and where is. If the contract covers equipment not
owned by Seller at this date it is sold subject to Seller's apquisition of possession and title.
Seller assumes no responsibility for design characteristics of special equipment manufactured to specifications-
supplied by or on behalf of Purchaser.
Seller shall not be liable for any expense whether for repairs, replacements, material, service or otherwise,:
incurred by Purchaser or modifications made by Purchaser to the Equipment without prior written consent
of Seller.
EXCEPT AS SET FORTH HEREIN, AND EXCEPT AS TO TITLE, THERE ARE NO WARRANTIES, OR
ANY AFFIRMATIONS OF FACT OR PROMISES BY SELLER, WITH REFERENCE TO THE EQUIPMENT,
OR TO MERCHANTABILITY, INFRINGEMENT, OR OTHERWISE, WHICH EXTEND BEYOND THE D'1=
SCRIPTION OF THE EQUIPMENT ON THE FACE HEREOF.
RETURNS AND EXCHANGES
Do not return any merchandise without our written approval and Return Authorization. We will provide'
special shipping instructions and a code number that will assure proper handling and prompt issuance of
credit. Please furnish complete details as to circumstances and reasons when requesting return of men-,
chandise. Custom built equipment or merchandise specially ordered for you is not returnable. Where return
is at the request of, or for the convenience of the customer, a restocking fee of 15% will be charged. AU
returned merchandise must be sent freight prepaid -and properly insured by the customer. When writing to
Gates Radio Company about your order, it will be helpful if you specify the Gates Factory Order Numl If
or Invoice Number.
WARRANTY ADJUSTMENTS a
In the event of equipment failure during the warranty period, replacement or repair parts may be provided
in accordance with the provisions of the Gates Warranty. In most cases you will be required to return the
defective merchandise or part to Gates f.o.b. Quincy, Illinois for replacement or repair. Cost of repair
parts or replacement merchandise will be billed to your account at the time of shipment and compensating
credit will be issued to offset the charge when the defective items are returned.
MODIFICATIONS
Gates reserves the right to modify the design and specifications of the equipment shown in this catalog
without notice or to withdraw any item from sale provided, however, that any modifications shall not ad-
versely affect the performance of the equipment so modified.

"L.
J
ADDENDA
FM -20113 FM TRANSMITTER
994'6746 105
A terminal board, TB -10 has been installed on the unierside of the
top of each transmitter cabinet (left, front corner).
Wires from the center cubicle of the FM -40H3 transmitter system
should be connected to the terminals of TB10 as designated on
drawing 838 9479 001. The drawing also shows the termination
of each wire in the center cubicle or the transmitter cabinet.
A blank panel with a terminal board has been installed in the
space normally occupied by the TE -3 FM Exciter. The wires usually
connected to the exciter are connected to these terminals. A
jumper has been added between AFC interlock circuitry wires 30 and
170.
The high voltage control circuitry of each transmitter has been
rewired as shown on drawing 838 9522 001.
The following items have been added to each transmitter's high
voltage control. circuitry.
DS10 Indicates when the recycle relay, K8, was
energized.
DS11 Indicates the transmitter automatic switching
circuitry ís in operation.
K13 Prevents the automatic switching circuitry from
turning ON the transmitter if the recycle relay
was energized.
K14 Is energized through normally open contacts of
the recycle relay. Its contacts control the
voltage to the coil of K13 when the automatic
switching is in operation.
S20 Will reset relays K13 and K14.
S21 Will energize the automatic switching relay
assembly on the side of the center cubicle.
S22 Will de -energize the automatic switching relay
assembly on the side of the center cubicle.

TB10 Terminal board for wire connections to the center
,cubicle. It is mounted on the underside of the top
at the left front corner.
The grid drive relay, K10, has been replaced with a new relay which has
2 form "C" sets of contacts.
Status lights have been installed in each transmitter, however, the
LOCKOUT light circuitry has been re -wired per the drawing 838 9522 001.
PARTS LIST
DSIO,DS11 406 0388 000 Pilot light
K10 572 0160 000 Relay, 2PDT
K13,K14 574 0224 000 Relay, 4PDT
S20,S21,S22 604 0391 000 Switch, P.B. SPDT
TB10 614 0020 000 Terminal Board
8389522 001 Schematic
852 7534 001 Schematic, Overall FM -20113
5/11/73 -2- 994 6746 105

INSTRUCTIONS FOR INSTALLING AND OPERATION
OF
GATES FM -20113 20KW FM TRANSMITTER
IB--888 1071 001 Gatos Radio Company
Quincy, I11inoi,s

TABLE OF CONTENTS
FM -20H3 20KW FM TRANSMITTER
SECTION 1 - GENERAL DESCRIPTION PAGE
1.1 Warranty 1
1.2 Safety Notice 1
1.3 Purpose of Book 1
1.4 Purpose of Equipment 1
1.5 Description 2
1.6 Vacuum Tube Table 2
1.7 FM -20H3 Technical Data 3
SECTION 2 -- INSTALLATION
2.1 Inspection 4
2.2 Packing Check List 4
2.3 Tube Handling & Operating
Precautions of the 4CX15,000A 4
2.4 Installation 5
2.5 Wiring Connection 7
2.6 Cooling 8
SECTION 3 - OPERATION
3.1 Pre -Operation 9
5.2. Test Data 9
3.3 Adjustment 9
3.4 Maiiitenanc e .I2
SECTION 4 - CIRCUIT DESCRIPTION
4.1 Power Amplifier 13
4.2 IPA 13
4.3 Exciter 13
4.4 Power Supply 14
4.5 Control Circuits 14
4.6 Metering & Fuse Indicators 16
SECTION 5 - ADDITIONAL INFORMATION
5.1 Remote Control 17
5.2 Stereophonic Operation 17
5.3 Door Interlock Switches 17
5.4 Low Pass Filter Position 18
5.5 Blower Location 18
5.6 Blower Rotation 18
5.7 IPA Plate Loading 18
5.8 Power Reduction 18
5.9 FM Noise Measurements 19
5.10 Second Harmonic Filter 10
Parts List
-1- FM -20H3

-- TABLE OF CONTENTS
SECTION 6 - PHOTOGRAPHS & DRAWINGS
PHOTOGRAPHS - Front View
Back View
Power Supply
DRAWINGS: Installation Details 838 1991 001
Block Diagram 814 6640 001
P.A. Efficiency Curve 814 9941 001
Low Pass Filter 814 8576 001
2nd Harmonic Filter 814 8554 001.
Overall Schematic 852 6553 001
SECTION 7 - FM EXCITER
-ii- FM -20113

SECTION 1 - GENERAL DESCRIPTION
1.1 Warranty
This equipment is guaranteed under the Gates Warranty. The
terms and conditions are explained in the Gates Warranty
which is printed inside the front cover of this manual.
Most Gates manufactured items are guaranteed for one year,
with the exception of tubes and moving parts, which are
subject to specific warranties based upon hours of usage.
The Warranty does not extend to "no charge" service in
the field.
1.2 Safety Notice
This equipment employs voltages which are dangerous and may
prove fatal if contacted by operating personnel. _creme
caution should be exercised when working with the equipment.
Observe safety regulations.
Do not change tubes or make adjustments inside equipment
with any voltages ON. While your Gates transmitter is fully
interlocked you should not rely on the interlock switches
for removing high operating voltages. It is always best to.
disconnect the primary power at the building wall switch and
discharge all capacitors with the grounding stick provided.
1.3 Purpose of Book
This instruction book has been pr.;ared to assist in the
installation, operation, and main 'nance of the Gates
FM -20H3 20KW FM Transmitter.
1.4 Purpose of Equipment
The Gates FM -20H3 is a .FM broadcast transmitter with the
capability of delivering 20,000 Watts power output to the
transmission line. The transmitter is designed for con-
tinuous broadcast operation in the 88.1 to 107.9 MHz fre-
quency band. Performance will meet or exceed those spec-
ifications required by the Federal Communication Commission
for standard FM broadcast service.
-1- FM -?_0H3

1.5 lescription
The FM -20H3 transmitter is housed in two cabinets utilizing
only ten square feet of floor space. The transmitter cabinet
is 42" wide x 32-3/4" deep x 78" high. The high voltage power
supply cabinet is 30" x 30" x 49" high, and it may be positioned
apart from the transmitter cabinet. Complete access to the in-
terior of both cabinets is possible by doors or removable panels.
Front doors are provided on the transmitter cabinet to offer a
pleasing and symmetrical front view appearance. All necessary
operational metering is provided by four meters located on the
meter panel above the front doors. The following controls are
also located on the meter panel.
(a) Filament ON
(b) Filament OFF
(c) Plate ON
(d) Plate OFF
(e) Filament Voltage Meter Switch (IPA or PA)
Controls listed below are located behind the front doors.
(a)
(b)
(c)
VSWR Cal
Pwr Cal
For Pwr-VSWR Cal-VSWR
(k)
(1)
(m)
Recycle
PA Remote Plate Current
PA O.L.
(d) NA F+'il arnent (n.) PA Remote Plate Voltage
(e) Multimeter (o) PA Plate Tuning
(f) IPA Screen (p) PA Output Loading
(g) PA Screen Raise -Lower (q) PA Neutralizing
(h) IPA Filament (r) IPA Plate Tuning
(i) Remote -Local (a) IPA Grid Tuning
(j) IPA U.L.
1.6 Vacuum Tube Table
Symbol No. Type Function
Vl 4CX15,000A Power Amplifier (PA)
V2,V3 40X250B Intermediate Power
Amplifier (IPA)
.-2- FM -20H3

1.7 FM -20H3 Technical Data (Monaural)
**
Power Output
Frequency Range
RF Output Impedance
Output Termination
Maximum VSWR
Harmonic Attenuation
Frequency Stability
Modulation Capability
Audio Input Impedance
Audio Input Level
Audio Frequency Response
Audio Distortion
FM Noise Level
AM Noise Level
Power Source Voltage
Power Source Variation
(Slow)
Power Factor
Power Input Requirement
Ambient Temperature Range
Altitude
Transmitter Cabinet Size
Power Supply Cabinet
Weight (Total)
Front Door Swing
PA Tube Dissipation
(Maximum)
Blower Air Flow
Size
10 to 20 kW
88.1 to 107.9 MHz
50 Ohms
Standard BIA 3-1/8" flange
1.7:1
-80 dB
+ .001%
+ 100 kHz
600 Ohms
+ 10 dBm, + 2 dB
+ 1 dB, 30-15,000 Hz
1,; or less, 30-15,000 Hz
65 dB below 100/ FM modulation
(400 Hz reference)
50 dB below equivalent 100'ió
AM modulation
208/230/240/250 V.; 3 phase,
60 Hz
115 V., 1 phase, 60 Hz
+ 5/%
90
30 kW., 240 V; 300 W. 115 V.
-20°C to +45°C
7500 feet
42" wide x 32-3/4" deep x 78"
high **
30" vide x 30" deep x 49" high
2000 pounds
21"
15,000 W., 850 BTU/Minute
1250 c.f.m. @ 6.3 :inches of
water pressure
* 32-3/4" is overall depth dimension. With rear
door, front door handles and meter trim strip
removed the minimum depth is 29-3/4"
R.7. plumbing extends 12-7/8" above cabinet top.

SECTION 2 - INSTALLATION
2.1 Inspection
The FM -20H3 is carefully packed at the Gates plant to ensure
safe arrival at its destination. The equipment is packed in
a number of heavy cartons and wooden crates. Open the crates
and cartons carefully to avoid damaging any of the contents.
Remove the packing material and search for possible loose items,
such as pilot lights, fuses, loose screws and bolts. A complete
visual inspection should be made of the equipment for shipping
damage.
If damage should occur during shipment, a damage report should
be filed promptly with the transportation company to receive
compensation. If a claim is to be filed, the original packing
case and material must be preserved. Gates Radio Company is
not responsible for damage occurring during shipment. Parts
or components shipped to replace the damaged items will be
billed to the customer plus transportation expenses, the cost
of which should form a portion of your claim to the transpor-
tation company.
Make sure all relay contacts are free and in good mechanical
condition. Make sure all mechanical connections are tight.
Check with a screw driver or a wrench, all mechanical and
electrical connections that are mechanically bolted together.
All tie do..rns or blocking used for shi.onin_e purposes should
be removed. A good overall visual inspection may save time
and trouble in placing the transmitter into operating condition.
2.2 Packing Check List
Certain components of the transmitter have been removed for
shipment and are packed separately to ensure safe handling.
These parts on the FM -20113 have been kept to a bare minimum
and are plug-in units, heavy components or tubes. Refer to
the Packing Check List to determine that all these components
are on hand.
2.3 Tube Handling and Operating Precautions, 1CX15,000A
Avoid bumping this tube. Due to its large mass, bumping this tube
will introduce resultant stresses which may cause internal damage.
Before operating this tube refer to the tune-up and operating
procedure given in SICTIOiI 3. It is recommended procedure to
adjust the equipment for operation under heavy plate loading
conditions, and with only sufficient R.F. drive to provide the
required power output and efficiency.
Extreme care should be taken during tune-up as well as in regular
service to avoid, even momentarily, operation of this tube under
conditions of insufficient plate loading or excessive R.F. drive.
These operating conditions, especially at the upeer end of the VHF
range, will produce excessively high seal and/or bulb teuperature
and will result in damage to this tube.
Fi :- 2.0ií3

In advance of actual placement of .the equipment, certain planning
should be accomplished. The use of the . nstallation Drawing will
assist in locating the power and audio input holes position of the
transmitter.
Either side of the transmitter may be placed against a wall or
other.equipment. Complete accessibility for maintenance and instal'
ation is provided in the FM-20H3by access from the front or the re___
of the transmitter and the high voltage power supply cabinets.
Mount the blower Bl on the shock mounts in the base of the trans-
mitter cabinet directly below the PA enclosure. Install the canvas
blower boot around the PA enclosure inlet and push boot up until it
is taut. Secure the boot tightly with the boot clamp. Rotate the
impeller by hand to check for freedom of movement.
The normal length of the interconnecting cable between the trans-
mitter cabinet and power supply cabinet is 15 feet. Other lengths
of 25 or 35 feet are available.
Wire No. From Power SurD12 To Transmitter
122 TB7-1 TB8-1
Gnd. TB7-2 TB8-2
123 T137--3 TIl8-3
124 TIl7-- TB8-4
125 TB7-5 TB8-5
126 T/i`/- .TB8...6
Coax E5 E4
Coax Shield Gnd. Stud Gnd. Stud
;`l AWG wire
(3 pieces) K5 T4 primary
Appropriate primary taps should be used on T4 and T5 (2000 V. Supp7.y.
for your 3 phase a.c. input line voltage.
Install the plug-in capacitor C37 in the transmitter bias supply
located on the side of the transmitter cabinet.
The R.F. output coupling assembly consists of a silver plated
3-1/8 inch outer conductor with a flange near the center and a
inner conductor with brass plugs in each end. The outer conductor
also.has two monitor coupling loops and an adjustable sleeve. Be-
fore installing, remove both monitor coupling loops and loosen the
set screws in the adjustable sleeve to temporarily position the
sleeve close to the flange.
Remove the output coupling loop that is connected to the output
loading capacitor C6.
Insert the R.F. output outer conductor in the hole in the top of
the PA enclosure from inside the PA enclosure. The flange will
be inside the enclosure and the monitor coupling loop cutouts will
be above the top of the cabinet. Secure the outer conductor to
.the top with bolts.
FM -20H3

Insert the inner conductor in the outer conductor from the top.
The 1/4 inch stud should be up. Position the notch filter and
directional coupler above the output outer conductor and thread
the stud of the inner conductor output assembly into the inner
conductor of the notch filter. With a crescent wrench tighten
the inner conductor from inside the PA. encl osure. Slip the
filter tee down over the output outer conductor and rotate to
final position. DO NOT POSITION ANY N.F. PLLTIBIN G 01U21' THE
PA AIR E:(HA ST. Tighten the stainless steel clamo securely
around the filter tee and the output outer conductor.
Remount the monitor coupling loops on the outer conductor.
Re -position the output adjustable sleeve to the distance
recorded in your test data sheets. This adjustment must
be accurate within 1/8 inch as measured from the tube deck
to the bottom of the sleeve.
Install the output coupling loop between the output loading
capacitor C6 and the inner conductor of the output coupling
assembly.
Bolt the low pass filter between the directional coupler and
the transmission line to the antenna. SUPPORT MUST BE PRO-
VIDED FOR THE NOTCH FILTER, LOW PASS FILTER AND Ti{ NShI.3$ION
LINE. (See Section 5.4).
Inspect.the IPA circuitry for possible shorts or loose con-
nections. DO NOT RE -POSITION ANY STRAPS OR BUSS WIRES SINCE
TH1 Y ARE AN INTRICATE PART OF THE EL O'23ICAL DESIGN.
The transmitter may or may not have two connecting
links from the PA tube to the plate line. Remove
the link toward the rear of the transmitter, if used.
Observing the tube handling precautions insert V1
(4CX15,000A) in its tube socket. A alignment line
on the tube anode should match a line on the tube
chimney. Press the tube firmly in the socket.
Examine the underneath side of the socket to make
sure the tube is properly seated and that the finger -
stock is making good contact with the tube conducting
rings.
Re -install the second connecting link if used. Place
the anode connector clamp around the outside of the
link or links and down against the tube chimney.
Tighten the clamp firmly, making sure the connect-
ing link or links are between the clamp and the t.at>e
anode.
Above 95 MHz the PA coarse tuning per operating frequency is
determined by the distance from the bottom of the rot?ry or
adjustable arm portion of the plate line to the PA tube deck.
The distance must be within 1/16 inch as recorded in the test
data. The arm should be fully extended from the plate line;
i.e., the tightening nuts must be at the: slot's ends on the
arm's swivel points.
Refer to the FM Exciter Installation Instructions for proper
module placement in the exciter cabinet.
-6- F;I-20ií3

2.5 Wiring Connection
After the transmitter is physically in place and the
components removed for shipment have been re -installed,
a.c. power should be brought to the transmitter.
Refer to the Installation drawing.
The conduit or wiring of the power leads should be in
agreement with local electric codes and be able to carry
the power requirements of the transmitter. Power leads
and program leads should not be run in the same conduit
or in the same wiring duct. If, due to necessity, the
program leads are in close proximity to the power leads,
the program leads should be separately shielded.
The 3 phase a.c. input enters the transmitter in the
right hand corner of the base and connects to K11, K12
and ceramic stand-off E6. Power source voltage should
be either 208, 230, 240, or 250 volts, 60 Hz with approx-
imately a 40 kVA capacity.
If the line voltage is below 203 V. add the dashpot oil
to the 3 phase line magnetic overload relays, ill and K12.
(Remove the reservoir cup and fill according to the direc-
tions on the oil container.)
The 115 V, 60 Hz single phase a.c. input also enters the
transmitter in the right hand corner of the base and con-
nects to Fl and F2. Approximately 500 VA capacity is re-
quired.
The audio input line enters the base of the transmitter at
the center, approximately 2" from the front. The audio
line connects directly td terminal board TS -1 of the FM
Exciter. Terminals 1 and 3 are the audio inputs, and
terminal 2 is ground or shield connection. If stereo it
used, the lines are connected in accordance with the
Stereo Generator instructions which is part of the FM
Exciter manual.
A good ground at these FM frequencies is mandatory in
keeping R.F. currents in nearby audio or monitoring
equipment to a minimum. A short length of unshielded
wire makes a very efficient antenna. If R.F. is trans-
ferred to audio equipment the results can show up as
noise or feedback. Gates recommends a single common
ground point from the transmitter cabinet to a good
grounding system, such as, a water pipe or an actual
earthing; ground.

2.6 Cooling
The RF amplifiers of the transmitters are forced air cooled.
Other components are convection cooled with the aid of fans
on the top of the transmitter cabinet and the high voltage
power supply cabinet. The temperature rise inside the equip-
ment must not exceed 20°C (68°F) above the room ambient and
must not be greater than 60°C (140°F) under any circumstance.
Heat is a major factor to electronic component deterioration.
A good system of removing the heated air from the transmitter
and the transmitter room and providing cool air for the air
inlet of the transmitter will prolong the life of the trans-
mitter and its components.
Maximum plate dissipation for the 4CX15,000A is 15,000 W
(850 BTU/minute), however nominal plate dissipation will be
around 6,000 W. (342 BTU/minute). Duct work to the top of
the PA enclosure air outlet, if installed, should not cause
'any back pressure. At no point should the duct work have less
of a cross sectional than the opening at the top of the trans-
mitter. Sharp, right angle bends are not permissible. Where
a bend is necessary a right angle radius type should be used.
The normal operating air pressure below the PA tube socket is
approximately áy1 inch of water. Under this condition the
blower is capable of delivering 1130 cubic feet per minute.
There are many installation possibilities. Contact a local
heating er.d cooling contractor for a detailed anal,; cis of
your particular installation.
* NOTE: The 20°C temperature rise is not applicable to the
PA enclosure inlet and outlet.
-8- FM -20H3

SECTION 3 - OPL3ATION
3.1 Pre -Operation
Before placing the FN --20H3 in operation, check once again the
points covered in SECTION 2.
1. Wires connected between transmitter and power supply
cabinet.
2e Primary power to the 3 phase input terminals.
3. 115 Volts to the 1 phase fuse block.
4.5Program line connected to the exciter.
115 Volts for the FIB exciter.
6. Transmitter connected to antenna or a suitable load.
If everything appears to be in order, then you may proceed.
3.2 Test Data
Your equipment has gone through many different kinds of tests
at the Gates factory and has operated for several hours on your
assigned frequency. This is to ensure correct adjustment and
proper setting of all controls. Refer to the test data supplied
with your transmitter. This data is attached to the front of the
transmitter when shipped.
3.3 Adjustment
Sot the dial settings to those recorded on the test data sheet.
Switch S19 in the high voltage power supply cabinet to the
4500 V d.c. position.
Primary power may now be
the Filament ON button.
should light.
Put the Filament Voltage
the IPA Filament control
meter.
applied to the transmitter by »pushing
The light behind the Filament ON button
switch in the IPA position and adjust
for 6 V as read on the Filament Voltage
Next, the blower should begin to run and come up to speed. After
the blower reaches operating speed, air pressure in the PA enclo-
sure will operate the air switch. Closing of the air switch will
turn ON the PA filament. Set the Filament Voltage switch in the
PA position and adjust the PA Filament control for 6.3 V as read
on the Filament Voltage meter. (See Section 5.6)
Turn the IPA Screen control fully counter -clockwise.
With the PA Screen Raise -Lower control in the Lower position run
the motor controlled PA screen voltage rheostats (R43 & R4':) to
the lowest voltage point (arm at 11 o'clock). The screen voltaze
rheostats Lay be inspected by opening the front access door.
Remember tLat a.c. voltages are present on components on the beck
of the access door.
-9-

Check the IPA bias voltage at TB3-5 and TB3-4.(ground) with an
external meter and adjust R28 as necessary to obtain the test
data sheet measurement. TB3 is located on a shelf directly
below the motor controlled PA screen voltage rheostat.
Check the PA bias voltage at TB3-3 and TB3--4 (ground) with au
external meter and adjust R29 as necessary to obtain the test
.data sheet measurement.
Place the Multimeter switch on the access door to the
position. If the exciter is RF driving the IPA stage
reading of approximately 10 to 30 mA will be indicated
Multimeter. Adjust the IPA Grid Tuning for a maximum
Check for an indication (5 mA) on the Multimeter when
meter switch is in the IPA Ik position.
IPA Ig
aon the
indication.
the Multi -
High voltage may now be applied by pushing the High Voltage ON
button. The light behind the High Voltage ON button should light.
Plate and screen voltage are applied simultaneously to the IPA
and PA stages.
Turn the IPA Screen control until 50% scale reading of IPA Ik
is indicated on the Multimeter. Resonate the IPA. stage by
adjusting the IPA Plate Tuning for a dip in the IPA Ik. Increase
IPA screen voltage for Test Data IPA Ik reading. k
The Power Output meter is the farthest meter on the right on the
meter panel. Its function is determined by the For Pwr-VSWR Cal-
VSWR switch. You may read:
1. For Pwr (Forward Power).
2. VSWR Cal (VSWR Calibrate) used for meter calibration
for maximum scale reading during VSWR measurements.
3. VSWR on the transmission line.
Power output of the" transmitter will be noticed if the PA Plate
Tuning and PA Output Loading are near their test data readings.
The For Pwr-VSWR Cal-VSWR switch will have to be in the For Pwr
position.
Increase the PA screen voltage with the PA Screen Raise -Lower
switch in the Raise position, until approximately 2 A of PA Plate
Current are indicated. Resonate the PA stage by adjusting the PA
Plate Tuning control for a dip in the PA plate current.
Adjust the PA Output Loading control for maximum power output
(4 to 6 kW).
Check the VSWR on the transmission line. With the For Pwr-VSWR Cal-
VSWR switch in the VS'.;R Cal position adjust the V3'.'R Cal control
maximum indication on the Power Output meter (it will be imnossible
to obtain a full scale calibration reference at 4 to 6 kW power
output). Position the switch to the VSWR position for the indica-
tion of VSWR on your transmission line. if a large mis -match is
present check the antenna or trans:_iission line for possible problea
before proceeding with the adjustment.
-10- 111-20H3

The low voltage check is now complete. Turn OFF the trans-
mitter and place S19 switch in the high voltage power supply
cabinet to the 9000 V d.c. position.
Turn ON the low voltage and lower the PA screen voltage to
minimum.
Turn ON the high voltage. Approximately 18 kW power output
should be noted.
Adjust PA Plate Tuning for a dip in plate current and the PA
Output Loading for the most efficient operation.
Re -adjust the PA Plate Tuning for a plate current dip and
then make one additional turn in the direction that causes
the power output to increase.
Increase the PA screen voltage for 20 kW power output which
should occur when the PA screen current is 75 to 125 mA as
indicated on the PA Screen Current meter.
Re -adjust the PA Plate Tuning and PA Output Loading controls
as indicated above for maximum power output and the most
efficient operation.
If the VSWR of the transmission line is satisfactory refer
to the test data sheets for operating condition you may expect.
Since the transmitter was checked into a 50 Ohm resistive load
at. the Gates factory any system with a mis -match will probably
change the tuning. Therefore, the recorded test dnta knob
readings may not agree with actual operation.
The PA Screen Raise -Lower control is used to compensate for
any a.c. supply voltage variations which cause power output
fluctuations.
The overloads are.set for 20 kW Operation at the Gates factory.
The IPA Plate Overload R36 is set for 500 mA of IPA Ik as read
on the Piultimeter. The PA Plate Overload R39 is set for
approximately 3.4 A. PA plate current. The controls are
located under a small cover plate on the front access door.
The RF output of the exciter is varied with a output control
of the 10 W. amplifier and is explained in the exciter section
of this instruction book.
-11-- ` FI1-20n3

3.4 Maintenance
Maintenance of the FM -20H3 should consist of the following:
1.
2.34.
Keeping the transmitter clean.
Changing tubes when emission falls off.
Checking mechanical connections and fastenings.
Lubricating the blower motor.
Keeping the transmitter clean from the accumulation of dust
will reduce failure resulting from arcing, dirty relay con-
tacts, and overheating of chokes, resistors and transformers.
Electrostatic fields are "dust catchers". Support insulators
in the PA enclosure and other locations are the worse offenders.
They must be kept clean and free of all foreign material at all
times. If not, arcing may result and the insulator shattered.,
The air filter should be clean at all times. The washable air
filter used in the back door may be purchased from the Gates
Radio Company under Part Number 827-5285-011. However, the
filter may be cleaned using warm water and a mild detergent.
Once a month the entire transmitter should be cleaned of dust.
The inside of the power amplifier should be thoroughly wiped
clean of dust. A small brush, soft rag, and vacuum cleaner
can be used very effectively in keeping the equipment clean.
All contactors and relays should be inspected regularly for
pitting and dirt. The contacts should be burnished and
cleaned if required. The cverlond relays are telephone type,
with sealed contacts and should require little attention.
The bearings for the motor of the PA blower are sealed and
normally give long trouble free operation. They are lubri-
cated for approximately 20,000 hours of operation. After
this period of operation the grease in these bearings should
be changed. This is done by taking the drain plug out of
the bottom of the bearing and a grease fitting attached to
the upper plug on the bearing. New grease should be applied
until clean grease runs out of the drain plug at the bottom.
It is suggested the blower be removed for this maintenance.
The PA tube and the IPA tubes should be removed once a
month and the fins cleaned of dust. Air may be blown
through the fins in the reverse direction, or the anode
cleaned with soap and water or denatured alcohol.
This transmitter is a precision electrical device, and
as such should be kept clean at all times and free of
dust and foreign material. Dust and moisture condensation
will lead to possible arc-overs and short co;iducti.°o paths.
A good preventive maintenance schedule is always the best
assurance for trouble free transmitter operation.
-12- FM -20H3

SECTION 4 - CIRCUIT Di SCRIx ION
The FM -20H3 circuits will be described in the following sections:
Power Amplifier (PA)
Intermediate Power Amplifier (IPA)
Exciter'
Power Supply
Control Circuits
Metering
See Block Diagram.
4.1 Power Amplifier
The power amplifier of the FM-20H3employs a single 4CX15,000A
tetrode in a common cathode amplifier circuit. The plate cir-
cuit is inductively tuned by varying a length of inner conduc-
tor of a transmission line within the rectangular outer con-
duccor. The plate line is approximately one-half wavelength
long, being foreshortened by the output capacity of the tube.
The large variable portion of the line is used for coarse or
approximate frequency setting and the end of the half -wave
line is made variable for fine plate circuit tuning which is
controlled from the front panel. The fine frequency control
covers approximately 3 MHz at the low end of the Fib band and
approximately 6 MHz at the higher end of the band. (The large
variable portion may not be used below 95 iúiz).
Output coupling is accomplished by capacity tuning a coaxial
coupler. The coupler inductively couples R.F. power from the
amplifier enclosure.
The PA grid circuit is common with the IPA plate circuit. The
/IPA plate inductance, L6, IPA plate tuning capacitor C24, and
the input capacitance of the PA tube form a pi circuit.
Bypassing of the PA screen and filaments is accomplished by
using a number of high voltage ceramic capacitors with lead
lengths kept as short as possible.
4.2 IPA
The intermediate power amplifier employs two 4CX250B tetrodes
in a common cathode circuit. The grid circuit is capacity
tuned. The plate circuit is common with the PA grid as pre-
viously explained. Screen bypassing is effected with the
built-in bypass of the 4CX250B air system socket. The cathode
of each IPA tube is bypassed with four ceramic button capacitors.
The two 4CX250B's are in parallel.
4.3 Exciter
The FE Exciter is described in detail in the Exciter instruction
book.
-13-