Harris GATES M-6576 User manual

stnnuccmi
m2ke/13m
HARRIS
INTERTYPE
CORPORATION
M-6576
FM- 7.5H FM TRANSMITTER
GATES
A DIVISION OF HARRIS-INTERTYPE

WARRANTY
Seller warrants new equipment manufactured by Gates 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 months
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 shall
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 an
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 acquisition of possession and title.
Seller assumes no responsibility for design characteristics of special equipment manutactured 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 DE-
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 mer-
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. All
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 Number
or Invoice Number.
WARRANTY ADJUSTMENTS
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.

INSTRUCTIONS FOR INSTALLING OPERATION
OF
FM- 7.5H, 7.5U FM TRANSMITTER
IB-888 0963 031
Gates Radio Company
Quincy, Illinois

FM HARMONICS IN THE TV BAND
The sharp upsurge in FM broadcasting has in some instances developed
unlooked for interference with local TV reception. In every instance this
interference is in so-called fringe areas for TV reception and where the
strength of the TV signal is weak enough that outside highly directional home
TV antennas are necessary. — When this condition develops, the TV viewer
quickly learns from his service man that the local FM station is the offender.
--- The FM broadcaster is immediately deluged with requests to eliminate
the interference. In some instances CATV ( Community Antenna Television)
systems are also offended as they pick up weak distant TV stations. ----
What is the FM broadcaster's responsibility? Answer: To meet FCC rules
and regulations as related to harmonic radiation of his FM equipment but not
to guarantee perfect TV reception.
Below is a chart showing the picture and sound frequencies of TV stations
between Channels 7-13 inclusive. Channels 2-6 are not shown. FM harmonics
do not fall in these Channels. In fact, commercial FM station harmonics will
affect only Channels 8 and above --- look at the chart.
TV Channel Picture Frequency Band --- Mc -- Sound Frequency
7 175.25 to 179.50 197.75
8 181.25 to 185.50 185.75
9 187.25 to 191.50 191.75
C10 193.25 to 197.50 197.75
11 199.25 to 203.50 203.75
12 205.25 to 209.50 209.75
13 211.25 to 215.50 215.75
The frequency range for commercial FM broadcasting is 92.1 Mc to 107.9 Mc:
--- To determine the second harmonic of your FM frequency, just multiply your
frequency by 2. Example: If your frequency is 99.9 Mc, multiplied by 2 would
make a second harmonic of 199.8 Mc. By consulting the above chart, you will
note the second harmonic falls in the picture portion of the TV Channel 11.
Correct FM Harmonic Radiation
The FCC stipulates that transmitters of 3000 watts power and over must have
a harmonic attenuation of 80 db. For 1000 watts, 73 db., and for 250 watts,
66.9 db. All reputable manufacturers design their FM transmitters to meet
or exceed these specifications.
Fringe Area TV Strength Versus FM Harmonics
4 %e
Let's take a typical FM station that radiates 70,000 microvolts per meter at
1 mile. At 80 db. harmonic attenuation ( as called for by FCC), this station will
radiate approximately 7 microvolts per meter at 1 mile on the second harmonic.
In the case of our Channel 11 example, it is estimated that a fringe area TV
station from 60 to 90 miles distance would have a signal strength of from 5 to
25 microvolts per meter. It can then be easily understood that a 7 microvolt
signal, well within FCC specifications, would definitely interfere with the TV
signal, yet with the FM broadcaster's equipment performing normally.

This is sometimes further aggravated by the FM station being located between
the TV station and the TV receivers. In this instance the TV antennas are
focussed not only on the TV station but your FM station as well. The home
TV antennas are beamed at your legal second harmonic as well as the fringe
TV station.
What To Do
When interference occurs, it will develop ragged horizontal lines on the TV
picture varying with the FM program content. If the TV sound portion is
interfered with ( usually not the case), then the FM signal will be heard in
addition to the TV sound.
1. It is not up to the FM broadcaster to go on the defensive. He did not put
the TV station 75 miles away nor did he select the TV Channel. -- In
most instances the condition is a natural phenomena that neither you, the
TV station, nor the FCC can correct.
2. Do not adjust the FM harmonic or " T" notch filters supplied with the
FM transmitter. These are factory adjusted and most FM stations do not
have the expensive equipment necessary for correct adjustment. Tampering
with this calibrated adjustment will probably make the condition worse.
3. Do not rely on TV service men's types of measuring equipment. They are
not built to accurately measure harmonics and invariably give erroneous
readings that invite the CATV or local service men's association to say
"I told you so." Remember it is difficult to radiate harmonics if the
equipment is built to suppress the harmonics and it is.
4. In many instances interference may be caused by overloading on the front
end of the TV receiver. This problem usually occurs when the receiver is
located close to the FM transmitter. This problem can be overcome by
installing a trap tuned to the frequency of the FM carrier. The TV service
man can and must learn how to do this. In most cases it works, while in
some instances, if not properly installed or tuned, it will not completely
eliminate the interference. In one case where interference of this type
existed, a TV station put traps for the fundamental FM frequency on
nearly every TV set in town. Not the FM transmitter.
Summary
The FCC is well acquainted with this nation-wide problem. If TV viewers write
FCC, complaining about your FM station, remember the FCC has received a few
thousand similar letters. — It is not the obligation of the FM broadcaster to
assure fringe area reception of a TV station any more than is the obligation of
the TV station to assure the FM broadcaster perfect reception in his TV city.
Probably your installation will not have problems as outlined above. If they do
exist, don't blame the equipment. Every transmitting device puts out a second
harmonic, even the TV stations. The fact that these harmonics legally fall into
the spectrum of a TV station many miles distant is coincidental, but not your
fault.
o
Gates Radio Company

TABLE OF CONTENTS
SECTION 1 - GENERAL DESCRIPTION
1.1 Warranty and Safety Notice 1
1.2 Purpose of Book 1
1.3 Purpose of Equipment 1
1.4 Description 1
1.5 Technical Data 2
1.6 Vacuum Tube Table 3
SECTION 2 - INSTALLATION
2.1 Inspection 3
2.2 Packing Check List 3
2.3 Installation 4
2.4 Handling the 4CX5000A 5
2.5 Power Connection 6
2.6 Cooling 6
SECTION 3 - OPERATION
3.1 Pre- operation - ( Also see Section 6)
3.2 Test Data
3.3 Adjustment
3.4 Maintenance
SECTION 4 - CIRCUIT DESCRIPTION
4.1 Power Amplifier
4.2 Intermediate Power Amplifier ( IPA)
4.3 Exciter
4.4 Power Supply
4.5 Control Circuits
4.6 Metering
7
7
8
10
11
12
12
12
13
14
SECTION 5 - ADDITIONAL INFORMATION
5.1 Remote Control 14
5.2 Stereophonic Operation 14
5.3 FM- 7.5H Parts List 15
SECTION 6 - M-6425 EXCITER -
SECTION 7 - ILLUSTRATIONS
7.1 FM- 7.511 Front View, Showing Operating Controls
7.2 Floor of Transmitter ( Rear View).
7.3 RF Output Balun
7.4 RF Cubicle
7.5 Installation Drawing
7.6 Graph - PA Tuning
7.7 Block Diagram, FM- 7.5H
7.8 IPA PA Simplified Block Diagram
7.9 Graph - PA Efficiency
7.10 Overall Schematic - Transmitter
7.11 Low Pass Filter
7.12 2nd Harmonic Filter
FM- 7.5H

SECTION 1 - GENERAL DESCRIPTION
1.1 Warranty and Safety Notice
This equipment is guaranteed under the liberal Gates
Warranty, terms and conditions of which are full set
forth in the standard Gates Warranty, available upon
request.
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 on
the field.
Switch to Safety - This equipment employs voltages which are
dangerous and may prove fatal if contacted by operating
personnel. Extreme 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 inter-
locked you should not rely on the interlock switches for re-
moving high operating voltages. It is always best to discon-
nect the primary power at the building wall switch and dis-
charge all capacitors with the grounding stick provided.
1.2 Purpose of Book
This instruction book has been prepared to assist in the
installation, operation, and maintenance of the Gates
FM- 7.5H, 7.5KW FM transmitter.
1.3 Purpose of Equipment
The Gates FM- 7.5H is an FM broadcast transmitter with 7,500
watts output delivered to the transmission line. The operat-
ing frequency is 88-108 MHz with characteristics exceeding
those required by the Federal Communications Commission for
standard FM broadcast service. The transmitter is designed
for continuous broadcast operation and consists of the exciter,
intermediate power amplifier, and the power amplifier, plus
their associated power supplies.
1.4 Description
Only one cabinet is required to house the entire transmitter.
This cabinet is 42" wide x 78" high x 32-3/4" deep. All
necessary metering is provided by four meters located on a
meter panel at the top of the cabinet. Ready access to the
complete transmitter is accomplished by removable rear door,
hinged access panel, and cover of the exciter unit. Front doors
-1- FM- 7.5H

arc provided to offer a pleasing and symmetrical front view
appearance. The following controls vre located on the front
panel ( see Illustration 7.1):
a)
b)
c)
d)
e)
f)
g)
h)
i)
j)
k)
Filament ON
Filament OFF
High Voltage ON
High Voltage OFF
Power Amplifier Fila-
ment Voltage Control
PA Screen Voltage
Control
Multimeter Selector
VSWR Calibrate
Power Calibrate
Power/VSWR Selector
IPA Screen Voltage
Control
1.5 FM- 7.5H Technical Data
Power Output
Frequency Range
RF Output Impedance
Output Termination
Frequency Stability
Harmonic Attenuation
Modulation Capability
Audio Input Impedance
Audio Input Level
Audio Frequency
Response
Audio Distortion
FM Noise Level
AM Noise Level
Power Source
Input AC Pwr Require
Pwr Line Variation
(Slow)
Power Factor
Altitude
Ambient Temperature
Range
Maximum VSWR
Overall Cabinet Size
Weight
Front Door Swing
1)
m)
n)
p)
r)
s)
Remote/Local Switch
Plate Tuning Control
Output Loading Control
IPA Plate Tuning Control
IPA Grid Tuning Control
Plus Five ( 5) Potentio-
meters for Adjustment of
PA Plate, Current Remote
Reading, Recycle Adjustment,
IPA Overload PA Plate Volt-
age Remote Reading Adjustment
PA Overload.
4.5 to 7.5 KW
88 to 108 MHz
50 ohms
Standard EIA 3-1/8" Flange
±.001%
-80 dB
±100 kHz
600 ohms
+10 dBm, ± 2 dB
±1 dB - 30-15,000 Hz
1/2% or less - 30-15,000 Hz
65 dB below 100% FM modulation
50 dB below equivalent 100% AM modulation
08/24V, 3 pi), 60 Hz, 115V, 1 ph, 60 Hz.
15 KW - 240 V., 500 Watts - 115 V.
±5%
90%
7500 Feet
20 oC to 45 o C.
1.7 to 1
42" x 78" H x 32-3/4" D*
1100 Lbs.
21"
*32-3/4" is overall depth dimension. With rear door, front door
handles, and meter trim strip removed minimum depth is 29-3/4".
Filter and RF plumbing extends 9-1/2" above cabinet top.
-2- FM- 7.5H

1.6 Vacuum Tube Table
The following tubes are employed in the transmitter.
Symbol Designation Tube Type Function
V1 4CX5000A Power Amplifier
V2 4CX250B Intermediate Power Amplifier
SECTION 2 - INSTALLATION
2.1 Inspection
The FM- 7.5H 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 small
items such as pilot lights, fuses, loose screws and bolts.
If damage should occur during shipment, all claims should be
filed promptly with the transportation company. If a claim
is to be filed, the original packing case and material must
be preserved. A damage report must be filed to collect for
shipping damages. Gates Radio Company is not responsible for
damage occuring during shipment. Parts or components shipped
to replace those damaged in transportation will be billed to
the customer plus transportation expenses, the cost of which
should form a portion of your claim to the transportation
company.
A complete visual inspection should be made of the equipment.
Determine that there are no loose connections, loose components,
broken insulators, etc., that may have been damaged in shipment.
Make sure all relay contacts are free and in good mechanical
condition. Make sure all mechanical connections arc tight.
Check with a screw driver or a wrench, all mechanical and
electrical connections that are mechanically bolted together.
All tie downs or blocking used for shipping 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- 7.5H have been kept to a bare minimum
are are plug-in units and heavy components. Tubes that are
not clamped down for normal operation are also removed. The
following components have been removed from the transmitter
for shipping purposes:
-3- FM- 7.5H

Quantity Part Number
Description
1 404 0073 000 Chimney SK606 ( 4CX250B)
1 813 9383 001 Exhaust Tubing and 0 Rings ( 4CX250B)
1 472 0397 000 Power Transformer, T4
1 374 0081 000 Tube, 4CX250B
1 Assy. 942 3928 001 Notch Filter & RF Output Balun
1 Assy. 992 1600 001 Low Pass Filter
As various components are removed, the wires connecting each
component are numbered or tagged for placement of these parts.
After determining that all these components are on hand, you
are ready to proceed with the installation in an orderly manner.
2.3 Installation
In advance of actual placement of the equipment, certain plan-
ning should be accomplished. The use of illustrations 7.2 and
7.5 will assist in locating the power and audio input terminals
of the transmitter.
Either side of the transmitter may be placed against a wall or
other equipment. Complete accessibility for maintenance and
installation is provided in the FM- 7.5H by access from the
front or the rear of the transmitter cabinet. Mount the power
transformer ( T4) through the back door of the transmitter. It
would be wise to remove the ceramic fuse block temporarily
when doing this to prevent accidental damage to this part.
Orient the transformer so that the secondary taps on the coils
face inward. Connect the primary power leads from contactor
K2 to the marked terminals at the top of T4. Secondary leads
are tagged for connection to their proper taps on the coils.
Install the plug-in capacitor in the transmitter bias supply.
Refer to the TE -1 Exciter Installation Instructions for proper
module placement - in the exciter cabinet.
Unblock the blower assembly.
The RF output balun assembly installs from inside the transmitter
enclosure. Before mounting, temporarily remove both monitor
coupling loops from the top portion of the balun. With the
innerconductor removed and the balun sleeve up, bolt the unit
beneath the top of the enclosure ( as shown in Figure 7.3).
Insert the : innerconductor from the top.
-4- FM- 7.5H

Take the notch filter and thread the stud from the balun
inner- conductor into the inner- conductor of the notch
filter. Tighten with a crescent wrench from the loading
capacitor connection in the P.A. enclosure. Slip the
filter sleeve down over the outer conductor and fasten
securely with the stainless steel clamp provided.
Remount the two monitor coupling loops on the exterior
vertical balun.
The lower end of the inner- conductor of the balun connects
to output loading capacitor,C6.
Loosen the Allen set screws on the adjustable portion of the
balun. The distance that this component is positioned verti-
cally from the tube deck varies with operating frequency.
Refer to your test data sheets for the proper measurement
on your assigned channel. This adjustment must be accurate
within 1/8" for proper operation.
The 40:250B, ceramic chimney, and exhaust tubing are installed
in the driver cubicle, Figure 7.4. Place the tube in its
socket, slip on the ceramic chimney, and clamp on the anode
connector. Drop the exhaust tube through the opening of the
upper tube deck to the top of the 40:250B. Hold the exhaust
tube in place with 0 rings above and below the deck surface.
Installing the 4CX5000A is simple. Handling of this tube is
covered in Section 2.4. The anode connector assembly secures
with a clamp to the tube and with a bolt to the plate line.
•Coarse frequency tuning of the plate circuit is determined by
the distance of the rotary portion of the plate circuit from
the 4CX5000A tube deck. This measurement is recorded in the
test data for your transmitter and should be checked before
operating the transmitter. Tolerance here is approximately
1/16".
Bolt the low pass filter in the transmission line between the
directional coupler and antenna coax, and your basic installa-
tion is complete. The weight of the low pass filter should
not be applied to the 1-5/8" coaxial components directly.
Provisions should be made at the transmitter site to have
at least two supports for the filter.
2.4 Tube Handling and Operating Precautions, 4CX5000A
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, please refer to the tune-up and
operating procedure given in Section 3. It is recommended
procedure to adjust the equipment for operation under heavy
plate loading conditions, and with only sufficient RF drive
to provide the required power output and efficiency.
-5- FM- 7.5H

Extreme care should be taken during tune-up as well as in
regular service to avoid, even momentarily, operating of
this tube under conditions of insufficient plate loading
or excessive RF drive. These operating conditions, especially
at the upper end of the VHF range, will produce excessively
high seal and/or bulb temperature and will result in damage
to this tube.
2.5 Power Connection
After the transmitter is physically in place and the components
removed for shipment have been reinstalled, AC power should be
brought to the transmitter. Referring to the installation draw-
ing, Figure 7.2, the 240V, 3 phase input enters the transmitter
in the lower right hand corner and connects to the 3 phase fuse
block immediately to the left. A 115 V, single phase fuse block
is located at the center and to the rear of the transmitter with
the input terminals for the 115 V, single phase towards the rear
of the transmitter.
The audio input line enters the base of the transmitter at the
center approximately 7-1/2" from the front. The audio line
connects directly to terminal board TB1 of the M-6425 exciter.
Terminals 1 and 3 are the audio inputs and terminal 2 is ground
or shield connection. If stereo is used, the lines are con-
nected in accordance with the information in the M-6533 stereo
generator instructions.
The power leads for the transmitter should come from a low
reactance power source of either 208, 230, or 240 volts, 60 Hz,
3 phase, with approximately a 20 KVA capacity. A power source
of 115 volts, 60 Hz with 500 watts capacity is also required.
The conduit or wiring of the power leads should be in agree-
ment 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.
A good ground at these FM frequencies is mandatory in keeping
RF currents in nearby audio equipment to a minimum. RF usually
shows up in one of two ways - feedback or high noise, and in
some cases both. It should be pointed out that even a small
amount of unshielded wire makes a very efficient antenna for
FM frequencies. If RF from the cabinet field is transferred
to the grids of the audio equipment, it is rectified and shows
up as noise or feedback. We strongly recommend a single common
ground point from the transmitter base to a good grounding
system such as a water pipe or actual earthing ground.
2.6 Cooling
The transmitter is air cooled and several kilowatts of heat are
developed and dissipated through the air outlet in the top of
-6- FM- 7.5H

the transmitter. It may be necessary to provide a means of
exhausting this air from the transmitter room or enclosure.
Heat is a major enemy 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 greatly prolong the life of the
transmitter and its components. Duct work, if installed, should
not provide any back pressure to the power amplifier enclosure.
At no point should the duct work have less of a cross sectional
area than the opening at the top of the transmitter. Sharp,
right-angle bends are not permissible. Where it is necessary
to turn a right-angle, a radius type bend should be used.
There are many installation possibilities. Each and every
installation is somewhat different. Therefore, it is not
possible to give complete detailed information on the trans-
mitter ducting. Only general information can be supplied.
As a suggestion, contact a local heating and cooling contractor
for a detailed analysis of the problem.
After the transmitter has operated in full output a number of
hours, a temperature rise inside the transmitter must not ex-
ceed a rise of 20 ° C above the ambient measursd at the air in-
take of the blower and must not rise above 60 C under any
circumstances.
SECTION 3 - OPERATION
3.1 Pre- Operation
Before placing the FM- 7.5H into operation, check once again the
points covered in Section 2. Have you mounted all components
physically and made these electrical connections:
1. Primary power to the 3 phase fuse block?
2. 115 volts to the 1 phase fuse block?
3. Program line connected to the exciter?
4. 115 volts for the crystal ovens?
5. 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 been operated for several hours
on your assigned operating frequency. This is to ensure cor-
rect 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.
-7- FM- 7.5H

3.3 Adjustment
Set the dial settings to those given on the test data sheet.
Turn the IPA screen voltage control fully counterclockwise.
Primary power may now be applied to the transmitter by push-
ing the filament ON button. The light behind the filament
ON button should light. Next, the blower should begin to
run and come up to speed. After the blower reaches maximum
operating speed, air pressure in the P.A. enclosure will
operate the air switch. Now run the P.A. screen toltage
control to the lower position ( counterclockwise) on the
screen rheostat. Check the P.A. bias voltage and adjust
as necessary to obtain the test data sheet measurements.
During the tune-up procedure it may be necessary to in-
crease the IPA voltage to prevent the P.A. from drawing
excessive plate current. The grid bias voltage on the P.A.
is a combination of the developed bias from the RF and the
constant voltage from the bias supply. The bias supply is
set at a compromise position to obtain the desired power
output and to keep the P.A. within its dissipation ratings
in case of RF failure.
Closing of the air switch will turn ON the PA filament volt-
age which may be read with the multimeter switch on the meter
panel in the filament voltage position. Set the filament volt-
age for 7.5 volts. Next, place the multimeter switch on the
meter panel to the driver cathode current position. ( This is
the UP position.) If the exciter is delivering power to the
driver stage, a reading of approximately 10% will be read on
the multimeter. Tune the IPA grid tuning for a maximum indi-
cation. As this meter is reading cathode current it will also
read the grid current. The high voltage may now be applied
by pushing the high voltage ON button. This supplies plate
and screen voltage to the IPA stage simultaneously with the
application of plate and screen voltage to the power ampli-
fier stage. Bring the screen control for the driver up until
the driver cathode draws approximately 50% scale reading on
the multimeter. Resonate the IPA plate circuit by tuning for
a dip in the IPA cathode meter reading. If the plate circuit
and loading are near their operating positions, power output
of the amplifier will be noticed.
Increase the screen voltage of the power amplifier by bringing
the screen control lever switch to the UP position until ap-
proximately 1.5 amperes power amplifier plate current is indicated.
Resonate the plate circuit of the power amplifier by adjusting
the plate tuning ( L2) for a dip in plate current. Next, check
the position of the output loading by rotating the output loading
control to give a maximum output indication.
-8- FM- 7.5H

The RF output meter is the farthest right-hand meter on the
FM- 7.5H. Its function is determined by the rotary switch
(Si) located on the upper portion of the access door. You
may read:
1. Forward power.
2. Meter calibration for maximum scale reading
during VSWR measurements.
3. VSWR on the transmission line.
Set this switch to position ( 1). At this time, if approximately
3 to 5 kilowatts of power are measured, you should check the
VSWR. If it is 1.5:1 or better, the screen voltage of the ampli-
fier and the IPA stage may be increased until both are at maximum
or near maximum. The plate tuning, output loading, and driver
plate tuning should be adjusted for the maximum output and the
most overall efficient condition. To reduce the RF output, the
amount of drive to the PA can be decreased by lowering the screen
voltage of the IPA. Also the output can be reduced by decreasing
the PA screen voltage.
The multimeter switch ( S10) located on the meter panel will give
an indication of the amount of drive to the grid of the PA tube.
This is a relative indication and is read with the meter switch
in the DOWN position. It will be noticed that maximum drive
condition will be very close to the same point of the driver
plate current dip. The driver plate tuning may, at some fre-
quencies and power levels, be different for maximum output and
for minimum driver plate current. A compromise should be made
on the plate tuning of the driver for a driver cathode current
of approximately 70% scale reading with a minimum or dip in
tuning. The tuning on either side of the dip may affect PA
efficiency as well as power output.
The operation of the transmitter is very simple and straight-
forward, and once adjusted should require only a nominal amount
of touching up the tuning at regular maintenance periods.
The overloads are set for correct operating level at the factory.
The IPA plate overload is set for nearly full scale reading on
.the multimeter. The PA plate overload is set for approximately
2.7 amperes plate current. The adjustments for the overloads are
located under a small cover plate located on the front access door.
They may be referred to by symbol number on the schematic ( Figure
7.10). Power output of the transmitter may be increased or de-
creased by three controls on the transmitter. The first is the
output loading. It is best to leave this control set for maximum
loading on the amplifier as this will give more stable operation
as recommended for any tetrode. The second control is the power
amplifier screen voltage. After the loading has been adjusted
for maximum power output the screen voltage may be raised or
lowered for the desired operating power. The third control is
-9- FM- 7.5H

the IPA screen voltage control. Reducing this to its minimum
value will reduce the drive and part of the bias to the final
amplifier causing it to overload and trip the plate voltage.
It may be operated in its maximum position without any detri-
mental effects. However, to give partial control to power
output and some tolerance on the power output of the IPA
stage it is recommended that it be run at approximately 80%
of its full scale setting.
The output of the exciter is adjusted with output control of
the 10 W. amplifier in the exciter and is covered in the exciter
manual.
The transmitter can easily be remotely controlled. Description
of the connections is covered in Section 5.
Two controls for setting the remote plate voltage and plate
current for external metering are located under the cover on
the hinged access door and are shown by symbol number on the
schematic.
The screen voltage of the power amplifier is motor controlled
and is also connected to the remote control Raise/Lower func-
tion for power output.
3.4 Maintenance
o
e
Maintenance of the FM- 7.5H should consist of the following:
1. Keeping the transmitter clean.
2. Changing tubes when emission falls off.
3. Checking mechanical connections and fastenings.
4. Lubricating the blower motor.
Keeping the transmitter clean from the accumulation of dust will
reduce failure resulting from arcing, dirty relay contacts, and
overheating of chokes, resistors, and transformers. Electro-
statis fields are " dust catchers". Support insulators in the
PA enclosure and other locations are the worst 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 filters should be clean at all times. Periodically, it
should be discarded and replaced with a new one. The filter is
a disposable type and may be obtained at a hardware or heating
supply store.
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.
-10- FM- 7.5H

All contactors and relays should be inspected regularly for
pitting and dirt. The contacts should be burnished and cleaned
if required. The overload 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 lubricated
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 tube 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 conductive paths.
A good preventive maintenance schedule is always the best assur-
ance for trouble free transmitter operation.
SECTION 4 - CIRCUIT DESCRIPTION
. The FM- 7.5H circuits will be described in the following sections:
Power Amplifier
Intermediate Power
Amplifier ( IPA)
Exciter
Power Supply
Control Circuits
Metering
See Illustration 7.7.
-4.1 Power Amplifier
The power amplifier of the FM- 7.5H employs a single 4CX5000A
tetrode in a common cathode amplifier circuit. The plate cir-
cuit is inductively tuned by varying a length of innerconductor
of a transmission line within the rectangular outer conductor.
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 rough or approximate
frequency setting and the end of the half- wave line is made
variable for plate circuit tuning.
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FM-7.5H

This is controlled from the front panel. The fine frequency
control covers approximately 3 MHz at the low end of the FM
band and approximately 6 MHz at the higher end of the band.
Output coupling is accomplished by capacity tuning a balun.
The balun inductively couples RF power from the amplifier
enclosure.
The PA grid circuit is common with the IPA plate circuit.
The IPA plate inductance, L-6, IPA plate tuning capacitor,
C-26 and the input capacitor of the PA tube form a modified
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.
In some transmitters, especially at the higher operating fre-
quencies, there is a capacitor connected between grid and
cathode of the PA. This capacitor is usually 24 pf. . or 50
pf: . The purpose of adding this component is to improve the
overall performance of the power amplifier.
4.2 IPA
The intermediate power amplifier employs a 4CX250B tetrode in a
common cathode circuit. The grid circuit is capacity tuned.
The plate circuit is common with the PA grid as previously ex-
plained. Screen bypassing is effected with the built in bypass
of the 4CX250B air system socket. The IPA cathode is bypassed
with four ceramic button capacitors.
4.3 Exciter
The FM exciter is described in detail in the exciter instruction
book.
4.4 Power Supply
Only one high voltage power supply is used in the FM- 7.5H. It
supplies 5.6 KV for the PA plate and 2 KV for the PA screen volt-
age, and the IPA plate. The basic configuration of the supply is
a 3 phase, full wave bridge. 2 KV is obtained from the center tap
of the transformer to supply the IPA plate and also supply the
source for the variable voltage of between 300 V. and 1500 V. for
the PA screen grid. Series limiting resistor, R47, prevents the
PA screen grid from over- dissipating in case the PA has a loss of
plate voltage.
o
The 0 to 270 V variable supply for the IPA screen voltage is also
derived from the 2 KV center tap source. R26 is the control.
Silicon rectifiers for this supply consists of three doublers, each
containing a number of diodes in series with proper resistors and
capacitors across the diodes. The diodes arc mounted in copper
heat sinks.
-12- FM- 7.5H

The PA bias supply is a single phase full wave bridge circuit
using silicon rectifiers. Grid bias between 90 to 175 volts
is supplied to the PA control grid. The bleeder resistor
across the supply, R41, will also provide additional bias
voltage if the PA grid current due to RF drive causes grid
current to flow about 40 ma with 160 volts fixed bias. R39
is the bias adjust control.
4.5 Control Circuits
The control circuits of the FM- 7.511 consist of the following: .
-Kl Primary Contactor - applies voltage to the filaments,
exciter, and energizes the blower.
-K2 Plate Contactor - applies primary voltage to the
plate transformer ( K2 is energized after K3 closes).
-K3 Step/Start Contactor - closes. Then K2 is energized,
shorting out the contacts of K3 and the 1 ohm resis-
tors. Step/Starting of the high voltage supply is
accomplished by K3 closing first and applying volt-
age to the transformer primary through 1 ohm resis-
tors, R22, R23, and R24.
-K4 Auxiliary Relay - applies holding voltage to the
Step/Start contactor K3, if the air switch and
door interlocks are closed.
-K5 Recycle Relay - energizes when either the PA overload
or IPA overload relay is energized a number of times.
The number of times is determined by control R25.
The two overload relay contacts are in series across
the relay circuit for K5. When either O.L. relay
energizes and the contacts open, C36 starts to charge.
If the contacts are open for a sufficient length of
time for C36 to charge to the point that the voltage
will energize KS, the contacts of K5 will break the
hold circuit of K4 and the plate voltage will be
switched off. If K5 does not operate, the overload
contacts will close after an overload and the plate
contact K2 will again energize.
-K9 The Underdrive Relay - will prevent application of
plate and screen voltage to the IPA and P.A. until
the grid current of the IPA reaches 8 ma or more.
K9 is located on the front access door below K4.
The contacts of K9 are in series with door interlocks.
In case of a plate voltage trip out due to low IPA
grid'current . the recycle circuit will not operate.
Only IPA and P.A. plate overload will operate the
recycle circuit.
-S9 Air Switch - closes after the air pressure in the
plenum reaches proper pressure, closing the inter-
lock circuit and switching primary voltage to the
PA filament transformer.
G
-13- FM- 7.M1

4.6 Metering
All necessary metering of the FM- 7.5H is accomplished with
four meters located on the cabinet meter panel. A multimeter
provides the following:
IPA Cathode Current
PA Filament Voltage
PA Drive
A metering rectifier circuit is calibrated at the factory to '
give PA filament voltage read on the multimeter. A PA drive
detector, coupled to the grid circuit, provides a DC voltage
to the multimeter to indicate the presence of RF in the PA
grid enclosure.
The second meter reads PA plate current and is located in the
Plate B-Flead. The meter is properly insulated and isolated
behind a protective plexiglass cover.
The third meter reads plate voltage and is located on the
low potential side of the meter multiplier resistor.
The fourth meter is for indicating power output and VSWR on
the transmission line. This meter works in conjunction with
the directional coupler mounted in the output transmission
line.
SECTION 5 - ADDITIONAL INFORMATION
5.1 Remote Control
Remote control facilities are built into the FM- 7.5H and
require only connection to either the Cates RDC-10C remote
control unit or the Gates RDC-200A remote control equipment.
The connections to the transmitter are made at TB- 6 located
in the base of the cabinet. Terminal connections for the
functions are shown on the schematic Fig. 7.10.
.The functions are:
1. Fail- Safe, Primary ON- OFF.
2. Momentary ON- OFF for plate voltage.
3. Raise- Lower for adjusting power output.
4. Plate voltage metering.
S. Plate current metering.
6. RF power output metering.
5.2 Stereophonic Operation
Provision has been provided for the installation of the
Gates M-6533 stereo generator in the FM exciter. Instruc-
tions for audio connections are given in the M-6425 manual.
With the addition of the M-6533 stereo generator the trans-
mitter is FOG type accepted for . stereophonic operation.
-14- FM-7.Sn