Ramsey Electronics FM25A User guide

FM25A •1
Ramsey Electronics Model No. FM25A
Own and operate your own FM Stereo broadcast station.
Definitely not a toy, the FM-25A has an exceptional synthesized
transmission range and improved audio quality that puts your
favorite radio station to shame.
•Great for transmitting your tape deck or CD player throughout the
house, yard or even your car.
•Powerful enough for college or neighborhood radio stations - in use
all over the world.
•Fantastic improved audio quality, sounds better than most stations
on the dial. And we’ll tell you why!
•Easily connects to the line-level outputs on any tape deck, stereo
system or CD player.
•Some users hook up one channel to the scanner and the other to
their two way radio. Now you can hear what’s going on around town
or a couple blocks away from your house with a simple stereo
receiver... and adjust the volume of each individually with your
balance control!
•Add a mike-mixer and tape or CD deck for a “PRO” sounding radio
station.
•Operates on 8 to 14 volts DC, 12 volt DC power supply provided.
•Easily tunable anywhere in the 88-108 MHz FM band.
•Clear, concise instructions guide you step-by-step to a finished
product that works FIRST time.
SYNTHESIZED FM
STEREO TRANSMITTER

FM25A •2
PARTIAL LIST OF AVAILABLE KITS
RAMSEY TRANSMITTER KITS
• FM25, MP3FM FM Stereo Transmitters
• AM1 AM Transmitter
• TV6 Television Transmitter
• FM100 Professional FM Stereo
Transmitter
RAMSEY RECEIVER KITS
• FR1 FM Broadcast Receiver
• AR1 Aircraft Band Receiver
• SR2 Shortwave Receiver
• AA7 Active Antenna
• SC1 Shortwave Converter
RAMSEY HOBBY KITS
• SG7 Personal Speed Radar
• SS70A Speech Scrambler
• MX5, MX10 Mixers
• MD3 Microwave Motion Detector
• PH10 Peak hold Meter
• STC1 Stereo Transmitter Companion
RAMSEY AMATEUR RADIO KITS
• FX 146 VHF Transceiver
• HR Series HF All Mode Receivers
• QRP Series HF CW Transmitters
• CW7 CW Keyer
• CPO3 Code Practice Oscillator
• QRP Power Amplifiers
RAMSEY MINI-KITS
Many other kits are available for hobby, school, scouts and just plain FUN. New
kits are always under development. Write or call for our free Ramsey catalog.
SYNTHESIZED FM STEREO TRANSMITTER KIT INSTRUCTION MANUAL
Ramsey Electronics publication No. MFM25A Rev 1.4
First printing: August 1996
COPYRIGHT 1996 by Ramsey Electronics, Inc. 793 Canning Parkway, Victor, New York
14564. All rights reserved. No portion of this publication may be copied or duplicated without the
written permission of Ramsey Electronics, Inc. Printed in the United States of America.

FM25A •3
SYNTHESIZED FM
STEREO TRANSMITTER
KIT
Ramsey Publication No. MFM25A
Price $5.00
TABLE OF CONTENTS
Introduction ...................................... 4
Circuit Description............................ 5
Parts Layout Diagram ...................... 7
FM25A Parts List.............................. 8
Assembly Procedure........................ 10
Schematic Diagram.......................... 16
Case and Knob Assembly................ 18
Choosing an Operating Frequency .. 19
Adjusting and Alignment .................. 19
At Home Use.................................... 21
Projects and Experiments ................ 21
Antenna Ideas.................................. 22
Troubleshooting ............................... 23
FCC Rules and Information.............. 24
Understanding Field Strength .......... 26
Summary.......................................... 28
Warranty........................................... 31
KIT ASSEMBLY
AND INSTRUCTION MANUAL FOR
RAMSEY ELECTRONICS, INC.
793 Canning Parkway
Victor, New York 14564
Phone [phone]
Fax [phone]
www.ramseykits.com

FM25A •4
INTRODUCTION
The Ramsey FM25A is a true SYNTHESIZED STEREO FM broadcast transmit-
ter, which any person may build and use in accordance with the rules of your
nation’s telecommunications authority. For U.S. residents, that authority is the
Federal Communications Commission (FCC). The FM25A’s low-power broad-
casting capability and other practical uses can be fun and interesting for people
of all ages, but the FM25A is not a toy. We will refer to the FCC regulations fre-
quently in this manual and provide you with some information necessary to en-
joy the FM25A's capabilities in accordance with the law.
Typical uses for the FM25A include the following:
•Extension of home stereo system - without wires.
•Listening aid for auditoriums, churches.
•Student-operated school radio station.
•College dorm favorite music broadcast service.
•Short-range, two-channel experiments and demonstrations.
We think you will be very pleased with the transmitting range, audio quality, fre-
quency stability and stereo channel separation of this build-it-yourself synthe-
sized FM stereo transmitter. If you follow our assembly directions carefully and
use your FM25A in accordance with applicable FCC rules, a whole new world
of sharing music, news and views with friends and neighbors awaits you.
Since the sharing of music and information is vital to the culture of our late-
20th-century global community, we realized that our FM25A low-power Synthe-
sized FM Stereo Transmitter Kit was certain to attract worldwide interest among
hobbyists, students and "pioneers." While the use of the FM25A may need to
be limited to "wireless stereo extensions" in some USA households (to comply
with FCC Rules, Part 15), we have seen it serve very well as a serious, though
simple, broadcast station for remote villages throughout the world where low
cost AM-FM receivers are available to people of all economic levels. After
you're done building your kit, sitting back and listening to your handiwork, con-
sider this: many other FM25A's just like yours are faithfully relaying news and
information to listeners in remote parts around the world. The FM25A is most
definitely not a toy!

FM25A •5
CIRCUIT DESCRIPTION
We will begin by talking about the power supply of your new FM25A. While a
DC power source is provided with the FM25A, the DC isn’t ‘proper’ enough to
provide us with the low-noise, stable supply that we would like for good audio
quality. Voltage regulator VR3 provides us with a means to take the raw DC
output from the wall transformer and ‘smooth’ it out, keeping it at a constant 12
volts. Filter capacitor C5 reduces ripple noise from the DC wall transformer.
VR1 provides us with a good clean +5 volts for the CMOS circuits of U1 and
U2, while VR2 gives us about 2.6-2.8 volts which is what the custom IC U3
likes to see.
The custom FM stereo IC (U3) is the heart of the FM25A. The control of U3 is
determined by its surrounding circuitry. Potentiometers R11 and R16 allow for
adjustment of the audio levels. Resistors R9 and R15 set the pre-emphasis
characteristics (75 µs for USA, 50 µs for Europe). R6 permits adjustment of ste-
reo balance and L1 and D21 form an adjustable resonant circuit to set the car-
rier operating frequency. Y2, C32, and C30 provide the 38 KHz subcarrier for
stereo transmission. C33, R22, R21 and C34 set the proper multiplexed audio
carrier levels for the modulator. The combined modulated RF signal is seen on
pin 7 of U3, and is amplified by Q3 and surrounding biasing components.
Q3 amplifies the output of U3. This signal is fed to U1 through C26 and R12
which is the transmitted frequency feedback for the PLL to compare the crystal
frequency to. It is also fed to Q2 through C29 and R19. Q2 provides further am-
plification before the signal goes to the low pass filter. The low pass RF filter
consisting of C35, L2, C36, L3, and C37 allows us to pass the fundamental
(operating) frequency while rejecting the harmonics. Harmonics are multiples of
the fundamental frequency, and in this case are undesirable in transmission
since they can transmit in critical areas of the RF spectrum.
U2 is the brains of the whole circuit. This micro-controller looks at the settings
of each of the dip switches S3 through S5 one at a time and from these it calcu-
lates the desired frequency. On these switches you add up the closed positions
1, 2, 4, and 8 to make to make any number between 0 and 9. For example clos-
ing position 1 and 8 on S3 (10 MHz switch) is equal to 90 MHz, plus closing 1
and 4 on S4 (1 MHz switch) is equal to 5 MHz, while closing 2 and 1 on S5 (0.1
MHz switch) is equal to 0.3 MHz. This makes the final frequency equal to 95.3
MHz. These switches may be set to any frequency between 88 and 108 MHz.
To set the frequency above 100 MHz, the S3 positions must add up to ten. Any
switch setting greater than 9, with the exception of S3, is invalid and will be
read as 0.
Once this frequency is determined, the information needed to control U1 is sent
serially from U2. This information is a string of binary data, (1's and 0's). In this
way data is sent one bit at a time to U1. The frequency information takes 16
bits, and there are an additional 32 bits sent for the internal control of U1. You

FM25A •6
may think that all this would take a long time but in fact the whole process from
the time you press the program switch until the data is completely sent is less
than 1/100th of a second!
U1 is a phase locked loop (PLL) synthesizer IC. It takes the 6 MHz crystal fre-
quency and divides it by 60 to obtain a stable reference frequency of 100KHz.
How does it know to divide the crystal frequency by 60? That is part of the data
that is sent from U2 each time the program button is pressed or the loop is out
of lock. U1 also takes the output frequency of U3, and divides it by a number,
N. N is the frequency data that was sent by U2. N is always equal to the de-
sired frequency in megahertz times 10.
Using the example from above, a frequency of 95.3 MHz gives an N of 953, so
the frequency from U3 is divided by 953 and then compared with the reference
frequency of 100 KHz. If the desired frequency is less than the reference fre-
quency U1 increases the output voltage on pin 13. This increases the voltage
across diode D21, a varactor diode. As the voltage across the varactor in-
creases, it causes a decrease in capacitance (Increasing reverse bias essen-
tially increases the distance between the capacitor’s plates by increasing the
depletion region in the diode (C = kA/d). The decrease in capacitance causes
an increase in U3’s RF oscillator (fo= 1/[2π(LC)½]), bringing the FM25A’s out-
put frequency back on frequency. If the desired frequency is higher than the ref-
erence, pin 13 is driven low. If the frequency is just right then pin 13 becomes a
high impedance, basically disconnecting it from the circuit so it will cause no
change in the voltage on D21. The voltage changes on pin 13 are filtered by
R8, C16, R10, and C19 to provide a steady, noise free tuning voltage for D21.
In this way the output frequency of U3 is "locked" to that desired by U2. When
the frequency is locked, U1 will cause led D1 to be brightly lit. If D1 is dim or off
there is a problem and the frequency is not locked. If the frequency starts to
drift for any reason (such as a temperature change) then U1 instantly corrects
the tuning voltage to bring it back to the proper frequency.
Notes on Operation
Any time the unit is turned "on" or the "program" switch is pressed the micro-
controller reads the frequency switches and sends the data to U1.
To change the frequency simply set the switches to the desired frequency and
press "program". The frequency is limited to within 88 to 108 MHz. Any setting
outside this range will be programmed as 88.1 MHz. If you set your frequency
and press program but don’t hear the signal on the frequency you expected try
listening on 88.1 MHz. If the signal is here there is an error in the switch set-
tings or maybe a solder bridge somewhere making the frequency setting inva-
lid.
A switch must be closed to be active. Add up the positions 1, 2, 4, and 8 to
make any number between 0 and 9. 10 -15 are invalid and will be read as 0.

FM25A •7
FM25A PARTS LAYOUT DIAGRAM

FM25A •8
PARTS SUPPLIED WITH FM25A TRANSMITTER KIT
Capacitors
8 .001 µF disc capacitors (marked .001, 102 or 1nF) [C10,C12,C13,C26,
C27,C28,C29,C31]
7 .01 µF disc capacitors (marked .01 or 103 or 10 nF) [C7,C8,C15,C23,
C24,C42,C43]
2 .1 µF disc capacitor (marked .1 or 104 or 100 nF) [C9,C16]
2 .022 uF ceramic capacitors (marked .022 or 223) [C14,C22]
2 5 pF disc capacitors (marked 5 or 5K) [C18,C20]
2 10 pF disc capacitors (marked 10 or 10K) [C30,C32]
2 27 pF disc capacitors (marked 27 or 27K) [C1,C2]
2 47 pF disc capacitors (marked 47) [C35,C37]
1 75 pF disc capacitor (marked 75 or 75K) [C36]
1 220 pF disc capacitor (marked 220 or 221) [C33]
8 10 µF electrolytic capacitors [C3,C6,C11,C17,C19,C21,C25,C34]
1 100 µF electrolytic capacitor [C4]
1 1000 µF electrolytic capacitor [C5]
Resistors
1 100 ohms (brown-black-brown) [R12]
2 220 ohms (red-red-brown) [R14,17]
1 270 ohms (red-violet-brown) [R13]
2 470 ohms (yellow-violet-brown) [R2,R10]
1 1K ohms (brown-black-red) [R31]
3 2.2K ohms (red-red-red) [R9*,R19,R15*]
2 3.3K ohms (orange-orange-red) [R9*,R15*]
3 4.7K ohms (yellow-violet-red) [R21,R28,R30]
9 10K ohms (brown-black-orange) [R1,R4,R5,R18,R20,R23,R24,R25,R26]
3 47K ohms (yellow-violet-orange) [R8,R27,R29]
1 100K ohms (brown-black-yellow) [R7]
1 150K ohms (brown-green-yellow) [R22]
1 10M ohms (brown-black-blue) [R3]
2 1K ohm potentiometer [R11,R16]
1 100K ohm potentiometer [R6]
Semiconductors
1 2N3904 NPN transistor [Q1]
2 2SC2498 or 2570 NPN transistors [Q2,Q3]
12 1N4148 diodes (small glass diodes) [D9 - D20]
1 Varactor diode (transistor shape with two leads, marked MV2105)[D21]
1 LED [D1]
Inductors
1 Shielded can inductor [L1]
2 Pre-wound spring style inductors [L2,L3]

FM25A •9
Hardware, Misc.
1 6 MHz crystal (thin shiny rectangle marked 6.00) [Y1]
1 38 KHz crystal (small silver cylinder with two small leads), taped to
paper. [Y2]
1 78L05 +5 volt voltage regulator [VR1]
1 78L02 +2.6 volt voltage regulator [VR2]
1 7812 +12 volt voltage regulator [VR3]
1 68HC705K1 Microcontroller IC (marked with white sticker) [U2]
1 16-pin socket for U2
1 18-pin socket for U3
1 145170 Serial PLL IC [U1]
1 BA1404 Stereo modulator IC [U3]
1 FM25A printed circuit board
3 RCA-style jacks [J2,J3,J4]
1 DC power jack [J1]
1 Horizontal push-button switch [S1]
1 Small black mini push-button switch [S2]
3 DIP switches (8 pin dip with 4 sliding tabs) [S3,S4,S5]
1 Whip antenna [ANT1]
1 12 volt DC power transformer
1 Two pin jumper and jumper block [JMP1]
Required, not supplied
Shielded stereo audio cables
Line level audio source (such as a tape deck or CD player)
Case and Knob Parts
Top Cover
Bottom Base Tray
4 - Short Phillips Head Screws
2 - Long Phillips Screws
Front and Rear Plastic Panels
Front and Rear Labels
4 - Rubber Feet
Appropriate Knobs for Kit
Required Tools
Small Phillips Head Screwdriver
Pen or Pencil
Sharp hobby knife or hand held paper punch
Ruler at least 6 inches long
Optional Tools
5/16” Drill

FM25A •10
RAMSEY "LEARN-AS-YOU-BUILD" ASSEMBLY STRATEGY
As you can see in examining the circuit board and components, there is a bit
more to this kit than just soldering a few parts. So that you don't spend extra
time "troubleshooting" instead of getting on the air, we strongly recommend that
you follow the assembly strategy and step-by-step procedures we provide.
Our strategy in installing parts on our PC board is to install the larger and more
obvious parts such as the connectors and controls. These parts will then act as
"landmarks" so that each additional device installed is seen in relationship to
them, or to others previously installed.
In addition, we'll discuss the purpose of most of the components or groups of
components as we go along. If you are new to the idea of building your own
transmitter, perhaps our explanations will help you understand and learn as we
go along. The assembly sequence will follow the circuit flow from input jacks to
antenna as faithfully as is practical, as part of Ramsey's "Learn-As-You-Build"
kit assembly philosophy.
FM25A ASSEMBLY
Use the boxes to check off your progress.
Check all received parts against the Parts list on page 8 and 9. The parts list
describes the various markings that may be found on the kit parts.
Since you may appreciate some "warm-up" soldering practice as well as a
chance to put some "landmarks" on the FM25A PC board, we'll first install
some "hardware" components, to make the up-down, left-right orientation of the
PC board as clear as possible.
In ALL the following instruction steps, our word "INSTALL" means this:
•Insert the part, oriented correctly, into its correct holes in the PC board.
•If helpful, gently BEND the part's wire leads or tabs to hold it in place, with
the body of the part snugly against the top "component side" of the PC
board.
•SOLDER ALL wires or pins of the part, whether the two wires of a resistor
or all pins of an IC socket.
•Nip or "trim" all excess wires extending beyond each solder connection,
taking care that wire trimmings do not become lodged in PC board solder
connections.
Enough said. . . Let's get building!

FM25A •11
1. Install S1, the DPDT horizontal push-button switch. It fits correctly only
one way. Ensure that the white plastic switch extends out over the edge of
the printed circuit board. Solder all six pins.
2. Install S2, the small black mini push-button switch. Solder all 4 or 2 pins
depending on the switch provided. This switch snaps into place and sits
flush with the top of the board.
3. Install J2, the Right Channel input jack. Solder all 4 points of the jack se-
curely.
4. Install J3, the Left Channel input jack. Solder all 4 points of the jack.
5. Install J4, the RF output (antenna) jack. Solder all 4 points of the jack se-
curely.
6. Install J1, the 2.5 mm phone jack (external DC input). Solder all 3 pins.
7. Install the 16-pin DIP socket supplied with your kit. There is no right or
wrong direction to this socket, but the U2 IC itself certainly needs to be in-
serted correctly (later). Taking care of this socket early in the project will
help you with positioning additional parts and will fine-tune your soldering
technique. Before soldering, make sure the socket body is flush against the
PC board, and that all 16 pins have been inserted. Bend the two corner
pins to hold it in place while you are soldering. After soldering make sure
that it is flat on the board. Solder all 16 pins and then CAREFULLY check to
ensure you have not caused any "solder bridges" between pins.
8. Now that the IC socket has your prime attention, carefully insert the mi-
crocontroller IC, U2 (marked with a white sticker) into the socket, taking
gentle care that ALL 16 pins get into their proper holes. The orientation of
the notched end, as shown on the Parts Layout Diagram is critically impor-
tant.
9. Install U1 (marked 145170). This IC does not have a socket. Don’t be
afraid of heating it too much. Bend the two corner pins as you did with the
socket for U2. Be sure that all of the pins are through the board and not
bent over on top. Place the notched end as shown on the PC board.
10. Install the 18-pin DIP socket as you did the 16-pin socket in step 7. Be
sure that all 18 pins are through the board before soldering. Install U3
(marked BA1404) in the same manner that you did U2. Watch the place-
ment of the notched end.
Progress Note: The preceding steps have secured a sufficient number of com-
ponents to your PC board to make general orientation around the board much
clearer for installing additional parts. Further parts will be installed in six phases
or groupings.

FM25A •12
Audio Input Circuits
We encourage you to refer to the schematic diagram and learn the functions of
the following parts. If you wish, insert the parts groupings as one operation,
then solder and nip all connections.
Most of the resistors will be installed in a stand up fashion. That is, one lead will
go straight into the board while the other is bent around to go back down into
the other hole. The straight lead goes into the hole with the circle around it.
1a. Install R16, one of the small yellow trimmer potentiometers marked
"102". Don't confuse it with R6 which is marked "104".
2a. Install R11, the other yellow trimmer marked "102". These two trimmers
will allow you to adjust the input level.
3a. Install R30, a 4.7K ohm resistor (yellow-violet-red).
4a. Install R29, a 47K ohm resistor (yellow-violet-orange).
5a. Install R27, a 47K ohm resistor (yellow-violet-orange).
6a. Install R28, a 4.7K ohm resistor (yellow-violet-red).
For use in Europe, use 2.2K ohm resistors in steps 7a and 8a. These resistors
set the pre-emphasis characteristic for the FM25A. Pre-emphasis is a tech-
nique used in FM transmitters to increase the high frequency signal to noise
ratio.
7a. Install R9, 3.3K (orange-orange-red).
8a. Install R15, 3.3K (orange-orange-red).
9a. Install C14, .022 µF disc capacitor (marked .022 or 223).
10a. Install C22, .022 µF disc capacitor (marked .022 or 223).
Electrolytic capacitors have a right and wrong way to be installed. Usually, ca-
pacitors have a wide black stripe which indicates their negative lead and the PC
board or Parts Layout Diagram will show the positive side of the capacitor's in-
stallation hole. Be sure to place the ( + ) capacitor lead into the PC board ( + )
hole and the ( - ) lead into the ( - ) hole. Observe correct polarity when installing
the following three electrolytic capacitors:
11a. Install C17, 10 µF electrolytic capacitor. Don’t forget... the proper ori-
entation is noted on the PC board or Parts Layout Diagram.
12a. Install C21, 10 µF electrolytic capacitor. Observe correct polarity.
13a. Install C25, 10 µF electrolytic capacitor. Remember correct polarity.
14a. Install C42, a .01µF ceramic capacitor (marked .01, 103 or 10n).
15a. Install C43, a .01µF ceramic capacitor (marked .01, 103, or 10n).

FM25A •13
Power Supply
1b. Install C5, the large 470 - 1000 µF electrolytic capacitor. Be sure to po-
sition for correct polarity.
2b. Install VR3, the 7812 12 volt regulator. Make sure the metal tab is ori-
ented as shown in the parts layout. The text side of the component should
be facing S1.
3b. Install VR1, the 78L05 5 volt regulator. Make sure the flat side of the
component is facing the same direction as shown.
4b. Install C7, a .01µF ceramic capacitor (marked .01, 103, or 10n).
5b. Install C3, a 10µF electrolytic capacitor. Pay attention to polarity!
6b. Install C4, a 100µF electrolytic capacitor. Watch polarity.
We’ll save the other power supply components for later in favor of ease of as-
sembly.
Transmitter
1c. Install R6, the small yellow trimmer potentiometer marked "104".
2c. Install R21, 4.7K (yellow-violet-red).
3c. Install R22, 150K (brown-green-yellow).
4c. Install C33, 220 pF disc capacitor (marked 220 or 221 or 220K).
5c. Install C34, 10µF electrolytic capacitor. Observe polarity.
6c. Install C28, .001 µF disc capacitor (marked .001 or 102).
7c. Install C12, a .001 µF disc capacitor (marked .001 or 102).
8c. Install C18, 5 pF disc capacitor (marked 5 or 5K).
9c. Install C20, 5 pF disc capacitor (marked 5 or 5K).
10c. Install C15, .01 µF disc capacitor (marked .01 or 103 or 10 nF).
11c. Install R7, 100K ohms (brown-black-yellow).
12c. Install D21, it looks like a transistor with only two leads (marked
MV2105). Notice the placement of the flat side, it should face L1.
13c. Install R10, 470 ohms (yellow-violet-brown). The outside lead on this
resistor is TP1, Test Point 1. You will be measuring voltage at this point.
14c. Install C19, 10 µF electrolytic capacitor. Observe polarity.
15c. Install shielded slug tuned coil L1. The larger tabs secure the shield
can to the ground foil connections, while the two thin leads are the coil con-
nections. If you find that the coil lead wires do not line up with the PC board
holes, simply "pull" the coil insert from the shield can, rotate it a quarter-turn

FM25A •14
and insert back into the can. Make sure L1 is squarely against the top of
the PC board before soldering. Solid installation of L1 is essential to the fre-
quency stability of your transmitter.
16c. Install R8, 47K ohms (yellow-violet-orange).
17c. Install C16, .1 µF disc capacitor (marked 104 or .1).
18c. Install VR2, the 78L02 2.6 volt regulator (TO-92 transistor style pack-
age). Make sure the flat side is oriented the same way as shown in the
parts layout diagram.
19c. Install C11, a 10µF electrolytic capacitor. Orientation!
20c. Install C10, a .001 ceramic capacitor (marked .001 or 102 or 1 nF).
21c. Install R13, 270 ohms (red-violet-brown).
22c. Install C31, .001 µF (marked .001 or 102 or 1 nF).
23c. Install C30, 10 pF disc capacitor (marked 10 or 10K).
24c. Install C32, 10 pF disc capacitor (marked 10 or 10K).
25c. Install C13, a .001µF ceramic capacitor (marked .001 or 102 or 1 nF).
26c. Install Y2, the small silver "can" crystal. Be especially careful when in-
stalling this part as its leads are very small. You may elect to put a small
"dab" of glue on the part to relieve stress and to keep it firmly attached to
the PC board, or use a piece of clipped off lead and form a loop to go
around the crystal and through the holes in the board.
27c. Install JMP1, a 2 pin header. A jumper block will be installed on this to
enable stereo transmission.
RF Amplifier
1d. Install Q3, the transistor marked C2498 (or 2570). The flat side must be
placed as shown on the PC board, facing away from R20. Mount Q3 as
close to the board as possible without forcing it.
2d. Install R20, 10K ohms (brown-black-orange).
3d. JMP2 is the high power jumper, made from a spare piece of component
lead. For normal operation in the U.S., jumper 2 should not be installed
since high power could put you in violation of FCC rules - better to play it
safe initially and see what your range is before you go hog wild.
4d. Install R17, 220 ohms (red-red-brown).
5d. Install C24, .01 µF (marked .01 or 103 or 10 nF).
6d. Install C29, .001 µF disc capacitor (marked .001 or 102 or 1 nF).
7d. Install R19, 2.2K ohms (red-red-red).

FM25A •15
8d. Install Q2, the other transistor marked C2498 (or 2570). Again watch
the placement of the flat side and mount as flush as possible.
9d. Install R18, 10K ohms (brown-black-orange).
10d. Install R14, 220 ohms (red-red-brown).
11d. Install C23, .01 µF (marked .01 or 103 or 10 nF).
RF Lowpass Filter
1e. Install C35, a 47pF ceramic capacitor (marked 47 or 47K).
2e. Install C27, a .001µF ceramic capacitor (marked .001, or 102). This is a
coupling capacitor for preventing DC from being present on the antenna or
J4, the external antenna jack.
3e. Install L2, one of the small pre-wound spring style inductors.
4e. Install L3, the other small pre-wound spring style inductor.
5e. Install C36, a 75pF ceramic capacitor (marked 75 or 75K).
6e. Install C37, a 47pF ceramic capacitor (marked 47 or 47K).
Synthesizer
1f. Install C26, .001 µF disc capacitor (marked .001 or 102 or 1 nF).
2f. Install R12, 100 ohms (brown-black-brown). Note this part is a laydown.
3f. Install R23, 10K ohms (brown-black-orange).
4f. Install R24, 10K ohms (brown-black-orange).
5f. Install R25, 10K ohms (brown-black-orange).
6f. Install R26, 10K ohms (brown-black-orange).
7f. Install D9 - D20, small glass diodes. The banded ends go toward the
inside of the board, as shown on the silkscreen.
8f. Install S3, a four posistion DIP switch. The DIP switches are used for
programming the exact transmitting frequency of your transmitter.
9f. Install S4 and S5 in the same manner.
10f. Install Y1, the 6.00 MHz crystal (marked 6.00)
11f. Install R3, 10M ohm (brown-black-blue).
12f. Install C1, 27 pF disc capacitor (marked 27 or 27K).
13f. Install C2, 27 pF disc capacitor (marked 27 or 27K).
14f. Install C9, .1 µF disc capacitor (marked 104 or .1 or 100 nF).
15f. Install R5, 10K ohms (brown-black-orange).

FM25A •16
16f. Install R31, a 1K ohm resistor (brown-black-red). Notice this resistor is a
laydown component.
17f. Install C8, .01 µF disc capacitor (marked 103 or .01 or 10 nF).
18f. Install R1, 10K ohms (brown-black-orange).
19f. Install Q1, the transistor marked 2N3904. The flat side must be placed as
shown on the PC board, facing C6. Mount it as close to the board as possible
without forcing it.
20f. Install C6, 10 µF electrolytic capacitor. Check polarity!
21f. Install R2, 470 ohms (yellow-violet-brown).
22f. Install R4, 10K ohms (brown-black-orange).
23f. Install D1, the LED. The flat side goes toward switch S1. Leave it stand-
ing about 1/2 an inch off of the board when soldering. After soldering bend it
over to a 90º angle at it’s mid point so that it faces the outside of the board.
D1 indicates that the frequency is locked.
ASSEMBLY INSTRUCTIONS FOR CUSTOM CASE
The enclosure is a key element to the overall pride you will have upon completing
your Ramsey kit. The enclosure will show how you were able to “build from
scratch” a commercial piece of high-tech electronics. For some of us, the enclo-
sure will also hide a number of “not-so-pretty” assembly mistakes. Once the kit is
enclosed, your friends will never know that you were new to soldering. Finally, the
enclosure will protect your electronics from many possible causes of damage so
that you can receive years worth of enjoyment using, talking about, and remem-
bering the fun you had building your kit. In short, TAKE YOUR TIME when as-
sembling the enclosure. This is the part that you and your friends will look at and
admire for years!
1. Lay the front and rear plastic plates over their corresponding labels.
2. Outline the holes that you need to cut using a pen or pencil.
3. Use your hobby knife or paper punch to cut the labels to their outlines.
4. Peel off the sticker backing on each sticker and place the labels over the
plastic plates. Make sure that they are aligned correctly before allowing them
to touch the plastic plates. Believe me, when they stick the first time, they
really stick!
5. Insert the board into the case with the knobs and LED extending through
the holes in the front panel.
6. Raise the rear portion of the PC board and extend the jacks through the
rear plastic plate. Insert the plate into the grooves on the base tray.
7. Secure the PC board to the bottom base tray with 4 short Phillips head
screws.

FM25A •17
CHOOSING AN OPERATING FREQUENCY
(A) It really is NOT sufficient to just "check" the FM band for an empty fre-
quency, using the FM portable radio closest at hand. It is your responsibility to
carefully research what FM stations can be listened to with a good system
within the transmitting range of your FM25A. This is especially important in the
low end of the FM broadcast band (88-92 MHz), where there are numerous me-
dium power National Public Radio stations. You may not be aware of these sta-
tions but your neighbors may be receiving them, using a good receiver and out-
door antenna. Interfering with such reception is a direct violation of federal law.
The most reliable way of finding a truly open frequency on the FM band is to
check the band with a very good FM receiving system using an external an-
tenna. If you do not have access to such a radio, most modern car radios (with
exterior antenna) are very sensitive and usable to help you know what stations
your neighbors really can be receiving on a particular frequency.
(B) In choosing an operating frequency, remember that most "digital-tuning" re-
ceivers, whether portable, mobile or hi-fi, are designed to tune in 200 KHz in-
crements and therefore might not receive well a signal operating between these
pre-tuned standard broadcasting frequencies. In order to comply with Part 15 of
FCC regulations, it is your responsibility to determine carefully that your opera-
tion will not cause interference to broadcast reception. Please study Appendix
A of this manual before using your FM25A.
ADJUSTING YOUR FM25A TRANSMITTER
Keep all tests very brief until you have carefully chosen an open operating fre-
quency in the FM broadcast band.
1. Transmitting Frequency
After finding a suitable "open" frequency in the 88-108 MHz FM band, program
DIP switches S3, S4, and S5 for the frequency desired and then momentarily
depress the ‘Program’ button. The DIP switches are programmed in BCD, a
form of binary that is very easy to use. Look at your circuit board or parts layout
diagram, you’ll see the numbers 1, 2, 4, 8 printed next to each DIP switch. On
these switches you add up the closed positions to make to make any number
between 0 and 9. For example: closing position 1 and 8 on S3 (10 MHz switch)
is equal to 90 MHz (1+8=9, 9x10 MHz = 90 MHz), closing 2 and 4 on S4 (1
MHz switch) is equal to 6 MHz (2+4=6, 6x1 MHz = 6 MHz), closing 4, 2 and 1
on S5 (0.1 MHz switch) is equal to 0.7 MHz (4+2+1=7, 7x .1 MHz = .7 MHz).
This makes the final frequency equal to 96.7 MHz (90+6+.7=96.7). These
switches may be set to any frequency between 88 and 108 MHz. To set the fre-
quency above 100 MHz, the S3 positions must add up to ten, you would close 8
and 2 (8+2=10). Any switch setting greater than 9, with the exception of S3, is
invalid and will be read as 0.

FM25A •18
After setting your DIP switches, recheck your ‘math’, depress the Program but-
ton and listen on a nearby FM radio for the carrier frequency. No audio input is
needed to make this first adjustment, you can simply listen for a "quieting" in the
normal background noise "hiss." Remember that if you have adjusted the
FM25A outside of 88 MHz to 108 MHz range it will default to 88.1 MHz. This
means that if you can’t hear anything on the radio near the frequency you are
trying to transmit at, go to 88.1 MHz and see if it is there.
If you still can’t receive anything then check the voltage at TP1. TP1 is located
at the lead of R10 that is closest to the outside of the board. With an operating
frequency of 107.9 MHz programmed into the DIP switches, there should be
around 4.5 volts at TP1. If there isn’t, adjust L1 until you read 4.5 volts. With a
frequency of 88.1 MHz programmed into the DIP switches, you should read
about 0.5 volts at TP1. If you can’t get either reading, see the troubleshooting
section.
2. Audio Connection
Adjust both Left and Right level potentiometers (R11, R16) to about half otf their
rotation. This is a good starting position. The best audio input for both testing
and general operation are the stereo "line-level outputs" of a cassette deck or
CD player. Most stereo systems have a variety of auxiliary output jacks of which
one or more are line-level outputs.
Hooking up an audio source to your FM25A is really quite simple. However,
there are some general rules:
•A terribly distorted sound is a sign of too much audio level. Simply rotate the
level potentiometers, R11 and R16, CCW to reduce the level. Make sure
you rotate each one about the same amount to maintain proper stereo bal-
ance.
•Stereo LP turntables are low-level output and will require the use of a pre-
amplifier for proper audio input to the FM25A.
•NEVER connect the FM25A audio inputs to speaker outputs of a high
power stereo system; such a connection will destroy the IC chip.
•Consider using a mike mixer for professional "radio station" sound. It will
allow you to easily fade, mix and switch between various audio sources.
3. Stereo balance
Adjust R6 for correct stereo balance. If you are transmitting in stereo adjust R6
to its center position. The better your ear for music and your understanding of
quality audio devices and interconnections, the better will be the performance of
your FM25A.

FM25A •19
USING THE FM25A WITHIN THE HOME
A most practical use for the FM25A would be to connect it to the main stereo
system within a large home so that whatever is playing on the main system can
also be tuned-in on portable FM radios in other rooms, the garage or in the
yard.
This connection consists of using shielded audio cables to connect the auxiliary
"line audio" output of your cassette deck, CD player or other stereo device to
the audio inputs of the FM25A. Consult the literature that came with your stereo
equipment.
Even if you intend use of the FM25A for your own home and family, it is still
your responsibility, in accordance with Part 15 of the FCC Rules, to ensure that
this operation does not cause interference to your neighbors.
EXPERIMENTAL "BROADCASTING" PROJECTS
To use the FM25A successfully as a "broadcasting" service to interested listen-
ers in a school or immediate neighborhood, most of your effort will be concen-
trated on smoothly "managing" or mixing the audio signals fed into the transmit-
ter input. Operation of the transmitter itself consists simply of the following:
1. Correct construction and adjustment.
2. Carefully checking for an open frequency between 88-108 MHz in accor-
dance with FCC Rules, Part 15.
3. Setting up a suitable antenna.
4. Connecting the audio source to the Left and Right input jacks.
5. Turning on the transmitter while you intend to be "on the air" and turning it
off when you are finished.
Explaining how to build a simple audio "mixing" panel or box, which is at the
heart of any studio operation, is beyond the purpose of this instruction manual.
We do recommend that you build a mixing system, for several reasons:
1. Parts to do so are readily available at Radio Shack.
2. It would be an excellent class, family or Scout project.
3. Commercially-made stereo mixing consoles, while much less expensive to-
day than a decade ago, will cost much more than did your FM25A kit!
The more home-built your complete setup, the more it is in conformity with the
spirit of FCC Part 15 regulations.

FM25A •20
RAMSEY’S HAS MIXER (MX) KITS!
You can greatly add to the versatility and professionalism of your transmitting
station by adding one of our audio mixers and processor. Mixers allow you to
smoothly ‘blend’ from one audio source to another just like the commercial sta-
tions do. Our STC1, Stereo Transmitter Companion provides full audio process-
ing in one easy package. The STC1 allows you to ‘sweeten’ the audio for more
impact and punch. Plus, it has brick-wall low pass filtering on the stereo inputs
to prevent that bothersome ‘swishing’ and squealing sound that is often found
when using lower quality CD players as the audio source! Check out our free
catalogue for all the details.
ANTENNA IDEAS
A simple, yet very effective, antenna for the FM25A consists of a "dipole", set
up either horizontally or vertically, and connected to the transmitter output jack
through a few feet of coaxial cable (either RG-58, RG-59 or miniature RG-174,
available at Radio Shack and other sources). Correct dipole lengths for major
sections of the 88-108 MHz band are:
88 MHz, each side: 2.7 feet; 5.4 feet total
98 MHz, each side: 2.4 feet; 4.8 feet total
108 MHz, each side: 2.2 feet; 4.4 feet total
You can see that there is not a great difference in antenna length from 88 to
107 MHz. Some antenna designers have the view that an "approximate" dipole
such as 2.5 ft. on a side will do fine, while others believe it is worth the effort to
calculate the length for your exact frequency, using the simple formula of
Length (of one side, in feet) = 234/Frequency in MHz.
If the dipole is installed vertically, the end connected to the center conductor of
the coax should be the upper (higher) end. If young children will be around the
set-up, a flexible wire antenna is preferable, rather than rigid tubing.
A "ground plane" antenna can be quite effective. A ground plane consists of
one vertical element, the same length as one side of a dipole, connected to the
center conductor of the coax. Four "radials" are connected to the shielded side
of the coax at a 90 to 135 degree angle to the vertical element. The dipole for-
mula is also used to calculate the length of the radial; since radials should be
slightly longer than the main element, use 240 rather than 234 in your calcula-
tions.
If you are equipped to make the field strength measurements required by Part
15 FCC rules, and if you think it would be best to aim or "focus" your signal in a
narrower direction, you can consult an antenna handbook and design a suitable
gain antenna. See Appendix A concerning FCC field strength limitations. An
FM- VHF TV receiving antenna could be modified for such a purpose.