Ramsey Electronics FM25B User guide

FM25B •1
Ramsey Electronics Model No. FM25B
Own and operate your own FM Stereo broadcast station. The
FM25B has an exceptional synthesized transmission range and
improved audio quality that puts your favorite radio station to
shame.
•Synthesized 88 to 108 MHz for no frequency drift!
•No annoying Hum with even better stereo separation than the
original!
•New design features ‘Line In’ and ‘Loop Out’ 1/8” Stereo jacks!
•‘F’ style RF output connector for easy connection to an external
antenna
•Fully adjustable RF output level for custom coverage capabilities!
•Kit includes case, AC adapter, 1/8” Stereo to RCA patch cable, and
whip antenna
•Great for schools, health clubs, realtors… or your back yard!
•The ideal campus or school radio station
•Clear, concise instructions guide you step-by-step to a finished
product that works FIRST time.
SYNTHESIZED FM
STEREO TRANSMITTER

FM25B •2
PARTIAL LIST OF AVAILABLE KITS
RAMSEY TRANSMITTER KITS
• FM25B FM Stereo Transmitter
• AM1, AM25 AM Transmitters
• 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
• DDF1 Doppler Direction Finder
• 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. FM25B Rev 1.2a
First printing: November 2001
COPYRIGHT 2001 by Ramsey Electronics, Inc. 590 Fishers Station Drive, 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.

FM25B •3
SYNTHESIZED FM
STEREO TRANSMITTER
KIT
Ramsey Publication No. MFM25B
Price $5.00
TABLE OF CONTENTS
Introduction ...................................... 4
Circuit Description............................ 5
Parts Layout Diagram ...................... 9
FM25B Parts List ............................. 10
FM25B Assembly ............................ 12
Custom Case Assembly................... 20
Choosing an Operating Frequency.. 20
Adjusting .......................................... 21
Home Use ........................................ 23
Projects ............................................ 23
Antenna Ideas.................................. 24
Troubleshooting ............................... 25
FCC Rules and Information ............. 26
Understanding Field Strength .......... 29
Summary.......................................... 30
Schematic Diagram.......................... 35
Warranty .......................................... 35
KIT ASSEMBLY
AND INSTRUCTION MANUAL FOR
RAMSEY ELECTRONICS, INC.
590 Fishers Station Drive
Victor, New York 14564
Phone [phone]
Fax [phone]
www.ramseykits.com

FM25B •4
INTRODUCTION
The Ramsey FM25B 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 FM25B’s low-power broad-
casting capability and other practical uses can be fun and interesting for people
of all ages, but the FM25B 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 FM25B's capabilities in accordance with the law.
Typical uses for the FM25B 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 FM25B 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 21st cen-
tury global community, we realized that our FM25B low-power Synthesized FM
Stereo Transmitter Kit was certain to attract worldwide interest among hobby-
ists, students and "pioneers." While the use of the FM25B may need to be lim-
ited 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 sim-
ple, 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, consider
this: many other FM25B's just like yours are faithfully relaying news and infor-
mation to listeners in remote areas around the world. The FM25B is most defi-
nitely not a toy!

FM25B •5
CIRCUIT DESCRIPTION
We will begin by talking about the power supply of your new FM25B. While a
DC power source is provided with the kit, its DC output isn’t ‘clean’ enough to
provide us with the low-noise, stable supply that we would like for good audio
quality. We wouldn’t want our FM transmitter ‘Humming’ with the music now
would we! I mean… it should know the words!
Special care has been taken to filter the input DC signal to maximize our work-
ing voltage while still offering a clean, stable supply. Take a look at the sche-
matic as we cover the kit’s circuit description. Right off the bat our input DC
voltage is channeled through an RF filter network composed of bypass caps
C35 & C36 and RF chokes L3 & L5. This filter isolates the plus and minus sup-
ply feeds coming from the wall transformer and helps to remove any unwanted
RF that might be coupled into your transmitter. The large electrolytic capacitor
(C32) that follows the RF filter stores energy so instantaneous peaks in de-
mand for power do not cause dips in the supply voltage.
The final parts of the FM25B’s power supply section are composed of a 12 Volt
ripple filter and a 5 Volt regulator to obtain a clean well-filtered power source.
The capacitive multiplier formed by Q1, R4, and C4 together make a handy low
loss ripple filter by in effect multiplying C4’s capacitive value by the DC current
gain of Q1. This increases C4’s ripple filtering capability to roughly that of a
10,000uF capacitor without the typical 2 to 3 volt overhead loss you would get
with a 12 Volt regulator! Now that we have a clean power source, let’s dive into
the rest of the kit!
The custom FM stereo IC (U3) is the heart of the FM25B. U3 is a microproces-
sor controlled FM stereo generator with lots of built-in performance features.
The surrounding support circuitry configures U3 for proper operation under a
variety of conditions.
Potentiometers R27 and R28 allow for adjustment of the input audio levels to
match a wide sampling of audio sources for the best possible sound.
Capacitors C24 and C27 set the pre-emphasis characteristics for that of the re-
gion you intend to operate in (75 µs for USA, 50 µs for Europe).
Capacitors C22 and C25 are part of the 15kHz audio low-pass filter (LPF) that
is internal to the chip. The Bessel filter has flat delay characteristics and re-
moves the higher frequency elements that would interfere with the stereo sig-
nal.
Capacitor C29 acts as a ripple filter for U3’s internal audio reference voltage.
This reduces any chance of audio distortion due to internal demands on its
power buss.

FM25B •6
Crystal X1 along with C9 and C12 form the timing reference of the transmitter.
The signal from the crystal network is divided down and used to generate the
stereo components of the transmitted signal as well as by the chip’s Phase
Lock Loop (PLL) circuitry to provide ‘rock solid’ frequency stability. More on the
PLL side of U3 in a moment.
Inductor L1, C13, and D8 form the Voltage Controlled Oscillator (VCO) stage.
The RF oscillator is a tuned circuit formed by these elements that sets the base
frequency of our RF transmitter. The DC voltage applied to varactor D8 causes
its capacitive value to change up or down as needed. This allows us to vary the
frequency output of the circuit up or down simply by changing the DC voltage
feed to D8.
Transistors Q2 and Q3 work in conjunction with C6, C8, and R17 to filter the
PLL correction pulses coming from U3 pin 7. The inverting LPF they form pass
a DC voltage component, as a function of the correction pulses, to the VCO to
vary the final operating frequency of the RF oscillator.
Varactor D7 is used to ‘wiggle’ the VCO voltage in accordance with the applied
audio signal. The resistor network around D7 serves a multitude of functions. In
a nutshell, D7’s capacitive value changes slightly with a given applied AC signal
(the processed composite audio signal) causing the VCO to deviate from its
base frequency. This is how we get our FM (Frequency Modulation) signal. By
using a dual varactor diode modulation scheme, we can achieve a very tight
deviation tolerance across the entire FM band and improve the overall perform-
ance of the system greatly!
Resistor R25 varies the combined modulated RF signal from U3 before its final
amplification stage. R25 gives the user full control to vary the final output level
to match their custom applications and coverage.
Amplifier U4 (the Gal-5 is phenomenally rated from DC to 4GHz operation!)
boosts the output level of U3 in one super clean stage without introducing har-
monics or other spurious signals even before the signal goes through a low
pass filter!
The low pass RF filter consisting of C34, L2, C37, L4, and C38 allows the fun-
damental (operating) frequency to pass through while rejecting any unwanted
harmonics. Harmonics are multiples of the desired fundamental frequency and
in this case, they can cause unwanted emissions in critical areas of the RF
spectrum. A cleaner RF output means happier neighbors and the ‘piece of
mind’ that you are not causing unwanted interference.
U1 is an LM358 opamp. This little work-horse can be found in use for all types
of different applications. Our application for this circuit gives the user feedback
about the operating status of the unit. The combined stages used with their
support components act as a dual purpose pulse detector. The opamp (U1) cir-

FM25B •7
cuit tells the user when their transmitter has locked on frequency and when an
applied audio signal is being transmitted. U1:A takes the PLL correction
pulses and amplifies them. The feedback resistor network formed by R6 and
R8 set the gain of the amplifier (remember that classic formula ‘G=1+Rf/Ri’ =
1+1 MegΩ/ 10KΩ). The output of U1:A is then rectified by D2 and the peak
voltage is stored by C2. R2 is used as a discharge ‘Bleeder’ resistor so that
the sampled peak-hold voltage on C2 will vary up and down fast enough to
give the reliable dual indication features we need. U1:B monitors the peak
voltage stored on C2 and turns on or off the D1 LED Frequency Lock / Audio
Modulation indicator. When a large number of correction pulses are present
on the PLL output, due to the user changing frequency or the unit become
unlocked, the resulting voltage on C2 will be high enough on the inverting in-
put of U1:B to swing the output Low. When the output of U1:B is low, the LED
(D1) is turned off indicating that the unit is unlocked. The similar scenario ap-
plies when audio is being transmitted. The audio being transmitted varies the
frequency up and down (FM – Frequency Modulation) in accordance with the
music. The PLL tries to correct for these deviations from the center transmis-
sion frequency by sending out short pulses. The positive feedback provided by
R9 adds a bit of hysteresis to U1:B’s response by changing the crossover trig-
ger point switching the output back and forth from High to Low. The resulting
effect is to smooth the response of the LED (D1) indicator making it more
pleasant to the eye. The detection circuit formed by U1 will indicate when au-
dio is being transmitted by flashing D1 along with the music. Not bad…two for
the price of one!
U2 acts as the brains of the whole circuit. This microcontroller looks at the set-
tings of each of the dip switches S1 through S3 one at a time and from these it
calculates the desired frequency. The switches allow you add up the closed
(down) positions 1, 2, 4, and 8 to make any number between 0 and 9. For ex-
ample closing position 1 and 8 on S3 (10 MHz switch) is equal to 90 MHz.
Closing 1 and 4 on S2 (1 MHz switch) is equal to 5 MHz. Closing 2 and 1 on
S1 (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 U3 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 U3. The frequency information
takes 10 bits of data along with an additional 6 bits sent for the internal control
and transmission mode (stereo / mono) selection. You may think that all this
would take a long time but in fact the whole process of sending the data takes
less than 1/100th of a second!
U3’s internal phase locked loop (PLL) synthesizer requires a 7.6 MHz refer-

FM25B •8
ence crystal (X1) as we discussed before. All the internal operations of U3 are
truly amazing! The reference signal of X1 is divided by 4 and then again by 19
to obtain a stable reference frequency of 100KHz that will be used to keep our
transmitter on frequency. U3 then internally samples the RF output and di-
vides it by a number (N). N is the frequency data that was sent by U2 and is
always equal to the desired frequency in Megahertz times 10. Using the previ-
ous example, a frequency of 95.3 MHz gives an N of 953. This means the
sampled RF output signal will be divided by 953 by U3 and then compared
with the reference frequency of 100 KHz. If the desired RF output frequency
happens to be too low (lower than the calculated reference frequency), U1
sends a series of controlled pulses close to the chip’s 0 Volt rail from a 1/2
VCC midpoint on pin 7. The inverting LPF (Q2, Q3 in conjunction with C6, C8,
and R17) in turn raises the DC control voltage on D8. As the voltage across
the varactor increases, it causes a decrease in capacitance (Increasing re-
verse bias essentially 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]), bring-
ing the FM25B’s output frequency back on frequency to match that of the ref-
erence. If the desired frequency is higher than the reference, U1 does just the
reverse and sends a series of controlled pulses close to the chip’s 5 Volt rail
from a 1/2 VCC midpoint on pin 7. If the frequency is just right then pin 7 floats
at a constant DC level, keeping the VCO voltage constant on D8. In this way
the output frequency of U3 is "locked" to that desired by U2. When the fre-
quency 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 (assuming of course that
no audio is being applied). If the frequency starts to drift for any reason (such
as a temperature change) then U3 instantly corrects the tuning voltage to
bring it back to the proper frequency.

FM25B •9
FM25B PARTS LAYOUT DIAGRAM

FM25B •10
PARTS SUPPLIED WITH FM25B: Note the extra chip capacitors included.
Capacitors
7 .001 µF disc capacitors (marked .001, 102 or 1nF)
[C13,14,18,31,33,35,36]
3 .01 µF disc capacitors (marked .01 or 103 or 10 nF) [C2,5,26]
3 .1 µF disc capacitors (marked .1 or 104 or 100 nF) [C7,21,23]
1 .047 uF ceramic capacitor (marked .047 or 473) [C6]
2 10 pF disc capacitors (marked 10 or 10K) [C11,15]
2 33 pF disc capacitors (marked 33 or 33K) [C9,12]
2 47 pF disc capacitors (marked 47) [C34,38]
1 75 pF disc capacitor (marked 75 or 75K) [C37]
2 100 pF disc capacitors (marked 100 or 101) [C39,40]
2 150 pF disc capacitors (marked 150 or 151) [C22,25]
2 2200 pF disc capacitors (marked 222 or .0022) [C24*27*] see text
2 3300 pF disc capacitors (marked 332 or 0.0033) [C24*,27*] see text
6 10 µF electrolytic capacitors [C1,10,20,28,29,30]
1 47 uF electrolytic capacitor [C8]
2 100 µF electrolytic capacitor [C3,4]
1 1000 µF electrolytic capacitor [C32]
4 .1 uF SMT capacitors [C16,17] Preinstalled!
Resistors
1 10 ohm (brown-black-black) [R1]
1 100 ohms (brown-black-brown) [R17]
1 120 ohms, larger, 1 watt (brown-red-brown) [R26]
1 270 ohms (red-violet-brown) [R15]
2 470 ohms (yellow-violet-brown) [R3,4]
2 4.7k ohms (yellow-violet-red) [R9,16]
8 10k ohms (brown-black-orange) [R7,8,10,11,12,14,19,22]
2 22k ohms (red-red-orange) [R5,18]
1 47k ohms (yellow-violet-orange) [R24]
3 100k ohms (brown-black-yellow) [R2,13,20]
1 220k ohms (red-red-yellow) [R23]
1 470k ohms (yellow-violet-yellow) [R21]
1 1M ohms (brown-black-green) [R6]
1 1k ohm trimmer potentiometer [R25]
2 10k ohm trimmer potentiometers [R27,28]
Semiconductors
3 2N3904 NPN transistors [Q1,2,3]
13 1N4148 diodes (small glass diodes) [D2, D3 - 6, D9 - 16]
1 1N4000 series diode (can be any number from 1N4002 to 1N4007
(black with white band) [D17]
2 Varactor diodes (transistor shape, two leads, marked MV2105)[D7,8]
1 LED [D1]

FM25B •11
Inductors
1 Shielded can inductor, 0.18 uH [L1]
2 Pre-wound spring style inductors, 44 nH [L2,4]
2 1 µH axial inductors (brown-black-gold) [L3,5]
Hardware, Misc.
1 PIC 16C505 microcontroller IC (marked with white sticker) [U2]
1 BH1415F Stereo generator IC [U3] Preinstalled!
NOTE: These surface mount parts may be pre-installed on your circuit board.
Check the solder side of the PC board for these parts.
1 7.6 MHz crystal (thin shiny rectangle marked 7.600000) [X1]
1 78L05 +5 volt voltage regulator [VR1]
1 14-pin socket for U2
1 LM358 Low Power Dual Operational Amplifier IC [U1]
1 GAL5 SMT Amplifier IC (surface mount chip with 4 leads, 3 on one
side) [U4] Preinstalled!
1 Two pin jumper and jumper block [J1]
1 ‘F’ type board mount connector [J3]
2 3.5mm stereo jacks [J4,5]
1 2.1mm DC power jack [J2]
1 DPDT push-button switch [S4]
3 DIP switches (8 pin dip with 4 sliding tabs) [S1,2,3]
1 Whip antenna [ANT1]
1 FM25B printed circuit board
1 1/8” stereo to RCA cable
1 AC125 12 volt DC power transformer
Required, not supplied
Line level audio source (such as a tape deck or CD player)
Case and Knob Parts
Top cover with drilled antenna hole
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
Multimeter for voltage adjustment

FM25B •12
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 pro-
vide.
Our strategy in installing parts on our PC board is to install the larger and
more obvious parts such as the ICs and connectors first. These parts will then
act as "landmarks" so that each additional device installed is seen in relation-
ship 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.
FM25B ASSEMBLY
Use the boxes to check off your progress.
Check all received parts against the Parts list on page 10 and 11. The parts
list describes the various markings that may be found on the kit parts.
Important Note! The surface mount parts in your kit have been prein-
stalled for you. Please don’t call the factory for your “missing” parts;
simply flip the PC board over and you’ll find them soldered into place.
Since you may appreciate some "warm-up" soldering practice as well as a
chance to put some "landmarks" on the FM25B 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.

FM25B •13
•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!
We’ll start with the surface mount parts on the “solder” side of the PC board.
That will get the tiniest, most easily lost parts “out of the way” . . . before you
have a chance to sneeze and blow them off the table! However, your PC
board may come with the parts pre-mounted for you, in which case skip ahead
to step 7. If they are not already in place flip the PC board over so that you
can see the side opposite the silkscreen. This is the side that has all the solder
pads and traces on it. Near the middle of the board is where our first part will
go. You’ll need tweezers for the next few parts, and possibly a magnifying lens
or eye loupe to see these tiny components. If you’re over 40, make sure you
have that magnifier! You know what I’m talking about; your arms just don’t get
long enough to read the writing on these babies. (Can you tell I might have
some personal experience with this?!)
1. Locate the BH1415F stereo generator IC, U3, by flipping the PC board
over. Note that it has already been installed for you, as has U4 and the
two surface mount capacitors. Makes things a little easier, doesn’t it?
We’ll install the ICs so that you can get them nice and flat on the board, then
we’ll place some of the larger components as landmarks.
2. Install the socket for U2, the PIC 16C505 microcontroller IC. It doesn’t
matter which way the socket is oriented, as long as the chip is placed in
the socket correctly. We’re installing this part now to make it easier for you
to get it nice and flat on the board. You can bend two of the corner leads
out slightly to hold the socket in place when you flip the board over to sol-
der it in. Be sure to solder all 14 pins.
3. Now insert U2 into the socket. Orient the notched end as shown on
the PC board silkscreen, toward R12. Before you push it down into the
U4J3
C16
C17
U3
PIN 1

FM25B •14
socket check to be sure that none of the pins are bent under or outside of
the socket. When you’re sure the pins are where they belong, press it
down so that it is seated flat within the socket.
4. Install U1, the LM358 dual op-amp. Again, orient the dotted or
notched end as shown on the silkscreen and be sure to solder all 8 pins.
5. Install S4, the DPDT push-button switch. You’ll want this part to be
seated correctly so that the power button moves freely when in the case.
Set it flush to the PC board before soldering all six pins.
6. Install J2, the 2.1mm DC power jack. You can gently bend the leads
to keep this part from falling out of the board when you flip the board over
to solder it.
7. Install J3, the PC mount “F” connector. This is another part that you’ll
want to seat correctly before soldering or it won’t fit through the back
panel properly. Be sure to solder the ground connections well. They may
require more heat but when working with RF, a good ground is essential.
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.
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.
8. Install D17, the 1N4000 series diode (4001 - 4007)(black with white
band). The body of the part should be placed in the hole with the circle
around it and the band or stripe should be up when oriented properly.
9. Install L3, 1µH axial inductor (looks like a fat resistor with brown-
black-gold bands).
10. Install C36, .001 µF disc capacitor (marked .001, 102 or 1nF).
11. Install L5, the other 1µH axial inductor (looks like a fat resistor with
brown-black-gold bands).
12. Install C35, .001 µF disc capacitor (marked .001, 102 or 1nF).
13. Install R28, one of the 10k ohm trimmer potentiometers (marked 103).
There are two of these parts; don’t confuse either one with the 1k trimmer
marked 102. Seat it flat before soldering.
14. Install the other 10k trimmer pot, R27 (marked 103).
15. Install J5, one of the stereo jacks. Proper positioning of this part will
help you later with case-up so be sure it’s where you want it before solder-
ing.

FM25B •15
16. Install J4, the other stereo jack.
Now we’ll install some electrolytic caps. Electrolytic capacitors have a right and
wrong way to be installed. Usually, capacitors 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 installation 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 electrolytic capacitors.
17. Install C32, 1000 µF electrolytic capacitor. Remember polarity!
18. Install C30, 10 µF electrolytic capacitor. The next two parts will be
identical, and will have the same polarity so be careful with this first one.
19. Install C29, another 10 µF electrolytic capacitor.
20. Install C28, yet another 10 µF electrolytic capacitor.
21. Install C40, 100 pF disc capacitor (marked 100 or 101).
22. Install C39, 100 pF disc capacitor (marked 100 or 101).
23. Install C37, 75 pF disc capacitor (marked 75 or 75K).
24. Install C38, 47 pF disc capacitor (marked 47).
25. Install L4, a prewound spring style inductor.
26. Install L2, the other prewound spring style inductor.
27. Install C34, 47 pF disc capacitor (marked 47).
28. Install C33, .001 µF disc capacitor (marked .001, 102 or 1nF).
29. Install C24 and C27, 3300 pF (marked 332 or 0.0033). For use in
Europe, use 2200 pF capacitors (marked 222 or .0022). These capacitors
set the pre-emphasis characteristics for the FM25B. Pre-emphasis is a
technique used in FM transmitters to increase the high frequency signal to
noise ratio.
30. Install C25, 150 pF (marked 150 or 151).
31. Install C22, 150 pF (marked 150 or 151).
32. Install R25, 1k ohm trimmer pot (marked 102). This potentiometer ad-
justs the final RF output.
33. Install C31, .001 µF disc capacitor (marked .001, 102 or 1nF).
34. Install R26, 120 ohms (large, 1 watt, brown-red-brown). You’ll notice
that this part is quite a bit bigger than the other resistors in your kit. This is
a 1 watt resistor and the others are 1/4 watt. That means this resistor can
dissipate 1 watt of power if needed.
35. Install C26, .01 µF disc capacitor (marked .01 or 103 or 10 nF).

FM25B •16
36. Install C23, .1 µF disc capacitor (marked .1 or 104 or 100 nF).
37. You’ll notice a silkscreen position for C19. This part is not used; don’t
install any component at that position.
38. Install C20, 10 µF electrolytic capacitor. This part was changed from
a disc capacitor to improve the low frequency response so you’ll have to fit
the electrolytic in where the disc would have gone. The positive lead
should be placed toward C13 and the negative or striped lead should be
placed toward R25. As with all the other components in this kit, keep the
lead lengths as short as possible.
39. Install C18, .001 µF disc capacitor (marked .001, 102 or 1nF).
40. Install R24, 47k ohms (yellow-violet-orange). Be careful! This part is
easily confused with the 4.7k resistors since the last color band is orange
and the 4.7k is red.
41. Install C21, another .1 µF disc capacitor (marked .1 or 104 or 100 nF).
42. Install R23, 220k ohms (red-red-yellow).
43. Install C15, 10 pF disc capacitor (marked 10 or 10K).
44. Install C13, .001 µF disc capacitor (marked .001, 102 or 1nF).
45. Install D8, MV2105 varactor diode (transistor shape with two leads,
marked MV2105). Follow the PC board silkscreen for proper placement.
46. Install R22, 10k ohms (brown-black-orange).
47. Install R21, 470k ohms (yellow-violet-yellow).
48. Install C11, 10 pF disc capacitor (marked 10 or 10K).
49. Install R20, 100k ohms (brown-black-yellow).
50. Install D7, the other MV2105 varactor diode (transistor shape with two
leads, marked MV2105). Again, be sure you place this part correctly.
51. Install C14, .001 µF disc capacitor (marked .001, 102 or 1nF).
52. Install R19, 10k ohms (brown-black-orange).
53. Install R16, 4.7k ohms (yellow-violet-red).
54. Install C8, 47 µF electrolytic capacitor. Remember to orient this part
as shown on the silkscreen and parts layout diagram. Bad things can hap-
pen when electrolytics are installed backwards!
55. Install R18, 22k ohms (red-red-orange).
56. Install R17, 100 ohms (brown-black-brown).
57. Install R14, 10k ohms (brown-black-orange).

FM25B •17
58. Install C6, .047 uF ceramic capacitor (marked .047 or 473).
59. Install R13, 100k ohms (brown-black-yellow).
60. Install Q3, the transistor marked 2N3904. The flat side must be placed
as shown on the PC board. Mount it as close to the board as possible
without forcing it.
61. Install Q2, another 2N3904 transistor. If you can’t remember how you
just did it, reread the step above!
62. Install L1, the shielded can inductor. It only fits in the board one way;
just be sure to seat it flat on the board before soldering.
63. Install C10, 10 µF electrolytic capacitor. Watch polarity when install-
ing.
64. Install R15, 270 ohms (red-violet-brown).
65. Install R12, 10k ohms (brown-black-orange).
66. Install C7, the last .1 µF disc capacitor (marked .1 or 104 or 100 nF).
67. Install C12, 33 pF ceramic capacitor (marked 33).
68. Install C9, the other 33 pF ceramic capacitor (marked 33).
69. Install X1, 7.6 MHz crystal (thin shiny rectangle marked 7.600000).
The next group of parts are all the same and all must be oriented the same
way. Be careful with this step! One diode installed backwards will make it im-
possible to transmit on some frequencies while having no effect on others.
Makes for interesting troubleshooting! Don’t worry, hardly anyone messes this
one up so we’re sure you’ll have no trouble installing these parts. Do a double
check before soldering.
70. Install diodes D3 through D6, and D9 through D16, all 1N4148 diodes
(small glass diodes). There are 12 installed in this step so you should
have 1 left over when you’re through. Be gentle when bending the leads
down to fit them in the board; these can be fragile. Orient the banded end
as shown on the PC board silkscreen. Solder them all in when you’re sure
they’re oriented correctly.
71. Install S3, one of the 8 pin DIP switches. Place the part so that it is flat
to the board and the switches (little white tabs) are toward the outside of
the circuit board. That will place them next to the numbers running along
the outside of the board and they should make it easier for you to tune
your FM25B. Also, if you decide to cut a hole in the side of the enclosure
so that you can reach the DIP switches without taking the cover off, you’ll
be glad they’re placed that way!
72. In the same way, install S2. As with the other parts, be sure this one is

FM25B •18
flat to the board before soldering.
73. Ok, you know the drill by now. Install S1, the last switch. If you have a
short memory, read the previous two steps. You didn’t think I was going to
type all that again, did you?
We’re getting there, aren’t we? Just a few more parts to install and you’ll be
finished! Take a break if you need to. Stretch your fingers and arms, get the
crick out of your neck and let’s keep going.
74. Install R8, 10k ohms (brown-black-orange).
75. Install R7, also 10k ohms (brown-black-orange).
76. Install R6, 1 Meg ohms (brown-black-green). As you probably already
know, “Meg” is another way of saying “million”. That resistor you just in-
stalled is a million ohms!
77. Install R3, 470 ohms (yellow-violet-brown).
78. Install D1, the red LED. Being a diode, this part has a polarity and
must be installed properly to work. You’ll see that it has a flat side and so
does the PC board silkscreen picture of the diode. The flat side indicates
the negative lead and you’ll see that this lead is shorter than the other one
too. Place the part so that the flat side lines up as shown on the silk-
screen. Leave the leads long; the part should sit up about an inch off the
board so that you can bend it over to fit through the front panel. This LED
indicates frequency lock, as well as the presence of an audio signal.
79. Install C5, .01 µF disc capacitor (marked .01 or 103 or 10 nF).
80. Install R9, 4.7k ohms (yellow-violet-red).
81. Install R5, 22k ohms (red-red-orange).
82. Install R10, 10k ohms (brown-black-orange).
83. Install R2, 100k ohms (brown-black-yellow).
84. Install D2, 1N4148 diode (small glass diode) following the silkscreen
for correct orientation.
85. Install C2, .01 µF disc capacitor (marked .01 or 103 or 10 nF).
86. Install J1, the two pin jumper. This is the stereo jumper and should be
left off if you want to operate in stereo and installed if you want to operate
in mono mode. If operating in stereo, simply place the jumper block on
one of the pins of the jumper so that you don’t lose it. That way you won’t
have to search for it if you ever want to operate in mono.
87. Install R11, 10k ohms (brown-black-orange).
88. Install C3, 100 µF electrolytic capacitor. Watch your orientation.

FM25B •19
89. Install VR1, the 78L05 +5 volt regulator. This part looks like a transis-
tor and like a transistor, it must be place correctly to work. Follow the parts
layout diagram and PC board silkscreen for orientation. Solder all three
pins.
90. Install C1, the last 10 µF electrolytic capacitor. Check polarity before
soldering.
91. Install C4, the other 100 µF electrolytic capacitor. Do I have to remind
you about polarity again?
92. Install R4, 470 ohms (yellow-violet-brown).
93. Install R1, 10 ohms (brown-black-black).
94. Install Q1, the last 2N3904 transistor. Again, be sure it is placed cor-
rectly before soldering.
That’s it! But before you grab the power supply and fire up the FM25B, take a
moment to check over your work. Check to be sure that all the parts are in-
stalled and the polarity sensitive ones are placed properly. Look for solder
joints that could be questionable and need a quick touch-up. Also look for po-
tential solder bridges; the parts layout diagram can help with that. You may
want to hurry and get the kit powered up but anything you find now will save
you a lot more trouble later. Unless you like the smell of smoke and the sound
of things exploding! It’s worth the few extra minutes. Looking over the PC
board is the first step our Ramsey technicians complete when testing units
here at the shop.
ASSEMBLY INSTRUCTIONS FOR CUSTOM CASE
1. Note that the case halves fit together only one way. Be sure you’re in-
stalling the board so that the case halves will fit together and the antenna
hole will be in the right spot. Insert the board into the case with the knobs
and LED extending through the holes in the front panel.
2. 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.
3. Secure the PC board to the bottom base tray with 4 short Phillips head
screws.
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 FM25B. This is especially important in the
low end of the FM broadcast band (88-92 MHz), where there are numerous
medium power National Public Radio stations. You may not be aware of these

FM25B •20
stations but your neighbors may be receiving them, using a good receiver and
outdoor 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"
receivers, whether portable, mobile or hi-fi, are designed to tune in 200 KHz
increments 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 operation will not cause interference to broadcast reception. Please study
Appendix A of this manual before using your FM25B.
ADJUSTING YOUR FM25B 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, pro-
gram DIP switches S3, S2, and S1 for the frequency desired. 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 S2 (1MHz switch) is equal to 6 MHz (2+4=6,
6x1 MHz = 6 MHz), closing 4, 2 and 1 on S1 (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 be-
tween 88 and 108 MHz. To set the frequency above 100 MHz, the S3 posi-
tions must add up to ten. To do this you would close positions 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.
After setting your DIP switches, recheck your ‘math’ and listen on a nearby FM
radio for the carrier frequency. No audio input should be applied while making
this first adjustment. You can simply listen for a "quieting" on an unused chan-
nel instead of the normal background "static" you would normally find. If you
don’t hear anything yet, don’t worry. We need to adjust L1 for the correct tun-
ing voltage at TP1 to ensuring proper operation across the band. TP1 is lo-
cated just off the exposed lead of R21 that is closest to the outside of the