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TUCoPS :: Radio :: fm_10.txt

Building an FM transmitter





                          WB8EOH Gizmo Report
                              (Very Long)

The Ramsey FM-10 is a low power FM stereo transmitter kit that is easy
to build and has many practical applications around the home and
ham/swl shack.  After living with one for several weeks, I could never
go back to life before I acquired this little "freedom machine".  The
audio quality and stability of the signal give the unit many HI FI
type applications, and indeed I am already planning to buy another
unit to patch into the Audio/Video system.

             Egad, you mean you have to BUILD this thing?

When my XYL saw that this package I had spent fifty bucks on consisted
of a couple of polyethylene bags full of plastic bugs and little
metalic button like pieces, she shook her head in dismay.  Though I
tinker with computer and radio gizmos all day long, I will admit that
the past 20 years have seen me become an "appliance operator" more
content to sit back and PLAY than get in there and tear into the
circuitry, much less build something from scratch.  Well, the folks at
Ramsey have honed the art of kitbuilding, and even a klutz like me CAN
have the satisfaction (like in the old Heath ads) to say "I built it
myself".

Constructing the FM-10 was in part a happy trip down memory lane.  In
my distant youth, one of my happiest memories was the construction and
operation of an Allied Radio Knight Kit #83Y706 three tube AM Phono
Oscillator.  This wonderful little gadget paved the way not only for
my interest in Ham radio, but also got me interested in broadcasting,
a field which put bread on the table for many years.  Well in the more
than 30 years between the two kits, things sure have changed.

First right off, don't even think about dragging out the old Weller
soldering gun and the old roll of Kester solder.  First thing I had to
do was borrow a low wattage iron, and get some super thin solder.  The
Lifetime Supply of solder I bought in 1968 (five pounds) was fine for
antenna work, and gobbing up audio connectors, but the old stuff is
actually WIDER than many of the separate connections on the FM-10 PC
board.  EGAD these components are TINY.

Kudos to Ramsey for the way their whole concept.  The manual is very
well written.  My only problem was I did not get the companion booklet
on how to build a kit (the generic HOW TO SOLDER book).  Well I hoped
that the instructions that came with my 1957 Knight Kit still applied,
because I remember them.  Heat the component, don't glob it all over
the place, and try not to melt the PC board.  I was on my own to
develop a technique.  Ramsey even takes this into account by having
you mount some of the larger "landmark" components first, namely three
RCA jacks (left and right audio and the antenna jack).

Now I wanted this thing to work.  I would not be able to face Elaine
if it became necessary to send the completed unworking mess back to
Ramsey for their $18 an hour bail-you-out plan.  So I took absolutely
extrordinary steps.  With a (borrowed) digital meter, I measured every
resistor, even though Ramsey gives the color code for each one in the
manual as you install it.

One of the nicest touches is the GIANT print of the PC board, upon
which you place all the components in the same place they will go in
the final kit.  I did this with most of the components, except for the
multitude of .01 capacitors, which I left neatly together (Ramsey
uses a masking tape type type medium for keeping similar value
components together).

The scariest moment for me was soldering in the 18 pin DIP socket that
houses the heart of the kit, a ROHM stereo transmitter on a chip.  We
are talking TEENY TINY little pins separated by seeming microns.
EGAD.  After each dot of solder, I held the PC board a quarter inch
from my eyeballs to make sure there were no solder bridges.  And I
measured all the components with the Digital meter a second time
before installing each one.  You probably won't take these
extrordinary steps, which make the simple kit an all night project.

But it was a FUN night.  The scent of melting tin/led/rosin... The
occaisional absolutely PERFECT joint...  Kitbuilding is not only an
art, it is positively a cosmic experience.  It is relaxing, creative,
and there is the anticipation of all the wonderful things you will do
with your kit upon completion.

3:25AM

Ah.  The moment of truth.

Connect the nine volt battery, push the power switch, and TUNE around
on the FM radio to find the carrier.

And I found.......

NOTHING.

It did not work.

Now this is where we separate the men from the boys.  How well you
manage not to throw the thing through the window.

So I went back through the whole manual.  checked EVERY joint.
Eyeballed EACH component.  I could find NOTHING wrong.

Elaine came into the shack as I was holding the PC board.  I hated to
admit defeat.  But I told her it did not work.... yet.

She held the board and looked at it with wonder.  She could not
believe that I had placed each of the little parts in their places.
She told me I would figure out what was wrong with it.

The next day, I went through the manual again.  I took voltage
measurements, and found all the proper voltages on the chip and RF
amplifier.  I was stymied.

Then I looked very closely at the OTHER side of the board.  I stared
at the ROHM chip, which Elaine had called a "train trestle".  Hmmm.  I
wonder if I pushed on the chip if it would go in any farther.

Push.  Click.  It snapped into place.

I pushed the power switch, turned on the FM radio, and found a nice
clean carrier at 102.3, with the stereo light blazing away in pure
clear silence.

Hot DAWG it WORKS!

First thing I did was to move the frequency.  Since it was
transmitting on top of one of the local stations, this seemed in
order.  Ramsey's manual stresses the importance of selecting a clear
channel so as not to bring the wrath of neighbors and the FCC.  Up
here in the boonies, there is a tremendous clear swath from about 98
Mhz to 102 mhz.  (I plan to pouplate it).

The next thing required is adjustment of the subcarrier frequency and
stereo balance.  Surprisingly mine was already right on for the
subcarrier adjustment, and I found out that this adjustment is
actually pretty critical.  There is a test point if you have a freq
counter to get it exactly on 19khz, but it can be done by just turning
the little variable capacitor until the stereo light goes on.

The "stereo balance" control takes a bit of explaining.  This is not a
simple LEFT-RIGHT adjustment, but actually adjustment of the level of
left MINUS right.  It is more of a separation adjustment, and also
seems to have a real effect on tonal quality.  It took quite a bit of
diddling to get it right, and there seems to be some interaction
between the two controls, as the stereo light will go out just when
you think you have the sound right...  The end result is a surprising
quality signal with amazing separation.  However, one thing to make
note of...  The RM-10 is designed with NO audio level controls.  This
is a bit of an oversight, because when using it with components that
have fixed level output, the unit is prone to overmodulation.  You
MUST be able to lower the audio level of whatever it is you are
feeding to the tranmitter, as it is too sensitive with EVERY component
I tried connecting direct.  Once you get the level under control,
though, it actually sounds better than many local FM stations because
the signal is not run through all sorts of "Enhancers" that
broadcasters use to be the loudest thing on the band.  I did find
though that adding an audio limiter (an old DBX 119) really helped
tame the overmodulation problem.

I finished the project by mounting it in the $12.95 Ramsey Kit
cabinet.  OK, this is where the Ramsey guys make a few bucks.  It does
give the kit a finished look, but I would have to say that this is a
bit dear for a simple plastic case.   I think the next one will be
built into a VIDEOTAPE plastic case or other cheapo cabinet.

One oversight is that there is no hole in the cabinet for the whip
antenna which mounts to the circuit board.  Ramsey suggests
constructing a dipole or groundplane antenna, which I did initially in
the final installation in my hamshack.  I have since stopped using
this antenna however (more on why later)

             OK NOW WHAT CAN I DO WITH THIS LITTLE GADGET

Ramsey gives many practical applications in the manual.  I found the
unit handy for listening to MDS stereo TV on a little sony walkman in
the wee hours without having to run headphone cords.  Remote listening
of ANYTHING on your main stereo system in any room in your home by
just tuning the radio is just ONE thing the kit can be used for.

My own application is a bit unique.  I have the FM-10 in my ham/swl
shack, and it is connected to a stereo mixer, to which I have several
receivers patched in.  I am a communications junkie and often in the
shack I listen to several things at once.  The only problem is that
one can not stay in ones hamshack all day long.  Well with the FM-10
and a pocket stereo receiver, you can monitor whatever you wish in
your home or yard.  Now since the Ramsey Kit is a STEREO transmitter,
you can do what I do (if you are insane enough)  I have a 2 meter rig
on the LEFT channel, scanner on the RIGHT channel, and the HF rig
panned dead center.  The ears and the brain manage to sort it all out.
I feel sorry for any of the neighbors who happen to tune in when in
this configuration (when I am in my active monitoring mode).

               No Code, No License, No Kidding It's LEGAL

Now speaking of neighbors, lets focus for a moment on the LEGALITY of
using this kit under part 15 of the FCC rules.  In 1989, the FCC
revised part 15, changing the way the measurements are taken to
determine if a device is legal.   The new standard is: 250 microvolts
per meter.  A calibrated Field Strength Meter is needed to make sure
the signal complies with this regulation (FCC rule 15.239).  Beyond
this, part 15 requires that the unit produce no interference to
licensed stations.  Basically those are the rules.  What you put on
the device is YOUR business.  It comes under the same type of
regulation as cordless phones, baby monitors, and walkie talkies.  the
only difference is that this unit operates in a BROADCAST band rather
than a semi-hidden part of the spectrum like the others.

The Ramsey manual has a chart that shows that even if one complies
FULLY with this measurement, the transmitter has a surprising range.
Doing the math of the inverse square law, we find that there is still
.41 microvolts at 5000 feet from the transmitter, nearly a mile.
Since the Ramsey kit has a FINAL AMPLIFIER, it is much more powerful
than a similar kit sold by another manufacturer which uses the chip
output only (that one can be heard well within only 20 feet).  I found
that the kit with a dipole antenna cut to the operating frequency has
TOO MUCH range for my own use.  Those who wish to "Play DJ" might be
interested in running the recommended dipole antenna (making CERTAIN
they make the Field Strength Measurement) to get the maximum range out
of the unit.  I found that just a small piece of wire gives me all the
coverage in my home that I need.

For someone interested in providing a broadcast type service, to a
college dorm, appartment complex or local neighborhood (Legally the
signal can be quite loud over a quarter mile away) It can be legally
done under part 15 of the present FCC rules (as ammended in June of
1989) by simply adding a mixer, microphone, and sound sources to the
Ramsey Kit.

An application I am considering is connecting the audio output of my
new satellite system and tuning it to the BBC audio feed and just
leave it run that way when I am not using the dish for other purposes.
This way I can enjoy BBC in FM quality and so can my immediate
neighbors.  Now before the flames begin, please remember that
copyrights, etc, do not apply to part 15 transmissions.  You can put
on ANYTHING you want.  Just as if you were listening on closed circuit
speakers.  The only difference is that your immediate neighbors CAN
also enjoy the transmissions as well.

FCC RULE 15.215(a) Says:  "Unless otherwise stated, there are no
restrictions as to the types of operations permitted under these
sections."  This general provision *APPEARS* to leave you free to use
the fm transmitter for just about ANY type of operation you desire,
including becoming a "legal low power broadcaster".

Now the nitty Gritty:

     RAMSEY FM-10 FM STEREO TRANSMITTER KIT

     Price:  Circuit Board and Components -- $29.95
             Cabinet (black plastic)      -- $12.95

     Shipping add 6%.

                          Ramsey Electronics
                          793 Canning Parkway
                           Victor, NY 14564

                        (716) 924-4560 (Voice)
                        (716) 924-4555   (FAX)

    o  Operates from internal 9 volt battery
    o  Choice of onboard whip or external antenna
    o  Stable output, from 88 to 108 MHz
    o  Left and Right channel RCA line audio input jacks
    o  Use with Mixers, cassette or CD decks etc.
    o  Clear, step-by-step assembly instructions
    o  Helpful information on FCC rules included

    TYPICAL USES:

    o  Extension of home stereo system without wires
    o  Student-operated school radio station
    o  Home or neighborhood radio station
    o  College dorm favorite music broadcast service
    o  Listening aid for auditoriums, churches

NOTE1:  I am in NO WAY affiliated with Ramsey Electronics other than
        being one of their very satisfied customers.

NOTE2: If you plan to use the kit as a "broadcasting" service, I would
       STRONGLY SUGGEST you have the output level CERTIFIED by an
       engineer assertaining for SURE it is no more than 250
       microvolts per meter.  This kit has an almost amazing range,
       and I imagine it could very easilly exceed LEGAL SPECIFICATIONS
       if you are not careful.

=============== WB8EOH = The Eccentric Old Hippie = WB8EOH ================




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