Five Essential Mods for the Ramsey FM10a Stereo Transmitter

Five Essential mods for the Ramsey FM10a stereo transmitter,
plus other useful bits of information

This information may be freely copied and distributed as long as credit goes
to the original author, G. Forrest Cook
Version 1.5: December 22, 2003 GFC
See for more information.

Driver choke mod:
A small RF choke (around 10 uH) should be connected in series with R13,
between R13 and U1 pin 7.  Lift the U1 pin 7 side of R13 and put one end
of the new coil in the opened hole.  Solder the free ends of the coil and
resistor together.  This mod improves the drive to the output transistor
and keeps stray RF out of the power supply circuit.  Keep the lead lengths
as short as possible.  It is possible to make your own choke, wrap 10-20
turns of small gauge, insulated wire around a 1K 1/4W resistor, strip and
solder the ends of the wire to the resistor leads.

Final amp choke and resistor mod:
Replace R9 (270 ohms) with a series combination of a small RF choke
(around 10 uH) and a 200 ohm 1/4 watt resistor.  This mod in combination
with the driver choke mod brings the output of the transmitter up to about
10 milliwatts and once again keeps RF out of the power supply.  The stock
FM-10a puts out around 5 milliwatts of power.  Don't do this mod without also
doing the lowpass filter mod, you wouldn't want harmonics showing up on the
police, fire department, or television bands.

In the USA this power level may be above the legal limit, so you should only
use this circuit to warm up a shielded dummy load.
The resistor value is set for maximum RF power with a 12 volt supply on the
FM-10a.  Smaller resistor values will gain you nothing and would possibly
overheat and destroy the transistor Q1.  Short lead lengths are a must.

Lowpass filter mod: *IMPORTANT*
Put the following circuit between C13 and the output jack, it eliminates
harmonics that may interfere with other frequencies and cause unwanted
attention to be drawn to you.  It is best to build the filter into its own
metal box and run coax cable to it via BNC connectors.  The capacitors may be
silver mica or disks.  The coils L1, L2, and (L3) can be made by wrapping four
turns of tinned bare #20 gauge wire around a 7/16" diameter form such as a
Sharpie pen.  Form the coil so that the wires come off of the same side of the
coil and at right angles to the coil.  Wind the coil tight and pull it apart so
that there is about 1/32" spacing between the turns.

Build this circuit on a small piece of copper clad circuit board
and make a wide solder connection between the filter board and the
main circuit board for a good RF ground connection.  I soldered one
filter board to the back side of the output jack with good results.
Some adjustment of the spacing may be necessary, the best way is to
connect the output to a cheap CB SWR meter with a 50 ohm dummy load on
the output, spread or squeeze the turns of the two coils to the point just
before the power level drops on the meter.  Beware that a CB SWR meter
may not work well at 100 Mhz, mine does.

If you have access to a network analyzer or a spectrum analyzer
or a scope and sweep generator, they can make tuning the filter easier.
Put the filter into a metal box with BNC or other coax connectors on
the input and output connections coming out of opposite ends of the box.

A useful modification of this circuit involves replacing the three fixed
capacitors with variable trimmer capacitors that cover similar capacitance
ranges as the fixed capacitors.  A trimmer capacitor in parallel with a
fixed capacitor may be used.
Tune all three variable capacitors to the point where the output level
just starts to fall off, this may take several tuning passes to get right.

This 2 section filter should be adequate for the FM10

To C12          L1       L2
Input  o-------uuuu-----uuuu-------o Output
           |         |         |
           = 33pf    = 56pf    =33pf
           |         |         |
Ground o---------------------------o ground

Even better filtering may be achieved with an extra section:

                L1         L2         L3
Input  o-------uuuu-------uuuu-------uuuu-------o Output
           |          |          |          |
           = 33pf     = 56pf     =56pf      =33pf
           |          |          |          |
Ground o----------------------------------------o ground

VFO capacitor mod:
The capacitor supplied with the kit for C16 is not very stable for use
in a VFO circuit.  Replace C16 with an equivalent value NPO ceramic capacitor
(zero temperature coefficient).  This will improve stability considerably.
NPO capacitors are available from Digi-Key and Newark in the USA.  If you can
find one, a ceramic Johanson coaxial trimmer capacitor will work nicely as a
replacement for C16.

VFO voltage regulator mod:
An inexpensive adjustable voltage regulator IC can be substituted for the
low quality temperature sensitive diode string regulator in the FM-10a.
This mod along with the VFO capacitor mod will improve the frequency stability
of the FM-10a greatly.  Installation instructions follow:

First Remove the BA1404 IC.  Remove one or all 3 of the 1N914 diodes,
D1 thru D3.  Remove resistor R8.  Install the input side of an LM317L
regulator in the hole where the S1 side of R8 was.  Install the output
side of the LM317L in the hole where the D1 side of R8 was.  Take two
220 ohm 1/4 watt resistors and twist them in series.  Connect one end
of the two resistors to the circuit board ground plane.  Connect the other
end of the two resistors to the voltage regulator output pin.  Connect the
voltage regulator reference input pin to the middle of the two resistors.
Connect an 0.1uF disk capacitor between the circuit board ground and the
input pin of the LM317L.

With the BA1404 IC (U1) still removed, test the operation of the regulator.
Apply 9 to 12 volts to the FM10a, turn the power switch on, and measure the
output pin of the regulator relative to ground.  If the regulator is working,
then it's output voltage is around 2.5 volts.  The output voltage should be
no more than 3.2 volts, higher voltages will damage the IC.  If possible,
try varying the input voltage, the output voltage should remain stable.
When everything checks out ok, plug the BA1404 back into its socket.

Parts list:
2 X 10 uH RF chokes, the value is not too critical but pick small parts
1 X 200 ohm 1/4 watt resistor
2 X 33 pf silver mica capacitors
1 X 56 pf silver mica capacitor
2 X 0.125 uH coil: 4 turns of #20 wire on a 7/16" diameter form spaced 1/32"
1 X 33* pf NPO ceramic capacitor *use the same value as the old C16 cap
1 X LM317L TO-92 case adjustable voltage regulator
2 X 220 ohm 1/4 watt resistors
1 X 0.1 uF capacitor, disk or monobloc type
All of the parts can be obtained from Digi-Key, 1-800-DIGI-KEY or other
electronics distributors.

Remember, as with all RF circuits, the FM10 will work much better if it
is securely mounted in a METAL box and wire leads are kept to a minimum.
Don't waste your time with plastic boxes.  Many thanks go to Mycal and
his FM10 faq for inspiring many of these mods.

- Addendum -
Several BA1404 based kits that I have seen produce a broad spectrum
output near the desired frequency of operation, this can cause interference
problems.  A simple fix to this is to put a tuned circuit right on the
output of the BA1404 (pin 7).  A 1nf DC blocking capacitor in series with a
parallel resonant tuned circuit can clean the signal up.

For the tuned circuit, I used a 5-45 pf compression trimmer (type 403) across
a 3 turn 3/8" diameter #20 AWG air coil.  The coil should be put right on
the trimmer with very small (< 1/4") leads.  Keep all leads short and solder
one side of the LC to a ground that is close to the BA1404 ground.  Connect
the 1nf capacitor between the top of the LC circuit and BA1404 pin 7.

Adjust the circuit for the highest output power, it may be necessary to
compress or expand the coil a bit to achieve a real peak.  If you have
access to a good scope or spectrum analyzer, these can help in the
alignment process.  Tuning should be fairly broad for this circuit.

A very useful test is to operate an fm receiver near the transmitter
antenna, try this before and after the mod, you should notice a drop
or complete elimination of your signal everywhere in the band other than
the desired transmit frequency.

If the BA1404 is used to drive an RF amplifier transistor, the best
impedance match for the transistor's base pin may be found by connecting
to the middle of the coil in the resonant circuit.  See the alternate
connection below.

BA1404-7   o-----||--------- (original) to next stage
             = 0.001uF
             |     |
     5-45pf  |     3   0.001uF
       #403  =     3---||--- (alternate) to next stage
    trimmer  |     3 \
             |     |  3 turn coil
BA1404 Gnd o--------

The BA1404 based kits that I have built have had problems with
RF feedback getting back into the circuit via the power and audio lines.
The following tips may improve this situation:

Enclose the BA1404 kit in a metal box.  Use a full metal BNC connector
(not an insulated ground type) for the RF output.  Install the BNC connector
close to the RF output on the circuit board and connect it with short wires.
Use metal spacers to mount the board to the box.  Be sure that the metal
spacers don't short out the circuit board traces.

Build low-pass filters on the audio inputs and DC power input.
Each audio filter consists of a 3 component pi low pass filter
with 10uH coils and 10pF caps, wired as follows:

Audio In o-----uuuu--------o FM10 input
           |         |
           = 10pf    = 10pf
           |         |
  Ground o-----------------o Ground

The audio should connect to the filter input via coaxial cables, the
filter should be located close to the BA1404 inputs, the components
should be grounded directly to the FM10 circuit board.

The 12V DC power supply should also connect to the FM10 board via
a similar pi filter.  For the power supply filter, the capacitors should
be 0.01uF (10nF) and the coil should be approximately 100uH.

All of this might seem like a lot of trouble, but it will save you time
in the long run.  I also recommend installing a temperature controller
circuit in the same box to keep the oscillator at a constant temperature,
this greatly reduces drifting.

Note: the BA1404 IC is (or at least was) made by the Rohm company,
A search for BA1404 on will likely yield many more resources.

Back to FC's Micro Power FM Broadcasting Circuits page.

Email Me