Details compiled by Harry, ZL1BK
Maxon speaker-microphone conversion.
We recommend you read this through
first to get the feeling of what it is you want to do.
This modification will produce
a diecast box radio, which is a rugged piece of gear and simplicity itself
It will, however have only a
maximum of two frequencies to choose from.
If more frequencies are necessary
a protocol converter can be added and with the aid of a computer interface
or a LCD display board.
We begin with removing the entire
radio out of the box to enable us to work on every part of it.
Remove the four screws in the
lid of the radio and put them inside the upside down lid in a safe place
in the workplace.
Undo the three screws in the
top of the controller board and put them in the same place.
Undo the ribbon cables by sliding
the connectors back towards the ribbon cables and carefully lifting the
board out of the radio.
Cut the blue wire, which is
attached to the bottom of the controller board, and place the controller
board in a safe place.
Remove the single screw in the
middle of the tin shield, remove the shield and put both in a safe place.
Unscrew the four mounting posts
with a flat bladed screwdriver and store them in a safe place.
Remove the four small screws
in the connector side of the radio and place them in a safe place.
The RF unit is now floating
in the housing and will move to accommodate removing the connector plate
from the unit.
Carefully pry the connector
plate away from its position in the diecast housing. It will be held there
with some Ados F2 glue, which is applied, by the factory to secure the
BNC locknut in place.
Separate the connector plate
from the RF board whilst being careful to not exert force to the vertical
PCB, which is the reason to have the RF board floating to accommodate in
the best position for separation.
Once the connector plate is
out the BNC connector needs to be de-soldered and the PCB removed so we
have an empty case to mount the LM317 voltage
Photo shows the removed RF PCB
with removed connector plate and de-soldered BNC connector.
It also shows one way of mounting
the LM317 regulator in the front under the connector.
The advantage is that the wiring
is kept to a minimum length and the space in the back of the radio is still
This method requires removal
of the output side of the (black) 2A use which connects to pin 3 of the
317, connecting pin 2 to the 12 volt side of the IN4007 diode to supply
the radio, soldering a 1.2 k. Resistor across pins 1 and 2 of the 317 and
connecting pin 1 of the 317 to a + 10 uf cap with the – connected to ground
and two resistors, 4k7 and 3k9 in series to make 8.6.k across the capacitor
for a 10.4 volt supply.
See LM317 specs for details.
Remove the green and white wires
Remove the blue wires and solder
a short piece of wire across the solder islands in place of R 410.
Photo shows the old PCB with
wire still attached
And new PCB with small wire
link in the place of R 410.
Put a blob of solder between
pin 12 and 13 of vertical PCB
The row of connections is 16
to the left and 9 to the right. 12 and 13 need to be linked.
The photo also shows the only
remaining wire (yellow) left in the radio.
Be careful when handling the
ribbon cable connectors as they are not designed to flex a lot and break
If you are planning to install
a protocol converter then this is a good time to find pin 8 on the DB9
connector, isolate it by removing the solder blob on pin 6 of the vertical
board and soldering the wire so it connects with pin 8 of the db 9 connector.
This, however removes the ability
to switch between the programmed two channels of the radio.
However in the modified state
it is still possible to operate the radio on the first programmed channel
in the mean time and progressing to the protocol converter at a later time.
This is a good time to consider
the mounting of the 317 regulator.
It needs to be mounted just
to the side of the board with its pins bent up like in Photo 1.
In practice it needs to be right
next to where the radius of the diecast box starts.
Drill a 2 mm hole and thread
it with a 3 mm. tap just enough so the screw can just fit in the thread.
Don't forget the isolating mica
washer and grommet because the LM317 heatsink is not electrically isolated
from pin 2.( Output)
Put the RF board back in its
position and re-solder the coax cable back on the BNC connector.
Re-connect the connector plate
with the header socket.
Re-install the 4 standoff posts
in the RF board with the shortest one in the middle of the board.
Use the 4 small screws to mount
the connector plate back in its original position.
Solder two wires, preferably
with small header connectors to the vertical boards' two outer pins of
the flat header connector, which go to pins 1 and 2 of the DB9 connector.
The one on the side of the BNC
connector is DB9 pin 2. These are the microphone and speaker connections
from the radio.
Re-install the tin shield and
put one screw back in the middle standoff post making sure that the access
holes line up correctly.
Re-install the ribbon cables
back in the control board and mount the board back in its original position.
Solder a small wire to the third
pin in from the ribbon connector closest to the DB9 connector.
This is used to connect the
microphone by means of the (removable) little header pin.
Use the next photo to find the
correct third connection from the right pin of the ribbon connector
Connect both wires for the speaker
and microphone as per the photo.
The speaker wire connects to
pin 1 of the little 3 pin connector with the brown connector
The microphone wire (grey wire)
connects to the soldered wire on the ribbon connector with the white connector.
These wires use small header
connectors so it would be possible to remove the control board without
The photo also shows the various
stages of work involved to mount the protocol converter.
The carved out and scraped copper
islands ready for mounting the IC socket onto.
Note that island 8 is not actually
an island but just a scraped part of the ground plane.
The radio is now modified and
ready to be tested for audio.