Speaker/Microphone modification
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 to operate.
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.

Step 1.  
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.

Step 2.
Undo the three screws in the top of the controller board and put them in the same place.

Step 3.
Undo the ribbon cables by sliding the connectors back towards the ribbon cables and carefully lifting the board out of the radio.

Step 4.
Cut the blue wire, which is attached to the bottom of the controller board, and place the controller board in a safe place.

Step 5.
Remove the single screw in the middle of the tin shield, remove the shield and put both in a safe place.

Step 6.
Unscrew the four mounting posts with a flat bladed screwdriver and store them in a safe place.

Step 7.
Remove the four small screws in the connector side of the radio and place them in a safe place.

Step 8.
The RF unit is now floating in the housing and will move to accommodate removing the connector plate from the unit.

Step 9.
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.

Step 10. 
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 

Regulator.

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 available.

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.
 

Step 11.
Remove the green and white wires altogether.

Step 12.
Remove the blue wires and solder a short piece of wire across the solder islands in place of R 410.  See photo.

Photo shows the old PCB with wire still attached  
And new PCB with small wire link in the place of R 410.
 

Step12,5.
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 easily.
 

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)

Step 14.
Put the RF board back in its position and re-solder the coax cable back on the BNC connector.

Step 15.
Re-connect the connector plate with the header socket.

Step 16.
Re-install the 4 standoff posts  in the RF board with the shortest one in the middle of the board.

Step 17.
Use the 4 small screws to mount the connector plate back in its original position.

Step 18.
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.

Step 19.
Re-install the tin shield and put one screw back in the middle standoff post making sure that the access holes line up correctly.

Step 20.
Re-install the ribbon cables back in the control board and mount the board back in its original position.

Step 21.
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

Step 22.
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 any soldering.

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.