Installing a serial controller to Maxon SP2550 and SP2850 radios

By Keith Dix ZL1BQE
 

The serial controller allows frequency to be changed on the Maxon SP2550 and SP2850 this interface is connected directly to the PLL device and by doing so disables the channel selector switch on the top of the radio. 

The controller consists of a small MCU controlling the Clock, Data and Enable signals into the PLL chip (MC145456) 

The MCU provides the conversion from standard serial ASCII to the PLL format 

Data is sent from/to the MCU in the following format: 

The radio must be connected to a PCs serial port set for 9600,N,8,1 no handshaking 

Start the PC running a serial terminal program turn radio on the display should show 

D 
MAXON SERIAL CONTROLLER 
T432500 
R432500 

To change frequency type the following 

R439000T434000 

PC will display 

T434000 
R439000 

If you type ? it will also return TX and X frequency 

Pushing the PTT on the radio will send 
T434000 
To the terminal and releasing PTT 
R43900 
Will be sent 
 
 

The Circuit and photos of the mods follow...

Fitting the MCU to the radio is fairly simple the most difficult part is lifting 3 pins on the surface mount chip on the radio controller board as can be seen for the photos pins 3, 5 and 7 are removed from the tracks and wires are connected to them (I have used Kynar wire) 

The MCU is fitted into an 8 pin socket which is surface mounted on a small piece of strip board don’t forget to cut all the tracks under the IC socket 

The solder resist is scrapped off the Maxon board to allow soldering the ground connection and allow a small surface mount cap to be fitted between pin 8 and ground pin 1 is connected directly to ground the ptt line is connected to pin 4 and a convenient point is found as per the photos. 

A 3.5 mm jack socket is fitted to the front panel and is a neat fit between the front and controller PCB. 

The level converter is assembled on another small piece of strip board and soldered directly onto a DB9 connector 

If you want to have a go at it let me know. I have a small supply of  IC's and  kynar wire,  the rest of the bits you'll have to find yourself.