Interfacing the Motorola Spectra to Allstar


This article describes how to interface a Motorola Spectra transceiver to Allstar. No modification or special programming of the radio is required! However programming of the radio for frequency, PL, Power, etc. does require special software and a DOS computer which is beyond the scope of this article.

Parts required

There are very few parts required for this modification. Two resistors, a capacitor, a FET, two connectors and shells, a 600:600 ohm isolation transformer, a small box, and wire are all that is required.

Modification Photos

Complete package showing cable connections

The red wire from the DB15 is for +12V for the ignition pin so the radio turns on. The Gray cable goes to the small interface box and then to the DMK-URI sound FOB. There is no requirement to use a DMK-URI. Any properly connected commercial radio ready or modified sound FOB would work. The FOB then connects to the Raspberry Pi which is not shown. The interface box also has a leaded speaker jack for monitoring if desired. Speaker leads must remain isolated. Neither speaker lead can be grounded!

The small interface box houses all of the discrete parts. See the schematic link for wiring information.

Description of operation

The Motorola Spectra has no COS line that is easily accessible, however the speaker leads both raise to about a >7 volts DC level when a valid PL'ed RX input signal is received. This works nicely to turn on a FET and gives a low (pulled down) signal to the COS line of the FOB so usbinvert can be used in the simpleusb configuration. It is also VERY IMPORTANT to note that neither speaker lead can be grounded! Doing so would destroy the audio output IC. An isolation transformer must be used to connect the receive audio to the FOB. PTT is directly connected and the mic input to the Spectra is connected via an electrolytic capacitor to the FOBs output. Receive audio setting was proper with a volume level of 5 on the Spectra, RX boost on, and level set to about 700. This will vary so make sure you use the simpleusb-tune-menu selection 2 to set it using the bar graph display. TX level was set at 450. Neither pre or deemphasis are used in Allstar. Both RX and TX audio quality are excellent and the interface works surprisingly well.