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Spectra MOD/APRS radio [message #989] Wed, 07 September 2011 02:00 Go to next message
root is currently offline  root
Messages: 208
Registered: December 2004
Senior Member
Using a Motorola VHF Spectra as an APRS radio on 144.390 MHz.

This mod was done in order for me to use a readily available Motorola mobile A5 model VHF Spectra as an APRS radio. In my case Imarried it to a TinyTrak3+. Spectras can be found at flea markets and elsewhere. They are basically bullet proof. Just make sure you have acquired the correct frequency range Spectra. A range 1 model is what you ideally want - 136 to 162 MHz. If you are proficient at HEX editing (bit banging) you can use an altered range 2 - 146 to 174 MHz. See the "Repeater Builders" and "Batlabs" web sites. They both contain a plethera of information with respect to Motorola radios and converting them for HAM use.

Most HAMS shy away from using a Motorola SPECTRA radio as obtaining the required COS voltage and suitable audio for use in most HAM projects can be difficult; APRS, links and repeater use.

When complete all required APRS interfacing connections can be taken from the rear DB15 on the Spectra. This mod will change the audio output on pin 11 of the DB15 from discriminator unfiltered audio to filtered squelch gated audio.

NOTE: If you are not proficient in the disassembly of the inner workings of a Spectra do not attempt this. Also, if you are not proficient at unsoldering and re-soldering tiny surface mount components do not attempt this mod. DO THIS MOD AT YOUR OWN RISK!

Writer assumes no responsibility for this mod, period, end of sentence. If you turn your Spectra into a door stop, that's your problem.

Carefully remove the first layer of circuit boards to gain access to the controller board. Remove all six (6) torx head screws that hold the controller board to the chassis. Yes there are six (6) screws, not five (5). Don't forget the screw in the middle of the board. Carefully remove the controller board and place it on a flat surface. Make sure your work area is clean and well lit.

A magnifying type work light is your best bet. Turn the board over so that the PA header pins are facing towards you. The area you will be working on is at the top right. See the circled area on the schematic, photo labeled "schematic".

You will unsolder and remove jumper JU9730. See photo labeled "JU9730" for it's lohoneyion prior to removal. You will then re-solder it at jumper lohoneyion JU507. See photo labeled "JU507" for it's lohoneyion. Photo labeled "JU507 installed" shows JU507 installed.

Spectra MOD for APRS
Spectra MOD for APRS
Spectra MOD for APRS
When re-installing the controller board into the PA header, do it gently and make sure it is fully seated.


Put the case back on and that completes the mod. Make up the interface cable to mate the Spectra with the TinyTrac3+ and your are ready to go. As there is DC voltage on the audio line don't forget a blocking capacitor.

Remember to follow the modem manufacturer's audio level setting instructions.

Have fun.

Bob - VE3DJ
Spectra MOD for APRS

[Updated on: Wed, 07 September 2011 02:03]

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Re: Spectra MOD/APRS radio [message #1792 is a reply to message #989] Sun, 13 January 2013 13:59 Go to previous message
KK4LQX is currently offline  KK4LQX
Messages: 1
Registered: January 2013
Location: Eastern Shore of Virginia
Junior Member
Did you experience any issues with your TX deviation? Right now no matter how high I turn up the audio gain on my Open Tracker USB I only am able to get 1.5 kHz. Didn't know if you had the same problem. Thanks for your help.

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