Astro Spectra

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The Astro Spectra is an Astro radio by Motorola which supports conventional analog and P25 operation as well as 3600-baud Type II trunking and, with the correct firmware, 9600-baud P25 trunking.


Audio Enhancements

Analog and digital audio is processed by the audio DSP before it is sent through the RF section and transmitted. The default settings are fine, and most people will have no complaints with the quality of audio from these radios. However, as found through testing and tweaking by members of, there exists a much better setting for Astro radios that can make the transmitted audio from these units sound even better. These audio settings can be found in the Radio Configuration -> Radio-Wide -> Audio Gain Options section of CPS.

The settings are as follows:

  • Analog: Checked
  • Digital: Checked
  • Type: AGC
  • Output: 0
  • Total: 6


W3 Cable Connections

If you want to homebrew a W3 control head cable, the connections are as follows:


Note that inside the rubber boot of the W3 head is an RJ45 connector. Motorola is very inconsistent with their RJ45 pin numbering, but in this case pin 1 is the standard RJ45 pin 1 (far left with tab facing down).


Available Flashcode Options

Flashcode Editing

FDBTool can update flashcode options on Astro Spectras.

Out of Band

The Astro Spectra CPS can be easily modified to accept out-of-band frequencies using the Hex-Edit Method. However, the design of the Spectra VCO circuitry is such that large deviations from the designed frequency range are difficult. In practice, maximum out-of-band tuning can be limited to anywhere from 3 MHz all the way up to 10 MHz, varying from radio to radio. This can present a problem for amateur use, as the easiest bandsplit to use in the ham bands is the 450-482 MHz model. While some models will tune all the way to 440 MHz with no problem, others will experience VCO unlock at around 443 MHz, leaving the lower portion of the band unusable. Luckily, a simple hardware mod can resolve this issue.

VCO Resistor Mod

To allow the VCO PLL circuit to lock at the lower or higher amateur frequencies (depending on the bandsplit of the radio), a resistor must be placed from the negative VCO steering line to ground, or from the negative SL to the positive SL. This slightly lowers or raises the operating frequency range of the PLL and allows for tuning of the frequencies desired. A detailed walkthrough of the mod can be found at the RepeaterBuilder website. The value of this resistor can vary, and values from 100K all the way to 400K have been shown to work. Individual results will inevitably vary, but the general idea remains the same across both the Spectra and Astro Spectra lines - the VCO boards are the same for both platforms.

S-Records & Binary Files

Astro Spectra S-Records can be read and written with MTSXLab. Similary (and much more easily) the binary conversions of these S-record files can be read/written with MotoTools. To convert between S19 and Bin files, I use the srec2bin tool from

Creating Codeplugs with Astro Spectra Depot

Astro Spectra Depot is able to save created codeplugs as .srec1 files. These are basically the standard s-record files mentioned above, with extra header information to allow them to be read/written by the Depot application. In order to actually write these files to the radio, we have to convert them to binary and make a slight edit. For whatever reason, the bin files from converted depot s-records have 5 extra bytes at the beginning of the file. This is enough for the radio to reject the record with a Fail 01/92, and become unresponsive in CPS until a new record is written. Removing the 5 bytes makes the s-records work perfectly.

Original Converted Bin File from Depot:


After removal of the first five bytes:


That's all there is too it!