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



Available Flashcode Options

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 amateur frequencies, a resistor must be placed from the negative VCO steering line to ground. This slightly lowers the operating frequency range of the PLL and allows for lower tuning of the frequency. 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 220K 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.