OpenRepeater Setup with Raspberry Pi

Greetings! follow along with me on my adventure to create a mobile repeater from Motoroa CDM750’s for Defcon 25 / 2017!

First off the list. This is is not complete, only the major components, there were small things needed, but otherwise a good list.

  • Motorola CDM750 | eBay $65 (or similar)
  • Raspberry pi | Amazon $35
  • Motorola interface cables | Hamgadgets $17.95
  • Notch Duplexer | Aliexpress $60 (do not do this, buy american)
  • Type N to Mini UHF pigtail | Amazon $15

To start the build I went to WA7GTU’s and had him test the notch and radios to ensure everything was working properly. It turned out the “factory tuned” duplexer was not tuned. The ebay radio did, however pass a simple watt output test.

After everything passed, its time to hook everything together!

The first task: program the radios. I scoured TPB and other places on the internet for the CPS software to program the CDM750 and found a torrent here:

Next I needed a programming cable, I decided to create one based on WA7GTU’s cable, as I wanted one of my own to fix things in the field. To make your own, create a UART cable with the following RJ45 pinout using TIA standard pin numbering, pin 1 is furthest left if holding connector with clip down, contacts up:

  1. spare
  2. spare
  3. RX (white)
  4. spare
  5. ground (black)
  6. TX (red)
  7. spare
  8. spare

Now that the cable is created, its time to program. There are two main bits of information needed, the frequencies used and the CTCSS tone(recommended)

I chose TX: 446.050 and RX 441.050 for my setup, following the Southern Nevada band plan. If you do not have a frequency pair, contact your area’s frequency coordinator. If this is not going to be a formal repeater, there really isnt need for coordination, but it is best practice. If you are not seeking coordination, take a look at your area’s band plan, it should have a section for repeaters. Note the frequency range for repeaters and also take a look in your areas repeater directory to find a frequency pair that is not in use. For me, I am doing a UHF setup with a 5mhz minus offset, that is, my transmit frequency will be 446.050 and the receive frequency will be 441.050. Tune to the frequency you are looking at using, monitor it. Try keying it up and identifying. Do this multiple times, possibly for a few days to ensure you are not going to be interfering with anything, as far as one can tell without knowing link frequencies. Now that you have a frequency pair, get your radios configured.

Now, the CPS software to program the CDM750 will NOT go into the ham bands, as it indicates it is “out of band”, bullcrap…. anyways, here is the fix for adjusting the available frequency range:



Now that we finally have the software, a cable, and the capability to program in the band we want, we can get started programming.



Now that programming is complete, its time to plug things in.

Connecting CDM750 to an ID o’matic using the Motorola cables from HamGadgets



Connect the TX radio to the receive side of the notch duplexer (for my setup this is correct)

Connect the RX radio to the transit side of the notch duplexer

Connect the Antenna

Connect Power

Bring that baby up!