Working on a multi mode repeater for GB3BZ

Amateur Radio, Build Blogs

GB3BZ, is a repeater located near Braintree in Essex and is run by members of the BADARS club.

I was asked by one of the members M0NAS if could assist with setting up a multimode repeater for use with both Analogue and DMR repeater.. Sounds like fun?! Right….?

Well it wasn’t till I started to think about this I realised it was a pretty complex set up.

The MMDVM modem boards which run so well for DMR, D-START, P25 etc, do not run any sort of analogue audio. So the trick is to run one system for Analogue and another for the DMR…

However, what do we run for the analogue side?

Doing some research I came across OpenRepeater which runs on a Raspberry Pi and provides a range of the key features you would need in a repeater. The ability to identify itself at regular intervals, Provide a time out on transmission, an end-of-over tone, and many more.

So I fired up a raspberry pi and in a little over an hour I had a working repeater.

So that is the controllers lined up, as for the hardware I had 2 GM660 UHF radios planned for this. But very soon into the testing, I realised there was a big issue.

No matter what we did to the audio levels, the RX radio would reboot after a few seconds of receiving a DMR signal. We worked out, with some help from google, that the way DMR works, only transmitting half the time, the Motorola would be switching the squelch on and off several times a second.  It can only do this a few seconds before the buffer overloads and the radio reboots itself.

Several frustrating hours later, we may of found one of two solutions.

  1. Updating a configuration byte in the radio would increase the squelch buffer allowing the radio to remain in RX mode without a problem.
  2. Use a different radio for the RX, and use the GM660 for the TX.

Whilst option 1 sounds the easiest, it is actually impossible without buying additional equipment and cables from Motorola at considerable cost. So lets explore the other option.

Luckily, Badars were donated 2 GM350 radios, power supply and MMDVM board. So the plan is to use one GM350 for the RX and a GM660 for the TX. This way we can make two working repeaters from the 4 radios.

The next challenge was finding the right software to program the Codeplugs into the GM350 (the GM660 i was given the correct software). Many google searches later I found the software, but it is so old it will not run on modern 64 bit machines.

So I spent the next few hours setting up a virtual windows 7 32 bit machine in Oracle Virtual Box. Luckily my old PC I use for doing all my testing has a COM Serial Port on the back so the Motorola Cable I borrowed fitted straight on.

Finally I was able to read, edit and upload a new code plug to the radio.

Next I had to work out how to connect both an Analogue Repeater and a Digital Repeater on the same radios. (as stated above the MMDVM does not do analogue)

The principle I used is based on the following logic.

  1. An Analogue repeater has a CTCSS access tone.
  2. A digital signal can be decoded by the MMDVM board


However, we don’t want both trying to TX at the same time hence we need to be able to “divert” or connect only one of the two repeater systems at a time.

As we have only 2 wires to switch, being the RX audio and the TX audio, a couple of relay board would work quite nicely.

And using the COS (Carrier Operated Switch) based on if the RX radio has received a CTCSS tone, which is ONLY present in an Analogue signal, we can switch between the two quite easily.


IF CTCSS Signal, direct audio pins to the Analogue repeater (AR)

ELSE, direct audio to the MMDVM.

So the MMDVM is normally closed contacts and the AR uses the normally open contacts.

Tested and worked, until…..

The AR has periodic Announcements, which give out the Repeaters ID in Morse and voice. If this happens whilst in a DMR QSO then it could cause problems. So we need to block the AR whilst the DMR is transmitting.

In order to achieve this, I put both PTT signals from AR & MMDVM through relays, in a way such that.

When in a DMR qso the AR PTT is disconnected &
When in Analogue QSO the DMR PTT is disconnected.

In this way, the repeater now is truly in either DMR mode or AR mode.


More to follow as I continue the build. Check back soon…..

UPDATE 10th July

Work on GB3BZ repeater is now complete.

Had to work around the fact that I could not use the radios CTCSS tone squelch otherwise it only worked when the CTCSS was sent, and of course it isnt with DMR signal.

So was born the CTCSS decoder project!

I expanding a lot on what I needed for this project into a full CTCSS decoder kit, which will soon be available to buy online at my store

So now the wiring has been tidied up, all i need do it test with the DMR ID and then arrange time to install.

Will put more pictures up in a new post about the install process.


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