Motorola Talk-Through Unit

Rob Brookes' Blog - MRMap and Communications Info

Motorola Talk-Through Unit

Postby Rob_Brookes » Sat Mar 10, 2012 7:35 pm

The final version of talk-through devices, this time using a pair of Motorola vehicle radios. These are fairly old models, a GM350, 4-channel low band set and a 128 channel, GM390 high band one. This is a cross-band rebroadcast/talk-through and so no diplexer is required. It will operate on any of the 12.5KHz UKSAR channels. Once again I've used an old Philips PRX10 power supply with an additional radio box bolted onto the top. The two Motorola radios are a very tight fit into this arrangement and are very nearly self-supporting. They've been secured using heavy duty Velcro pads under each radio and an 'L' bracket bolted behind each set which prevents them sliding out the back should the Velcro give way. They are a very tight fit through the front of the unit and unlikely to escape that way.
Motorola Talk-Through#3.jpg

The speaker in the PRX10 case isn't required as both radios have built-in speakers. Both were programmed to allow zero volume so that the unit runs silently in use. A slight squelch tails is still audible however. The white lead from the PRX10 power supply is the ignition sense connection for the FM1100 radios which this PSU was designed for. This also isn't required and all three leads can be taped back out of the way.

The Motorola cross-over cable is rather flimsy and has been protected using insulating tape around the bare circuit boards and by passing the cable through the ventilation slots already in the radio mounting boxes. This ensures that the cable is inside the case and as well protected as I can make it. Same goes for the 12 volt supply leads which have been routed the same way. The PRX10 case is ideal for this type of task as it has numerous holes and slots that all conveniently line up so allowing you to tighten bolts that are on the inside of the assembly. They are becoming quite rare these days however. They do have a tendency to not switch back on once switched off if they've seen a lot of duty. This is due to failure of the start-up resistors which are under rated for the job they have to do. In use they heat up and fail. Until the unit is switched off and then on again, this often goes un-noticed but it's easy to replace the two offending components with ones of a higher wattage rating. One major advantage of this particular housing is that even if you come across a dead one, one with a faulty power supply, they are still worth getting as a standard, modern switched mode PSU will fit inside the case where the PRX10 PSU currently resides. It's really the double radio box that's important.
Motorola Talk-Through#2.jpg

As you can see from the rear photograph, the radios themselves are quite small and don't come to the back of the unit. This makes fitting antenna connections difficult unless you have very small hands or a set of long-nosed pliers. In use, I'll fit a short BNC plug to BNC socket tail onto each radio so that I can connect antenna cables a little more easily. I might even fit bulk-head sockets and add a bracket to the back of the unit rather than have them flying.

In use the device performs reliably and accepting that the programming software is as old as the radios and still needs Windows 98 in order to run, it's easy enough to program them. In my case, it's a different program for each radio but if you used two of the same model then this is less of a problem. GPS data passes through unhindered and the voice audio quality is very good as you would expect of Motorola equipment. Sensitivity is good, better than the Icom version I built some time ago. The radios are about the same age as the Icom F-510s I used.
Motorola Talk-Through#1.jpg

An advantage of using the GM390 is that the channel name can be given a 3-digit alias. Here I've used the UKSAR channel number so '854' is indicating the backup TWC. The low band set uses whatever licensed channel your team uses. The low band GM350 could of course just as easily be a UHF one or indeed, another highband one. In this case a diplexer would be needed in the manner of the previously described Icom-based repeater.

If anyone needs further details on this device please get in touch and I'll try and answer any questions.

Later addition:-
One thing I've noticed now I've had this device running for a few days is that the switch between receive and transmit is very fast. There is no lead in delay or hang time on this rebroadcast and any signal received is immediately passed through for transmission, there's no noticeable lag and for those who use the double PTT click method of acknowledging a signal, this is perfectly possible using this device. Not many I've seen can manage the transition between Rx and Tx quickly enough. All in all, this is one of the better versions of this device and I'd imagine that a conventional repeater based on the same radios would be equally impressive. If I can get hold of another high band set then I'll give it a try.
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