Motorola radio - Canford 2-4 wire interface

[aidso]

Hello I'm trying to connect two Motorola radios into our wired comms system using a Canford 903 2 wire to 4 wire.

 

The problem I am having is that I am getting a lot of noise both in the comms and the radios.

 

Now I know the Canford unit is transformer isolated but the impedence is 10k both in and out. Am I right that if I put a 10k resistor across the hot and cold pins of the 1k mic input and 8r speaker output of the radios then that should bring them closer therefore reduce the noise?

 

Cheers in advance.

[TAG1960]

At the risk of stating the obvious, you are using the wrong device. 903 is an interface for other types of wired comms to interface into the Tecpro system. You need an AD913 which will interface duplex walkie talkies in to the Tecpro system... Linky

I have used this interface before, and it does work>

 

Good Luck

 

Neil

[paulears]

The 903 2 to 4 wire unit is actually the correct one to use with a duplex base station, while the 913 is the one to use with a simplex hand-held radio. The difference is really PTT - so you press the yellow call button, wait a fraction of a second then talk and the people with radios hear you. The system just waits for anything coming in from a radio and everyone hears it. The duplex system with a two frequency base station is better, as everything on the ring is permanently transmitted, but somebody prodding their PTT also gets heard by everyone. This is the usual way it works on TV and radio OBs. I find the simplex one, using a walkie-talkie pretty horrible, because people talk before the transmitter starts - there's just enough lag to nip off the first syllable resulting in lots of "say again?". The base station is much more expensive to add in, but is hugely nicer to use!

[aidso]

I am running duplex using a mobile set from a car for TX and a Motorola gp320 for RX.

 

I put a 10kR inline with hot out off RX and bridged + & - with a 4.7kR. Seems to have lifted the most of the noise. Still a low faint earth buzz.

[paulears]

The two to four wire is the key. the only thing to watch for is the power output of the mobile TX - not all are rated for continuous operation and get very hot. Some of the Motorola 300 series are fine on 5W or less, but 25W is too much (and you could also suffer from receiver de-sense as the handheld receiver may not have much filtering in the front end and be a bit 'wide'. Worth checking if the buzz is power supply induced?

[aidso]

Right so got ground loop isolator today from maplins. That and running RX handheld unit on battery has completely removed buzz. Just some tweaking to do.

 

Paul currently running TX at 5w. Planning on mounting large fan on heatsink if I need 25w. Joys of new build theatre with foot and a half poured concrete walls.

[paulears]

The power supplies on Icom and Motorola are quite prone to hums - trying a different power supply could help. The chargers were not really meant for powering the radios, and never press transmit while on one - there is a HUGE hum. many are simply a resistor to limit current, a diode and a capacitor. Charges fine, but not much use for anything else. I'd also suggest just moving the receive aerial a bit further away and sensitivity will improve too. I used to have some Police UHF back-to-back kit and simply moving the aerials apart a couple of feet doubled the effective range on walkthrough.

[mackerr]

Right so got ground loop isolator today from maplins. That and running RX handheld unit on battery has completely removed buzz. Just some tweaking to do.

 

Paul currently running TX at 5w. Planning on mounting large fan on heatsink if I need 25w. Joys of new build theatre with foot and a half poured concrete walls.

 

If you need 25W Tx, I guess you don't need any Rx, because only the base station is going to be transmitting at that power. The handhelds will still be at 2-4W trying to talk back over the same distance, probably with their antenna being detuned by close body contact.

 

The base should be run below 5W so it can withstand constant transmit duty cycle, and should have a remote antenna so you don't desensitize the Rx handheld nearby. A mobile mag mount on something metal is what I usually use. If the handheld RX unit is in a poor location, a remote mag mount on it will help it as well. It will have better sensitivity than the built in whip.

 

If there is a lot of cuing going on via radio constant Tx is OK, but for general communications I would rather not listen to the open transmitter all day. Most of the walkie talkie comm interfaces can use the call signal to key on the transmitter. Just train the show caller to use it properly.

 

Mac

[paulears]

Most of that is already further up the topic mac? The semi-duplex system just makes it possible to interrupt or join in when you need to - something I always find a pain with PTT style operation, where you can't say NOOOOOOOO when the person calling is about to tell you to GO. If you have a few people on radios, it's also really annoying when the person speaking lets go of the call button and umpteen people who have been waiting chirp up at the same time - and you lose the entire point of the call button.

 

25Watt is overkill for the distances we're talking about here - even 5W is more than you really need. local site coverage on 1-2W works fine - and is usually enough to do the job. I left my one in Great Yarmouth on one evening, and I could hear it at home 10 miles away - and that was only 5W.