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Using a GM300 and a M120 together. How to blank one another?

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KI4FKW

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So, say that I have a GM300 and a M120 in the same room and would use them for two stations to use the same repeater. Obviously, the radio that TX's is going to get feedback from the one that is RX'ing. How would you cause them blank each other?

Also, I apologize for the juvenile nature of my understanding these commercial radios.
 

cmjonesinc

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I believe there's a way to do it on some Motorola radios with the accessory plug but I'm not sure on either of those. Something like this would work if you were using an external speaker on the one you are trying to mute. Or I suppose you could use two of these and external speakers on both radios if you needed that. Any way you go about it you'll likely need wiring between both radios so that may not be exactly ideal. Pure curiosity on my end, but why do you have 2 radios in close proximity transmitting on the same channel? AI-External Speaker Muter (ESM) | Scanner Master
 

KI4FKW

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It appears that it would be possible by connecting two of the pins on the radios, but I don't pretend to understand exactly which.

Reason is that it is a backup VHF system for a dispatch center that utilizes a trunking system mainly. There is currently only one VHF radio, but 3 consoles. I'd like to make two of the consoles usable when the VHF system is used (generally when the trunked system is down for maintenance).
 

mmckenna

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What you are looking for is commonly referred to as "cross muting" in the industry.

You would need to program one of the I/O pins in each radio to trigger a relay when that radio is transmitting. That relay would be used to open the external speaker lead on the other radio to mute it when the other is transmitting.
 

KI4FKW

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Thank you for giving me the term. Part of my difficulty in researching it myself was not knowing what to search for.
 

N5XPM

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A little puzzled here on the configuration. A dispatch center often uses one radio with a tone remote or dispatch console. A dual head radio might be an economy approach (one radio with two control heads, one at each dispatcher position). But most GM300s and M120s are wide-band, so, while I believe there was a dual head option for those (might have been aftermarket), it might be hard to find anymore. CDM 1250s are discontinued but newer and wide/narrowband capable. There was in a dual control head option for the CDM series on page 14 here: https://www.motorolasolutions.com/c...static_files/md-eu-cdmseries_brochure_new.pdf
I would be a little careful on a custom-engineered solution on 20 year old radios in a dispatch application that might fail at just the wrong time.
 

jeepsandradios

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Reason is that it is a backup VHF system for a dispatch center that utilizes a trunking system mainly. There is currently only one VHF radio, but 3 consoles. I'd like to make two of the consoles usable when the VHF system is used (generally when the trunked system is down for maintenance).
If this is a PS Backup system its most likely not wideband and those radios need replaced with narrowband radios.
 

KI4FKW

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The VHF repeater has been on the air since probably the 80s. One of these mobile radios has been being used as a base since then. The other came from a car that had been removed from service. All of our cars have a VHF radio, as well.

We used a P25 system as our main system. In the last decade, it has been taken off air only a hand full of scheduled times. Usually less than an hour. I know of one incident where one of the tower sites was down and there was no service in that area. Those are the only times the VHF system is used.

That said, I'd just like to make two consoles usable instead of just one. Mainly in the even that the trunked system goes down for any extended time, or for whatever reason we walk away from that system.

You're right about narrow banding. I hadn't thought about it. Our repeater has been narrow banded. I suspect that by design, the mobile radios we have purchased for cars in the last several years are NFM, too. Some of our older ones (such as the two I'm talking about), aren't. Am I grasping that right?
 
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mmckenna

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You're right about narrow banding. I hadn't thought about it. Our repeater has been narrow banded. I suspect that by design, the mobile radios we have purchased for cars in the last several years are NFM, too. Some of our older ones (such as the two I'm talking about), aren't. Am I grasping that right?
Check the model number of the radios. There were some narrow band GM300's, but they are pretty rare.

Any modern radio made in the last 15 years or so would be narrow band -capable-, but question would be if the person who programmed them set them up correctly.

This is something that should have been checked back in 2013.
 

MTS2000des

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Also the narrowband GM300s do not to "splinter" frequencies as their synthesizers aren't capable. The "narrowband" models only have a narrow IF bandwidth and correct TX deviation. Motorola also sold retrofit kits to "narrowband" a "wideband" GM300, but again, if you're using any "splinter" channels (e.g. 154.8225) the radio will not accept programming as the synthesizer just can't do those steps.
 

KI4FKW

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Our frequencies aren't splintered.

I suspect this is why the radio was missed:

We remodeled our dispatch center back around 2011. The VHF radio was put away in a filing cabinet during this remodel and forgotten until around 2017. We had a scheduled maintenance of the P25 system, so I hooked it back up. It's sat there on, but other than a kerchunk from time to time, has been without activity. Me doing this project has brought to light the need to replace it.

I have plenty of M1225s that I guess I could use to replace it. I do have confidence that they were programmed properly in respect to narrow band. At that time, we were using a professional company in our area for all of our radio needs. We still do, really. This was just a project I was tinkering with myself.
 

MTS2000des

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GM300s are 20+ year old radios and for ham, GMRS, scanning- fine. But no way I would put a 20 year old radio that hasn't been tuned and aligned in a mission critical or dispatch application, narrowbanding issues aside.

The 1225s are a better choice for your use. But any radio you're using if it hasn't been PM'ed should be before you allow people to use it for something like dispatch ops. At our agency, I PM all backup radios (including portables) once a year. Having just transitioned to our backup center and deploying said radios, due to current events. It's good to know equipment is up to spec when it's needed.
 

jeepsandradios

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What consoles do you use for dispatch ? Is it possible to add a Tone Remote Adapter to your M1225 in the back room and control it via consoles when your P25 is down ? I have seen this in the past and gives you the benefit from controlling it from any console.
 

KI4FKW

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We discussed this earlier. I’ll check in to it.

I want it to work if the 800mhz P25 system is down.
 
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