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No TX PL on GM300

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No TX PL on GM300

Postby kb3tix » Wed Aug 23, 2017 6:36 pm

Hello -

I have an old Motorola GM300, model number M43GMC29CsAA which I'm trying to use on amateur 2m frequencies. I've searched far and wide and can't seem to figure out what's going on.

I programmed it using RSS and / or Radio Doctor for some simplex frequencies and a few local repeaters. On simplex, it works beautifully, but on the repeaters, it only keys at the end of the transmission.

What confuses me further is that when I use a known good HT configured to require a PL tone to receive, it works fine.

Repeater Scenario:

While monitoring a repeater on a separate radio, I begin a transmission. I key the mic and begin to talk. Nothing is heard on the radio monitoring the repeater. I finish saying something and unkey and then the repeater gives me a roger beep heard both on the monitoring radio and the GM300, so perhaps the PL is being engaged too late? I reverse the split on the monitoring radio and transmit from the GM300 and hear myself just fine, eliminating the repeater essentially. I've tried this on two different repeaters with the same net result.

HT Scenario:

I program an HT to require a PL tone of 131.8 and program a mode on the GM300 to transmit on a simplex frequency with a TX PL of 131.8. I transmit from the GM300 to the HT and hear my voice.

Now I program the HT to require a PL tone of 88.5 and keep the GM300 using 131.8. I transmit from my GM300 to the HT. The RX light lights up and no audio is heard, as expected.

I revert the HT to require a PL tone of 131.8 and all is well again.

I've gone through RSS fairly exhaustively and I don't have any kind of PL inhibit enabled. As seen in the HT scenario, *some* kind of tone is being generated or I wouldn't be able to hear myself on the HT.

I'm sorry if the solution is obvious, but this one has me stumped. Thank you very much for any suggestions...


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Re: No TX PL on GM300

Postby jackhackett » Thu Aug 24, 2017 3:33 am

Have you tried hitting the repeaters with the HT or another radio to verify that the problem is in the GM300?
Are the repeaters 12.5KHz narrowband?

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Re: No TX PL on GM300

Postby Bigred » Thu Aug 24, 2017 6:28 am

You need to separate the HT from the GM300 to test through the repeater. Better yet test with a buddy that is away from your location.
Look up "receiver desense" for an explanation.
Lots and lots of watts...

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Re: No TX PL on GM300

Postby kb3tix » Thu Aug 24, 2017 10:58 am

I was able to hit the repeaters with the HT and both are wideband.

I will research receiver desense. I have friends in the area who can help me test. Thank you both for your suggestions!

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Re: No TX PL on GM300

Postby kb4mdz » Thu Aug 24, 2017 4:29 pm

Do you have a way to check the level of the transmitted PL on the GM300? Or check what level of PL deviation the repeater needs? (tho, that's getting far afield in terms of ease of checking these things out)

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Re: No TX PL on GM300

Postby kb3tix » Sun Aug 27, 2017 5:04 pm

I solved this mystery today. Bigred was absolutely correct. I bought a 2m radio at a hamfest and wanted to test it. The HT exhibited the exact same behavior, so I asked my friend several miles away to listen on a known good radio and everything worked correctly. Thank you for all of your suggestions.

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Re: No TX PL on GM300

Postby Jim202 » Tue Aug 29, 2017 7:29 am

Welcome to the world of radios and repeater operation. The problem with swamping, overloading or desense of a near by receiver is a common problem in trying to see if your radio is working through a repeater correctly. It's a real life lesson that is hard to explain to a new person without that person actually experiencing it.

Now that you have learned what desense is all about, you will have a better understanding the next time you try to see if your making it into a repeater. The closer you are to the repeater, the less of a problem this will be. But if the repeater is miles away, it is a common problem all of us hams have.

A few of us are lucky enough to have test equipment on the bench that we can test the radio with, without putting an antenna on it. The majority of the hams around the country have to do it the hard way like you have managed to work through.

Again, welcome to the repeater activities. If you run into problems, there are a number of people on the chat groups like this one that are more than willing to provide suggestions on trying to resolve the problems you might run into.


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