GM300 UHF no TX audio, does have power out

[radiocat37]

I have a 25W GM300 UHF 438-470 that has full power out (carrier), but no mic audio. I also hooked a DTMF mic to it and no tones were present when I pushed the buttons. I cannot find anything else wrong. Model number is: M34GMC29C3AA.
I got this radio in a lot of equipment. I am not sure if it is a jumper setting or an actual problem. Can someone guide me where to start?? I have an IFR service monitor and most everything else needed to test.

[jackhackett]

There is a jumper JU651 for setting the mic gain, if that's missing you'd get no mic audio.
You can find where the jumper is in pictures here:

http://www.repeater-builder.com/maxtrac/gm300-info.html

Another possibility is that someone had a third party board such as a voice scrambler in there and it's since been removed. Those usually require the audio path to be broken at some point and wires from the scrambler board soldered across the break. That'll be harder to find. They usually break the RX audio path as well though, and you didn't indicate a receive problem.

Go to the bottom of the page I linked, click the Maxtrac Index link, and scroll down towards the bottom of that page, you should find GM300 info there that might be a big help.

And of course there's always the ever popular " bad feedthru connector", those have been discussed on here numerous times, the tx audio signals go through that connector, so it's *possible* you could lose audio if it's bad, but I'd say it's a slim chance.

[kcbooboo]

If you tried a Motorola DTMF mike, remember that when you push the PTT button, the DTMF pad is disconnected and only the microphone will work. To get the DTMF to work, just push one of its buttons; it will key the radio by itself and send the DTMF tones. There may not be anything wrong with the radio itself.

Also, some DTMF mikes with an RJ45 connector may be wired differently depending on application, so unless you know for sure that the mike works on other Motorola radios, it may not be the best thing to test with.

Bob M.

[jackhackett]

To add to what Bob said, the newer Moto DTMF mics for the CDM series radios will *not* work on a GM300/Maxtrac as they do not actually generate the tones within the microphone, so make sure you've got one that's compatible with the older radios.