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GP300 with TinyTrak 3

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Project25_MASTR

Millennial Graying OBT Guy
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I've got a couple of spare GP300's that I've been wanting to configure for APRS. The thing that stumps me is what is the best way to go about modifying a GP series radio for such a task?

Is their a way to go through a header similar to that of the J1 header on the Radius/Maxtrac/GM300 series? I've set up a few trackers based off that, but never a Motorola HT. Based off a quick look at the schematic of a speaker/mic, grounding the tx and rx connectors would let the magic smoke out...I'm guessing.

Another quick curiosity question, do the pads at the bottom of the radio (where the front shield is cut away) have any function? Just strikes me odd as them being there.

I don't know much about these radios, they are about as old as I am...
 
cmdrwill

cmdrwill

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The PTT is a resistor on the remote mic line. You will need isolation on the MIC hi with a capacitor in series.
The earphone/speaker line is NOT referenced to radio's ground, use a series coupling capacitor or you will blow out the speaker amplifier in your radio.

SO: a series capacitor in the Mic high side, a 2.2 k resistor from radio mic hi to the TinyTrack's PTT which is already in the TinyTrack3 at pin 1.
. a blocking capacitor on the remote speaker out put which is already in the TinyTrack at pin 5

Pin 1 on the TinyTrac3 RADIO connector to GP300 remote mic hi.
Pin 5 is GP300 remote speaker
Pin 6 is ground.

You will have to do a compensation calibration to get the low and high tones to be the same level on transmit and out of limiting.
 
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