Motorola talkabout T5720 repair

Hello, I’m an undergrad electronics and communications engineer.

I have a pair of Motorola Talkabout T5720 radios, one unit I think is fully functional. It transmits fine and I haven’t tested how well it can receive. I can receive from it on a software defined radio receiver on my pc. The other unit doesn’t receive or react when the first one is transmitting and when I press PTT, there is RF power output but there is no carrier. I can see that on the spectrum of the SDR receiver.

I think it might be a local oscillator problem, any pointer on how to start diagnosing that?

If you really are a communications technician, then you will NEED a good frequency counter and RF strength meter. A scanner will not tell you the strength or “privacy” tones of the transmitter. You will need a frequency counter.

If you then find the second Talkabout radio does not have tones enabled and still does not work, then you will learn quickly that lesson one of consumer FRS radios is that they are not designed to be repaired. If they don’t work, they are thrown away. The cost of your time and the price of the parts will easily exceed to cost of a cheap two-way consumer-quality radio.

I have a software defined radio, I can see the whole spectrum from 25 MHz to 1.7 GHz, I can measure frequency down to 0.5 PPM and I can measure RF power relative to the background noise.

The second unit, the one not transmitting, when I press the PTT, it raises the noise floor seen by the receiver significantly. I can post a screenshot when I get to it. I can conclude that the rf power amplifier inside it is working but is amplifing noise. That may point that the local oscillator is not working but I’m not sure.