The PA blow out is happening due to the power amplifier going into self-oscillations when it goes from transmit back to receive. We think there is a fix for this that involves making a cut in the PCB and soldering a wire.

Here is what is going on, briefly:

On receive the K9 relay and the U3 (electronic switch) provide the path for the RF from the antenna to directly to the input of the LPF (L16 to L21). Unfortunately, this is also the input to the PA. At the end of transmission, when the TX_LINE goes low. For a brief while, the drive and the PA are switched on while the caps discharge and it has a return path, setting up these huge oscillations.

The fix is to first power down the PA and then flip the relays over. For this, we disconnect the Q10 that controls the power to the PA from the TX_LINE and we use a fresh line from the raspberry Pi (we call as TX_POWER) to switch the power to the PA in sequence to avoid the oscillations.

All the details of the modifications are on:?

It just needs you to make a small cut in the PCB and solder a wire.

- f?