Hello Mike,?
You may also try to see If the Arduino is communicating with the si5351 before changing the nano.
In the files section I uploaded some time ago a diagnostic sketch that tests if the i2c bus is locked (short on bus or other device pulling it down), if it can find the si5351 at the expected address and if it can talk to it.
It uses the serial monitor so the test can be done with the Raduino unplugged from the board to eliminate as many variables as possible.
73, John (VK2ETA)
