Pi Servo pHAT (v2) Hookup Guide
QCPete, santaimpersonator 
Troubleshooting Tips
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Double check your connections. On a Raspberry Pi, you may get this is indicated with an OSError: [Errno 121] Remote I/O error readout.
On a Raspberry Pi, also make sure that the I2C hardware is enabled. This is usually indicated with an Error: Failed to connect to I2C bus 1. readout.
Checking Your I2C Connection
A simple method to check if your Raspberry Pi can communicate with the Pi Servo pHAT over I2C is to ping the I2C bus. On the latest releases of Raspbian Stretch, the i2ctools package should come pre-installed. If it isn't run the following command in the terminal:
sudo apt-get install i2ctools
Once the i2ctools package is installed, you can ping the I2C bus with the following command in the terminal:
i2cdetect -y 1
You should see a table printed out in the terminal. If the Servo pHAT is connected/working properly you should see the address space for 0x40 marked with 40.
Current Draw Issues
If your servos are drawing more current that your power supply can handle, your Pi Servo pHAT will not operate correctly and the Raspberry Pi may reboot/brown out intermittently.
Otherwise, if the power isolation jumper is cut, the Raspberry Pi will continue to work since its power supply is isolated from the Pi Servo pHAT. However, the PCA9685 will reset intermittently. A good indicator for this is if you see/hear the attached servos respond to the default configuration for the PCA9685. Another option is to ping the Servo pHAT, as mentioned in the issue above. If you ping the board fast enough (use the up arrow key on your keyboard to pull up a previous entry), you will occasionally see that the Pi Servo pHAT address will disappear from the address table.