First check that the serial connection coming out of the back of the master is in place
Check that the power supply in one of the slaves is grounded to the case.
Use an ohm meter and check from the power supply (Output) black wire is connected to the metal case of the sign; meter should show zero ohms or a value less than 1 ohm. This has to be done on a slave as the master is always connected to ground through the antenna extension.
If value is correct replace the master control card
If the value is wrong (High above 1 ohm) order and install all new master and slave control cards and a GPS ground wire kit