I posted this a while back . I got this info on the phone years ago from Raymarine tech support. (Sorry I wasn't quicker Charlie)

To perform the check, you will need a multimeter, and a DC power supply. You will need to supply 8v to the masthead across the red and shield connections. Normally the ST50/ST60 display head would be energized and provide this 8V, however in your case, you may need to power the mashead transducer seperately.

With the multimeter, check the following:

Set the multimeter to the 20V DC scale.

Test at the connections on the rear of the display head, or at the base of the mast (if you have a junction box installed.)

Red to shield should read 8 volts DC steady. This is the masthead power supply, coming from the ST50/ST60 display head. If the head is damaged, you may need to provide this power from another source to test the masthead.

Blue to Shield should read anywhere between 2 and 6 volts DC. This is the port side directional element. The voltage changes as the vane turns.

Green to Shield should read anywhere between 2 and 6 volts DC. This is the starboard side directional element.

Yellow to shield should read between 0 and 5 volts DC. This is the wind speed circuit. The faster the wind is blowing, the higher the voltage will read.

If any of these readings are out of spec, you should assume the masthead is damaged, and it should be repaired or replaced.