The Hydra and hercules units have 2 boards, one for WIND and one for DEPTH (only wind is shown in pictures), there are fuses on each board. Battery is a BR2325. Both boards have a seperate EPROM, the depth EPROM is smaller (27C256 ), the wind is a 27C512 EPROM. Difference between Hydra and Hercules is a different front casing.
Changing the Hydra/Hercules wind and depth eprom
This is not so hard, remove the cover from the connectors, remove the 4 screws that hold the top cover.
Now you see the bottom of the Wind board, there are 4 white plastic washers, remove them from the 4 holders. slowly remove the depth board from these 4 holders, and flip it on the side with the flexible connector. This should be enough to change the EPROM, and access the battery. Remember the notch on the EPROM, place it in the same direction.
For the Depth board, slowly move the depth board up (it also has a flexible connector) the 4 holders will come loose of the bottom of the housing. Replace the EPROM and assemble in the reverse order.
The performance unit has an 27C512 EPROM, and a fuse. Battery is a BR2325. Changing the EPROM is very simple, see instructions from the wind board. Please note the notch on the EPROM.
There are 2 types of Full Function Displays, the normal and the NMEA version. Not much difference between them. They have the same 27C512 EPROM with the same firmware.
When you take an FFD or 20/20 apart, be careful when removing the snap ring on the frame...its easy to break it.,Then undue the screw on the connector to allow air in, then use a pair of dull knife blade to separate the frame with its rubber seal from the can.. its tight... also its very easy to snap off the mounting ears. Be careful of the orientation of the connector in the unit... you can re-connected it backwards...
Carefully remove the EPROM and replace it with the new one, be sure to insert in the same orientation (little notch to the center of the board)
when you reassemble, I use a very small amount of o-ring grease on the gasket and I bake the desiccant crystals bag in the unit at 150deg F for 1-2 hr in my oven to re-activate it.
its also a good time to change the memory keep-alive batteries in the units, I do it with power on to keep the memory safe. (Thanks to Tom)
Hydra 20/20 circuit board
Not much info on this one. Has an EPROM.
Linear Expansion unit
No information or pictures
Polar Speed, VMG on the displays
NMEA FFD should be at least version 9 to accept Polar Target Speed information from an NMEA source.
$PBGTTBS - Polar Speed knots, Polar performance %
$PBGTLAY - Distance to Layline (Nm), Time to Layline (hhmmss)
$PBGTVMG - VMG upwind angle AWA (polar), upwind heading for best VMG (polar), downwind heading for best VMG (polar)
$PBGTSTR - Distance to Start Line (metres), Time to Start Line (hhmmss)
(appears in PERFORM - POL menu)
(appears in PERFORM - LAYLINE menu)
(appears in WIND - AWA and NAVIGATE - TARG)
(appears in WAYPOINT menu)