Thanks - I have continued troubleshooting and it looks like multiple problems after all.
Here are specific situations I have isolated:
One of the displays (RL70RC) "kills" all seatalk communications when it is plugged into the seatalk network.
Autopilot still indicates "seatalk fail" even when seatalk cable is disconnected from the rear of it. Won't go into autopilot mode from standby.
GPS Receiver is not sending out any data.
The instruments are sending data ok, and the second display (RL70) displays depth and speed from them when the RL70RC is disconnected. The RL70 does display chart data from the RL70RC (sent over hsb), and radar works ok.
Any suggestions as to next steps I can take? This would be a great opportunity (or excuse
) to replace everything but I can't afford it, so need to somehow patch things back together.
Hmmm. Having more than one faulty instrument is strange, there might have been a lighting strike nearby, you don't need a direct hit to cause problems. Actually, a direct hit would probably fry all electronics. If the autopilot won't go into auto with only power and the fluxgate compass connected, then i'm pretty sure there is a problem with that. Since you have noting to loose, you might try taking apart the instrument to check for water ingress, as i just said in an other thread the case of the st4000+ can crack where the studs are screwed into it and salt water can gradually destroy the circuit or simply create problems by its conductivity. This might happen over a long time, even when the boat is out of water) I have just recently repaired such an instrument. It is quite easy to take it apart, just unscrew the screws on the back, pull the back over off (remove the buzzer's wire, by pulling off the connector inside) an check for signs of water, especially where the fixing studs are. If you see white deposit or darkened conductors on the circuit boar, you have a problem. Not all is lost, though, remove the circuit board (3 screws on the back, where the buttons are, i think), be careful not to drop and crack the LCD, and note how everything is to ba able to assemble them later. Soak the whole board in distilled water to remove salt, use a fine brush if needed. Then soak it in isopropyl alcohol (or pure alcohol) for a couple of minutes to remove water. Let it dry for a day, and reassemble. You might need to patch up the circuit board, if the copper is corroded, but that is better left to someone very good with soldering. I have resurrected four or five dead instruments and VHF radios this way, because usually the damage is not permanent, but conductive salty deposits prevent proper operation. Of course, you will need to investigate and fix the root cause of the water ingress.
For the RL70, it might be useful to see what happens on the seatalk line electronically, but if you don't have proper troubleshooting equipment, it might not be worth it. Most modern stuff uses application specific circuits, which are basically impossible to get as a replacement so even if you see that it is faulty, there is nothing to do by yourself.
I would simply buy a new one on ebay, lots of them are available, most without leads and bracket, but you have those anyway.
But still, check that the cables and connectors are ok.
Unless of course you need to convince wife on spending cash on new electronics, in this case any helpful repair shop would give you expert testimony that all of these are beyond repair and have to be replaced