When I bought my Sealine F33 in August 2019, I had a few issues with the compass and autopilot.
I tried the usual things without success; swinging the compass (aka. slowing driving round in circles while it calibrates the deviation), checking the resistances between the compass wires, ensuring no metal stored neared the compass etc.
It even took me a while to locate the actual instrument (it was under the floor between the bathroom and mid cabin doors), but as it turned out that wasn't all that useful anyway.
Eventually, I managed to get hold of the service manuals for the compass and SmartPilot, and this is where things became interesting.
Raymarine published a technical update for the SmartPilot S1 & S1G, TU228 in 2005 (extract included below), subject "Fluxgate Compass Circuit Fault". Having exhausted all other avenues, I took the unit out and had a look. I was quite surprised to find a defect listed as urgent hadn't already been sorted a lot earlier.
"Please be advised that infield feedback has highlighted a potential fluxgate compass circuit fault, which can affect current S1 and S1G SmartPilot course computers.
The fault will cause the compass heading to drift, jump segments or show large inaccuracies.
The fault is associated with the wrongly reverse placement of the fluxgate drive AC coupling capacitor C43. The fault does not affect all S1 Course Computers and is only triggered when the tolerance of the capacitor is at the limits of its value specification."
It sounds like this problem may not actually show up straight away - perhaps it's something that dealers decide to leave alone until it occurs.
Anyway, here's the problem and the fix.
The offending component is at the bottom left of the circuit board near the bottom left of the Murata Gyro (newer models have a Kionix Gyro that looks more like a daughter board).
Here is a close up showing the C43 capacitor (marked J225) - notice that the white bar is at the bottom. :
The Wrong Way
The service procedure that I carried out is to desolder the capacitor from the board, turn it round the correct way, and resolder it in:
The Right Way
It's pretty fiddly because the component must be all of 3mm long x 1.5mm wide but the good news it's surface mount stuff so I didn't need my desoldering pump; just heat on one side and pull gently with tweezers, then do the other.
Having made this modification, I'm pleased to confirm that the compass has worked perfectly ever since.