st4000+ compass issues

[Lyra]

I have an original, 20 ears old, ST4000+ autopilot system.
The drive was replaced several years ago with a type 1 linear unit, and till now the system worked perfectly.
Today I noticed that although engaged, it would lose course. I noticed that although the boat turns, the course display does not change.
I see that the detected heading is simply incorrect (wrong heading compared to magnetic or GPS, heading does not change according to the amount I turn the boat etc). Though there was problem with calibration and tried to go through the procedure, but this does not seem to be the issue (the procedure failed)
So, I believe the problem is either the flux gate compass or the head unit (electronic section that reads the heading, because everything else seems to work fine).
Any suggestion as to how I can establish where the problem is?
Anybody knows what signal does the compass provide the head unit (DC? AC? voltage levels?)
Anybody ever opened the compass unit  - I have a tendency to believe that the problem may be there because as far as I know it is suspended on some kind of gimbals

Thanks

[Yngmar]

Either a magnetized item messing with the fluxgate compass (take your handbearing compass down there and check all around it).

Or the fluxgate is stuck and can't swivel. We've had that once. Opened it up, gently cleaned and lightly grease the moving parts and it worked ever since.

Or something is actually broken!

[Lyra]

Thanks,
Since nothing changed around the bulkhead where the compass is mounted, I tend to suspect the second option  - will open it and take a look.

[primozk]

Hi Lyra,

I had some issues with the fluxgate compas some years back. The autopilot compas did not turn normaly - it got stuck at some course then it jumped 150 degrees and similar behaviour on the at other courses.

The advice i got was to measure resistance between the cables. First time it helped when i disconected the compas, measured the resistances and connected back. It worked again for half a year. Then again same issue. I had a spare fluxgate and replaced the two.

To test:
Disconnect the cables from the Autopilot

Aprox resistances should be:
- shield <> blue = 8 ohm
- yellow <> red = 4 ohm
- green <> red = 4 ohm
- yellow <> green = 8 ohm
- shield <> red = open

The resistances are not exact but should be in proportion.

If they would not be similar is probably the issue with the compass. I was also advised to open, clean but i hesitated to make it by myself.

Hope it helps.
P.