Raytheon ST4000+ Autopilot Problem

[Mike Sibley]

My Raytheon ST4000+ Autopilot has suddenly developed a problem. When it is turned on, it goes through its usual routine and then goes into STANDBY as usual. However, when AUTO is pressed (to engage autopilot at the current heading), the unit starts turning the wheel to the left (steering to port) and will continue turning the wheel until STANDBY is pressed again or until the wheel can move no further. It turns the wheel in little steps at the rate of approximately 1.4 steps/second. In ten seconds, it will turn the wheel through an arc of approximately 65 degrees.

At present, the control unit is simply wired, with no Seatalk or NMEA connections that might complicate things. The only cables attached are: 1) the wires from the compass, 2) the power supply wires, and 3) the drive wires to the wheel pilot. All of these appear to be sound.

Any idea what could be causing this problem?

[senormechanico]

Assuming everything is wired correctly, (did it work ok before?) I suspect bad solder connections in the head unit.
Mine worked ok but would occasionally make a hard left or right turn for no apparent reason, then it would slowly correct back to course.
I finally got serious about fixing it and found that eight of the twelve solder connections in the middle of the board were bad. Those connections go to the four power transistors which drive the steering motor.

The solder connections had developed microscopic cracks which were only visible under magnification and high brightness. If you have ANY doubt, resolder them.

Now it works great. Cost to fix besides a little solder and electricity?, zilch.

[hillary]

Funny you should mention that! Our autopilot had gone berserk and was doing as you say - and if left to its own devices would just take the boat in circles. We were looking on the internet today to find possible solutions and I came across your post. The autopilot (ST4000) had been working fine for the trip before last down to Busselton and back, but on the last trip it just wouldn't do the right thing. Had to hand steer all the way - 24 hours out, 22 hours back on our Easter trip to Jurien up the coast of West Oz. Jots of fun when you HAVE an autopilot!! After much discussion about possible causes, today we went back to basics and did the Sherlock Holmes thing - if all else fails, look for the simple things. We had done a lot of work on the boat between the two trips, and one thing we did was to change the non-functioning cockpit speakers for new ones, fitting the new speakers in the exact same location as the old ones. But today we removed all metal items in the lockers to no avail, (the fluxgate compass is on the starboard side on the lazarette stern bulkhead). So we then removed the new speaker in the starboard locker cockpit wall and - the autopilot instantly began working correctly again. Just a thought - it has solved our problem. Have you changed anything in your boat??? I hope you find a solution a simple (and cheap !) as ours! (Just have to relocate the speaker now.)

[speciald@ocens.]

Make sure that the rudder position device is still connected to the quadrant.

[GordMay]

Do you have the ST4000+ Autopilot Service Manual
Available here ➥ http://www.ganssle.com/jack/st4000.pdf

[senormechanico]

Originally Posted by GordMay

Do you have the ST4000+ Autopilot Service Manual
Available here ➥ http://www.ganssle.com/jack/st4000.pdf

Ooohhh!!
It's even got the schematic!

Thanks Gord!

[MMSI 316------]

Hello Mike & Senormechanico,
I had the exact same issue that Mike had, the unit kept steering to port. I followed up with Mike to find out if/how he resolved it. He said it was an electrical connection problem (not surprised, they usually are).

So I tried a direct connect to the battery to isolate the problem, however, it still steered to port, but this eliminated a possible bad power suppy connection.

I then measured the voltage coming out of the motor drive terminals on back of the control/diplay unit, it was intermittently sending the same polarity voltage which the drive motor obligingly interpreted as an intermittent signal to turn to port. So this indicated the problem was with the control/display unit.

So I removed the unit and the flux gate compass and took it home and dissassembled it on the kitchen table. I had a look at the power transistors as Senormechanico mentioned, sure enough, I could see where the solder joints had failed.

These switch control transistors are power MOSFETS, and you can tell that they give off heat when operating because Raytheon has them mounted on a metal bar as a heat sink.

However, over time, the constant heating/cooling of the MOSFEts during operation results in fatigue of the solder joints becasue of the thermal mechanical stress of heating/cooling.

Long and short of it is, I re-soldered the transistor contacts as Senormechanical advised, and lo and behold, the unit is working again.

So thanks for your help guys.

Cheers!
Tom King