STEC 60-2 Roll Servo

[Peter Rejto]

My STEC 60-2 suddenly lost roll steering ability but altitude hold is still working. Without the engine running I can hear a sound when moving the heading bug. I can't tell if the sound is the motor spinning freely or a hum from a stalled condition. The clutch is not engaging as the controls remain free.

EDIT: After a bit of reading I'm now feeling more certain that the motor is spinning but the solenoid powering the clutch is not engaging. I can hear a distinct clunk sound engaging the pitch servo, but nothing at all engaging the roll servo.  I understand that the clutch is operated from pins 1 and 2 which I am going to test. I will also test if 12 VDC is being sent to the solenoid. 

I found the servo but I'm quite unsure if I should investigate any further. I'm guessing that the screw shown holds the cover in place. If I remove that screw is it likely that I will be able to possibly tell what has failed? I also want to be sure that I can get that screw back in place.

I do have an appointment with an avionics shop but not for a week+. I'm hoping I might learn something useful that could speed up the repair.

Thanks for any advice!

Peter

 

 

 

Edited July 1, 2021 by Peter Rejto

[Peter Rejto]

I managed to rough up a way to test if voltage is being sent to the solenoid. As I cannot yet find a spare DB9 M&F I used a pair of paper clips to access the female DB9 that runs from the autopilot to the servo. Pin 2 has a solid connection but Pin 1 appears not to be connected to anything internally. I connected the neg lead of my volt meter to ground and with heading mode engaged measured 6.8 VDC. I expected to read 12 VDC but perhaps this is because I'm not correct about Pin 1 being ground, or maybe the solenoid operates at 6.8 volts., or maybe 6.8 volts is incorrect and the reason the solenoid won't engage. Currently I'm a 3+ hour drive away from an electronics part store so unless I find a DB9 I cannot directly test the solenoid.

Peter

[Rwsavory]

Most likely your servo will need to be overhauled.  Here in the U.S. the cost is about $1,500 at S-TEC.  In the mean time, I would not try to work on it yourself.  The avionics shop can troubleshoot the entire system and pinpoint the issue.  You don't want to be "that guy" who screws something up and makes things more expensive.  In the mean time, this is a good thread on one pilot's experience:  https://www.beechtalk.com/forums/viewtopic.php?f=21&t=61657

[Peter Rejto]

Thanks!

The article you posted a link to is the one I was referring to when I mentioned the pin numbers. 

In the light of day I realized that I'd connected to the wrong pins! I now show 12.3 VDC supplied to the solenoid when heading mode is engaged. Clearly the servo is at fault and will need to be pulled.

Edited July 2, 2021 by Peter Rejto

[Bob Weber]

Peter there is only a solenoid and wires inside that servo, you may consider taking a quick peek inside it, but first try pulling the the back shell apart and inspect the servo connector first (servo side).

If you don't see anything loose, check across pins 1&2 for the impedance of the solenoid, if you find an "open", then get the peek inside.

There is a set of solder terminals in there used to connect the solenoid to the servo harness, try to measure for the impedance there as well, if still open the solenoid coil is bad.

It is very rare to see one of those coils fail.

Give me a call 616 822 1999, I can help you thru this.

 

[Peter Rejto]

Hello Bob,

Thanks for your suggestion. I found a female DB-9 and used it to measure impedance across pins 1 and 2 of around 40 ohms. So, not open. I used an external power supply to put 12.5 VDC across pins 1 and 2 with no response. Perhaps the clutch mechanism has jammed or failed?

It's so hard to work in the space available through the inspection opening that I'm reluctant to go any further.

Peter

[Peter Rejto]

Just a final post on the roll servo issue. The avionics shop just rang and the plane is good to go. Turns out 2 screws holding the motor on the servo backed out and one jammed the solenoid! The servo has been untouched since installation back around 1993

I put the "!" because my other recent thread was about 3 screws backing out of the cap on recently installed Surefly Magneto. Unrelated, of course, but....

Peter

 

[David Lloyd]

Glad your problem was solved.  Stec still has my roll computer for repair.