Anyone have ideas to throw on top of all this?

First, for Colyn:

 

S-Tec admits that the 55-X tray connector has had some problems.  The 50 does not have a tray connector and in the worse case scenario - connections made thru my breakout box - I have not had a connector problem.

 

Factoid:

 

S-Tec did have a problem with some early "new, improved" altitude sensors - that is, they locked up or failed to operate properly below 700 feet MSL.  If you have a new sensor with "AA" ending the serial number, it should operate properly at any altitude.

 

For Ed,

 

My sensor readings match yours.  S-Tec said sea level should be about 8.2 volts. I have sucked it to 7.5 and blown to about 8.3.  If I have to do further testing, I will plumb my spare altimeter into the pressure test setup.

 

OK, yesterday I was doing ground tests with S-Tec on the phone.  The setup had two multimeters attached, one on the sensor output and one on the pitch servo power pins (3 and 4 at the servo).  With the AP activated (not altitude hold) there was a rapidly fluctuating voltage at the servo from -.5 to +1.5 volts.  S-Tec said that that was OK.  Note:  There is a selenoid that must be activated to engage the pitch servo gears but maybe it is this voltage that slowly destroys the motor operation as it sits bouncing back and forth with the AP on but not on alt hold.

 

Whether the sensor was reporting a higher or lower altitude different from the alt hold output, a negative voltage (up to about 9 volts - somewhat flaky looking though) was delivered to the servo resulting in a dive - always a dive.  "Aha," said S-Tec, "You must have wired the motor incorrectly, swap pins 3 & 4."  That accomplished, the same voltages arrived at the servo and now it would always climb (Duh....).  Today, the computer is being sent to S-Tec even though it was checked "good" by them on 9 September 2004.

 

For Andre,

 

I asked about the sensitivity adjustment you made by altering R57.  S-Tec thought reducing the resistance would decrease sensitivity - apparently they are not experts on this since you have shown that increased resistance reduced sensivity.  Anyway, I will be requesting further guidance from them in my letter attached to the computer.

 

Note to all:  I have never had a problem reaching S-Tec Technical support.  The only problem I have had is consistently getting the same person so I don't have to re-explain the whole problem again.

 

I will post results when next available.