Air Variable Capacitors and Autocontrol II

There is a feedback air variable capacitor mounted on the servo.  I am suspicious that this is cracked or perhaps otherwise problematic.  It is certainly very dirty. 

This capacitor is used in the operation of the feedback control mechanism.  If the attitude indicator goes off level then a variable induction pickup inside the indicator changes.  This change in inductance will then alter the frequency of one of the radio frequency oscillators.  The benchmark radio frequency oscillator is then compared to the variable frequency.

The difference in frequency is detected by a discriminator circuit and this is amplified and turned into a DC current that is sent to the servo motor.  This then turns, and as it does, the feedback capacitor also turns along with the ailerons.  Eventually, the capacitor will vary the frequency until it matches and the current to the servo motor goes to zero, and the aircraft begins to bank.

As the bank angle returns to zero the frequency shifts back, and so does the servo motor.  In straight and level flight the ailerons are also calibrated to be straight and level.

Now, if this capacitor's insulator was cracked, and in the vibration of flight the plates touched (they are only 0.012 inches apart), then the capacitance would go to zero and a turn would be commanded at all times.  Not particularly helpful!

It turns out that Cardwell Condenser is still in business, and still manufactures an item that is very similar.  I took several photographs of the condenser in order to calculate the capacitance and find matching sizes, and I end up with a maximum capacitance that is very close to the rating of a Caldwell Condenser Johnson Series M160-110-1 type capacitor.

And then I found a surplus equipment dealer that sold me two of them new in a box for $8.00 each!  Just in case...