Royer Oscillator Input Stage for Piper Omni-Locator Coupler
I managed to locate a service manual with a schematic and I am working my way through it. The first portion is the input oscillator. This is used to excite the linear variable displacement transformers used in the heading indicator.
The 5 kHz square wave it produces is also used later in circuitry to detect phase differences and so forth.
This appears to be a version of a Royer oscillator except that the frequency is not determined by the saturation of the core transformer but rather by L5, C1, and L4. Per a service manual for a similar autopilot this is about 5 kHz.
When power is applied one of the transistors will conduct first (like set by R2) and this will provide a spike of current into one side of the input coils L1 and L2. This is then reflected into L4 and the oscillations are filter by L5 and C1 to provide an oscillating input into Q1 and Q2. Once the voltage drops (from the coil saturating to ground) the other transistor will turn on the cycle repeats.
D1 and D2 snub the input voltages into the base of Q1 and Q2. D3 is a zener diode at 9.1 volts. The resistor network and Q3 form a voltage regulator to ensure that a consistent voltage drop is present between the supply and the center of the coils. Q3 is shown on the schematic biased in reverse active mode and this might not have been as destructive to an old germanium transistor as it is to a modern silicon one. I've simulated this in the forward bias position and it seems to work fine.
L3 and R3 are there as a dummy load for the simulation. It appears to maintain a steady frequency, a nice crisp square wave, and a constant amplitude across a wide range.
Q1 and Q2 are shown on the schematic as a 2N1192 and the datasheet is still available for this. A 2N2907 is a good fit.
Now, I would use something like a 555 timer with a driving output stage to do this and have it all driven by a proper voltage regulator. But this is quite clever and has very few components.
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