I've extracted a reasonably complete description and schematic from the document Das Gerät A4 Baureihe B Gerätbeschreibung (The A4 Device Series B Device Description) published by the Oberkommando des Heeres, Heereswaffenamt, Amtsgruppe für Entwicklung und Prüfung (Army High Command, Army Weapons Office, Office Group for Development and Testing), Berlin, 1 February 1945.
I'm quoting using OCR and machine translation from German, edited for typos and awkward translation of engineering jargon.
Cutting to the chase, the Mischgerät schematic I sought is Figure 79, below.
Also, as user @Forward Ed noted in SE Engineering, the digipeer.de site hosts a number of digitized copies of A4 design drawings, including the basic circuit diagram. While it would ideal to order copies, I haven't yet figured out how to do so on the Deutsches Museum site, so I've zoomed the on-line copy and stitched together a reasonably high resolution example. A low resolution example is at the bottom of this answer.
(from page 173, Section 18, "Device Control")
Similar to a projectile, the missile is held arrow-stable during its flight not by spin, but by fixed fins. However, since the device can be forced out of its flight position by external influences despite its arrow stability, which also results in a change in the direction of the drive in relation to its desired flight position, and because it has to fly through a curve in a certain direction during the flight, the device must [also] be controlled by additional control elements.
In the normal case, the control is carried out by a three-axis control system installed on board. For the special case with remote control, a guidance beam radio receiver is also available, which is connected to a guidance beam on the azimuth of the target during the propulsion part of the trajectory.
(from page 173, Section 181, "The Control System")
The task of the A4's control system (Fig 80) is to force the device to follow its prescribed path while it is being driven and to avoid oscillating and rolling movements. After motor burnout during the free flight path, control is switched off and the device flies on like a bullet.
The control system consists of the following parts:
- 1 gyroscope D
- 1 gyroscope EA
- 1 command control battery
- 1 mixer
- 4 power steering engines
- 2 rudder drive motors (together called rudder drive or also trim rudder spreader)
- 4 graphite vanes, 2 of which (vanes I and III) with a potentiometer arrangement for actuating the rudder drive motors
- 4 rudders (2 roll rudders and 2 trim rudders)
- 4 rudder trim pots
Every steering action on the A4 device causes a turning movement around the center of gravity. All possible rotational movements can be represented by rotations around three mutually perpendicular axes. These are named as follows on the A4 (Fig. 84):
A - axis or roll axis is the longitudinal axis of the device.
E - axis or yaw axis is the straight line running parallel to the axis of rudders I and III through the center of gravity.
D - axis or pitch axis is the straight line running parallel to the rudder axis II and IV through the center of gravity. It is also perpendicular to A and E.
The task of the control system is to prevent any unwanted rotation around the A (roll) axis and around the E (yaw) and D (pitch) axes.
(from page 181, Sub-section 8, "The Mixer Device", Figure 79)
(The active stabilization problem:)
In addition to the command voltages of the A and E potentiometers, the
pressure pieces of fins I and III also have to react to the voltages
of the [radio] beacon device if necessary. Mixing these 3 command
voltages into 2 control currents is one task of the mixer.
However, the main task of the mixer is as follows:
If the command voltages of the D [pitch], E [yaw] and A [roll]
potentiometers corresponding to the incorrect angular position of the
device were given directly to the steering gear as control currents,
the following picture would result (Fig. 82, case a).
As long as the tip is on the left when the unit oscillates, the
steering gear receives a command current that is proportional to this
right position. The steering gear then runs to the right at more or
less high running speed in order to bring the unit back into the
target position via the pressure piece (ie. a graphite vane or fin
rudder). The unit also reacts to this pressure piece movement that has
meanwhile taken place and returns to its zero position.
During the entire time that the tip of the unit was to the left of the
zero position, however, the steering gear constantly received a
command to continue coasting. The unit has now probably returned to
its zero position, but the thrust pads are still on the right. The
consequence of this is that the unit now not only swings over the zero
position to the other side as a result of its momentum, but is even
supported in it by the pressure pieces that have run out.
During the first period, in which the tip is now on the right, the
pressure pads are on the wrong side for a long time, as long as the
steering gear needs to pull the pressure pads back and turn to the
other side as a result of the new command. In this way, the deflection
of the tip increases with every change of direction, and the
oscillation builds up. Care must now be taken to ensure that the
pressure pieces return to the zero position before the unit tip
returns to this position.
(The active stabilization solution:)
It must now be ensured that the pressure pieces [control actuators]
return to the zero position before the unit [missile] tip returns to
it. The steering gear must therefore receive the command to run back
some time beforehand. Since these are oscillations, i.e. sinusoidal
processes, the required lead of the rudder current before the unit
oscillation is called a leading phase shift (Fig. 82, case b).
The advance of the rudder current is generated by sending the command
picked off at the direction indicator through an electrical network of
resistors and capacitors. An amplifier picks up the new command
obtained in this way and sends it amplified to the steering machine.
The processes taking place in the network of resistors and capacitors
are also referred to as double electrical differentiation of the
It is therefore necessary to measure not only the misalignment, but
also the angular velocity and the angular acceleration with which the
device rotates from the target shot direction into the misalignment.
The capacitors are used to measure this angular velocity. A capacitor
consists of two mutually insulated conductive layers.
As long as the resistance shown in the picture is not changed, the
voltmeter on the right will not show any voltage. But as soon as the
voltage is changed on the left, the voltmeter deflects on the right,
which is a function of this voltage. In the mixing device (in a
simplified representation) the command voltage from a straightener is
located on the left-hand covering.
If this voltage changes, ie. the angle changes continuously, ie. if
there is an angular velocity, a voltage corresponding to the angular
velocity is generated on the right-hand pad, which is used for
damping. This procedure is called 'Electrical Differentiation'. In the
mixing device, the voltages generated from the angular velocity are
differentiated again by a further capacitor circuit and the angular
acceleration is also used for damping.
(Basic Circuit Diagram)