MA 2. The MA 2 circuit is shown in figure 202. The amplifier consists of three pento-des types RV 2.4 P 700, which make up two channels: the firing channel and the pro-tecting channel. The output of the microphone is fed to an amplifier pentode (34). The output of this amplifer is fed, through coupling transformer (2) to the two channels. Pen-tode (34) and its associated circuits make up the firing channel. Pentode (35) and its associated circuits make up the protective channel. The input circuit to (34) has a time-delay circuit of 1 to 2 seconds. The input circuit to (35) has a very short time-constant and is designed to handle steep fronts.

MA 2 Cam Switching System. The rate-of-change feature of the unit is provided by means of a motor-driven potentiometer on which is mounted a group of cam-switches. The potentiometer (8) is driven through an arc of 280 degrees by the armature (7) of the motor. In this arc the resistance of the potentiometer increases arithmetically from 150 to 2,000 Ohms. When the motor armature is energized, its field coil (9), its magnetic brake (Br) and the blocking relay (S) are also energized. The magnetic brake is designed to stop the motor instantly when de-energized, and the blocking relay (S) breaks the circuit to the detonator during motor running. The cam-switching assembly consists of two groups labelled "n1" and "n2" respectively. The switches labelled "n1" are closed im-mediately after the motor starts, and open immediately when the motor has driven the potentiometer back to the zero-position. The switches labelled "n2" are closed immedia-tely when the motor has driven the potentiometer back to the zero-position. The swit-ches labelled "n2" are closed immediately after the motor starts, but re-open again when the potentiometer has been driven to the 280 degree position.

Figure 202 – MA 2/MA 3 Unit Circuit

MA 2 Operation. When the hydrostatic clock (UES II) runs off, switches a-g and b-c close, putting the detonator (Z) into the circuit. A short time later, e-f closes and puts the 15-volt battery across the (+) and (-) terminals of the M 1 component through f1 and (4-5) of the ZK II mechanism. The M 1 runs through latitude adjustment and be-comes normally alive. A magnetic actuation closes switch I (needle switch), and switch II (Z switch) is closed due to relay action. Closure of switch II allows the battery to energize the trigger coil of the ZK II, figure 203. All "blind" actuations on the ZK II me-chanism are run off on magnetic actuation alone (11 maximum). After the last "blind" actuation, the ZK II breaks 3-1 and makes 1-2, putting the acoustic-triggering system into the circuit.

Figure 203 – MA 2/MA 3 Unit Circuit

Triggering. When the magnetic actuation occurs, closure of switch II allows the 15-volt battery to energize the microphone, and the heaters of the three pentodes. The acoustic component is now alive for the first acoustic impule, figure 204.

Figure 204 – MA 2/MA 3 Unit Circuit

Automatic Adjustment. If sound is at this point incident upon the microphone and it is of the proper frequency (100-200 cps.) the output of the amplifier stage (34) is fed through the coupling transformer (2) to the two channels. If the signal is of firing level, it is rectified by rectifier (19), passed through the time-delay circuit to the grid of pentode (36). The resultant increases in plate current of (36) will operate relay R1. Operation of R1 closes contacts r1a and r1b. Closure of r1a connects the plate of (35) to 96 volts through 100 K Ohme resistor (28). This current does not operate R2, but is designed to replace blocking-signal current which, by this time, would normally have been dissipated. Closure of r1b allows the 15-volt battery to energize the motor (7), its magnetic brake (Br) and the operating coil of relay (S), trough n2a. Closure of s1 makes a maintaining circuit for the motor through r1b. Opening s2 breaks the detonator lead to the battery. Closure s3 bridges the time-delay circuit in the input circuit to the firing tube. Thus, the motor dreives the potentiometer (8) as long as the circuit to the motor is maintained. The variation of the potentiometer resistance varies the grid potential of the grids of pentodes (34) and (36). When the plate current of (36) is reduced below the maintaining point of R1, r1b, breaks the motor circuit and the braking coil brings it to a stop immedia-tely. The system is capable of compensating for an initial sound level of 150 mv. or less.

Cam-switching. As soon as the motor starts, all switches "n" are switched over, figure 205.

n1a puts the 15-volt battery across the heater coil of the thermal delay switch T.

n1b arms the motor-resetting circuit which comes into play when f1 operates.

n1c connects the potentiometer to the (+) side of the 96-volt battery, thus causing battery drain only when the unit is live to acoustic actuation.

n2a arms the firing circuit, but, since s2 is open during this process, the mine cannot fire.

n2b shuts switch II of the M 1 unit.

Firing. If, after the adjustment has taken place, the sound level arises so that the plate current of (36) reaches the operating point of R1, R1 operates, closing r1b and since n2a, s2 and r2 are closed, the 15-volt battery fires the detonnator Z, figure 205.

Figure 205 – MA 2/MA 3 Unit Circuit

Blocking. If at any time during the acoustically active period, the sound level rises too rapidly and actuates the blocking circuit of pentode (35), operation of R2 opens r2 and breaks the firing circuit.

Re-stetting. If satisfactory acoustic actuation does not take place after acoustic ad-justment occurs, heating of the thermal delay switch T causes its contact f1 to switch over and allow the 15-volt battery to energize the motor and associated gear through n1b. This will continue until the motor reaches the 0° setting, at which point the cam switches return to normal and the motor is de-energized. In addition, this will occur if the initial sound level incident on the microphone is higher than 150 mv. In this case, rotation of the potentiometer is incapable of reducing the plate current of pentode (36) below the maintaining point of R1. Therefore, the motor current will be maintained through r1b until the potentiometer is returned to normal. At this point it will stop unless the magne-tic component is still in an actuated condition. If this is the case, the triggering will go through another cycle.