Copied from D.R.Osborne's Training Note Book.

Overall dimensions with a spool box removed are 2'4" x 16" x 20" high weight 2cwt.

BATTERY SWITCH is a double pole type and closes the circuits to both batteries also single pole switches control fork and mike current independently.

LINE RELAY is mounted on rear of control pane, the coil of the relay being connected across the mike battery when the main battery switch is on . Thus if the batteries (of the line) are connected the wrong way round, the relay will not operate and no line current is connected, this is in order to protect the electrolytic condensers.

TELEPHONE JACK permits communication with AP’s (Advance Posts) on radio link.

MOTOR GENERATOR provision is made for supplying mike current via this method and is conveyed by means of the 3 pin mike plug, contact No 3 supplying LT (Low Tension) for the generator. The wiring of the instrument at present make it impossible to use a generator and line batteries must be used.

LINE RESISTORS each resistance consists of a ganged pair of variable resistors, there is one ganged pair for each mike line in order to adjust the current and also maintain symmetry with reference to earth of the mike lines. The maximum value of each resistance is 800W Thus a pair will have a resistance of 1600W for each mike line.

ATTENUATORS are used in order to maintain a constant resistance across the transformer windings and at the same time make it possible to vary the sensitivity of each string independently. The mike battery is connected across the one end of the attenuator so that the potential on the condensers is reduced and so also the direct leakage through the condensers and transformers is reduced.

SAFETY SWITCH operated by panel cover cuts off the galvo strings by breaking 6 point relay circuit thus preventing damage to strings will making adjustments, switch is open when lid is open.

GALVO TEST pressing the small knob causes 0.5ma to flow through the selected string and the sensitivity noted.

TWO CONTROL SWITCHES one of 3 position type (START, STOP, RUN MOTOR) controls the camera motor, clutch, galvo lamp and six point relay. The other switch (lamp, lamp and relay) in one position closes the lamp circuit only and in the other closes the lamp and the six point relay for testing purposes only. When used in AP control the switches should be left in mid position.

WARNING LAMP is a 2.5v 0.2a bulb with a 60W resistor in series and indicates that the AP relay has opened and the film should be cut.

TRANSFORMERS have to respond to frequencies of from 2-30cps and the large inductance required to do this is obtained by using nickel iron alloy cores Electrolytic condensers are placed in series with the transformers primary windings so that DC will not pass through them and so reduce the inductance, in actual fact the leakage through the condensers is still about 2ma maximum . At the more sensitive settings of the attenuator this leakage is reduced still further. The condensers are 250m fd. 100v working electrolytics.

EINTHOVEN GALAVNOMETER (Harp) has six strings of 0.02mm copper wire suspended under slight tension in the 1mm gap of 16,000 lines per sq. cm. Permanent magnet. The gap is inclined at 80^o to the plane containing the strings so the strings are deflected in a plane 10^o to their common plane and on large impulses can pass each other without entangling . Strings 0.5mm apart.

OPTICAL PATH includes two reflections between the strings and camera film the length thus giving a magnification of 18 times. A third mirror reflects the string image onto a ground glass viewing screen. A left to right laterally inverted image is obtained on the screen. On the record No 6 string appears at the top, the top being in the direction of the string kick.

TIME MOTOR & LAMP are situated on a single casting and held in position by 3 screws the electrical connections are via a 4 pin plug.

THE LAMP is a 12v double filament type with a special base which ensures that the lamp is in correct position for sharp focus.

TIME MOTOR is a modified clock motor and is energised by 12v DC interrupted by the contacts on a tuned fork. The time motor is geared to a vane wheel with ten vanes of three different widths, these interrupt the light to the galvanometer and produce shadows on the film at intervals of ten, fifty and one hundred sound metres.

FORK frequency is 50.64cps, vibration is maintained by means of a pair of contacts on one of the tines and interrupts a coil supplied by the 12v battery. A further pair of contacts, on the opposite tine are in series with the time motor and battery. Condenser and resistors are fitted to the fork contacts to prevent interference with radio.

CAMERA MOTOR supplies power to the three rollers of the paper drive i.e. fixing tank, and two in the developing tank. The motor is a 1/16 HP 12v shunt wound motor of 3500 rpm. Coupled through worm reduction gear.

SPEED CONTROL consists of resistances in both field and armature circuits. The armature resistor is a permanent one with a shorting switch, the field resistance is a variable in series with the field winding. The range of paper speeds varies from 4 to 7 cms per sec.

CLUTCH RELAY is energised by (1) the closing of the AP Relay or placing motor switch to "Start". The circuits closed by the relay contacts are by contact "A" the coil of the motor relay . By contact "B" the clutch , lamp and 6 point relay. Also when the motor relay is closed the double contact "B" causes the warning lamp to light.

MOTOR RELAY is energised by the closing of clutch relay and the circuits closed by the motor relay are :- By contact "C" an additional circuit is completed to the coil of the relay so that it remains closed after clutch relay has opened. By contact "D" the camera motor circuit.

FOCUSING STRINGS Adjust the lens mounted on the side of the harp further form the time motor so that strings are sharply focused. Focus vanes by lens mounted on harp nearest to motor.