a relay coi;„ The relay completes the 110- volt AC circuit to a female 

 receptacle ' ocated on the rear of the switching unit chassis and marked 



360 The re ay also completes the 110-volt AC circuit to a cord which is 

 plugged int the strip at the rear of the sound level recorder directly 

 below the a tomatic switching unit,, This receptacle supplies power to 

 the sound 1 ;vel recorder motor and to the AC relay controlling the trans- 

 fer of power to the time indexing system, A second cable bridges across 

 the twist~lock receptacle cable at the rear of the sound level recorder 

 and carries the AC current to the motor and AC control relay of the 

 graphic level recorder in Rack 3o 



37 „ Another cable at the back of the switching unit chassis (P~601 in Fig, 

 18) supplies 110 volts AC to the automatic switching unit ivhich has a self- 

 contained power supply built on its chassis The power switch (S-602) is 

 on the front panel (Fig„ 17) <> A voltage regulator tube prevents fluctuations 

 in voltage from firing the unit, 



38o The signal input comes from the system bus and is led in through a. 

 terminal strip marked ' on the right rear corner of the chassis (Figo 

 18),, A control, located on the left of the front panel (Fig, 17), 

 controls the level of the signal input into the unit. Normally, this 

 control is placed close to its iraximum settings 



Sensitivity Adjustment 



'39o The sensitivity of the Automatic Switching Unit is adjusted as follows <> 

 After a 15-minute warm-up period, the control (Fig, 17) is 



rotated clockwise until the thy rat r on fires,, It is then turned back very 

 slightly and the unit is reset, The maximum sensitivity will be just below 

 the point where self-noise fires the thyratron and at the point where the 

 operator can reset the unit without reducing the sensitivity further <, The 

 sensitivity will probably be close to 9 on the scale -when this adjustment is 

 achieved „ 



40. To reset the unit after it has fired, push in the button 



on the front panel (Fig» 17) and hold it in for about five seconds to 

 deionize the 2050 tube If the tube fires again when the reset button ia 

 released, reduce the sensitivity setting slightly and repeat the resetting „ 



41c The automatic switching unit is bolted to the front panel and can be 



- 11 - 



