88 



USRL TEST STATIONS 



No one is allowed to work on a pier alone when 

 weather conditions are hazardous. 



Electrical. The power is turned on about 3 hours 

 before any measurements are made so that thermal 

 equilibrium is reached to minimize any drift in fre- 

 quency or level. Check observations are always made 

 at the beginning of each day and, if necessary, re- 

 peated at intervals. The performance of each regu- 

 lated rectifier is checked by means of a built-in 

 metering circuit, which permits the observation of 

 output voltage and load current distribution among 

 various tubes. This is required frequently as faulty 

 tubes result in a nonuniform load distribution. 



The calibration of the 30A transmission measuring 

 set is described earlier in this section. 



The signal generator is adjusted for continuous- 

 wave, noise, pulse, or other types of tests. 



The wide-band receiving amplifier, detector, and 

 recorder are checked as a unit. Individual circuits are 

 tested only when the unit fails. To observe the over- 

 all performance and adjust the sensitivity, the oscil- 

 lator is set at 10 kc and adjusted to 130 db vs I0 -10 

 watt with the 30A set. The signal is fed through the 

 wide-band amplifier and the detector with zero gain, 

 and the recorder is adjusted to the same level entering 

 the amplifier. 



The resolution, stiffness, and over-shoot of the re- 

 corder are adjusted and the rate of response to chang- 

 ing levels observed. Any substance that increases the 

 friction between the sliding surfaces reduces the re- 

 sponse rate and may soon cause serious wear. 



The carrier frequency can be balanced out at the 

 first modulator of the detector circuit with the signal 

 generator set at c. The controls for balancing the 

 rejection circuit are adjusted until the recorder sig- 

 nal is at a minimum. 



In addition to these individual adjustments, it is 

 desirable to determine the overall gain of the test 

 system. The circuit and the levels used are shown in 

 Figure 28. A level of 160 db vs 10 _le watt is estab- 

 lished at the projector side of the impedance match- 

 ing coil with the 30A set connected to this coil 

 through a 20-db matching pad. The oscillator and 

 power amplifier gain controls are adjusted until the 

 correct power level is indicated on the set. Then the 

 oscillator frequency dial and recorder paper scale are 

 synchronized at c and a response is taken up to 150 

 kc. Since irregularities in the performance of any of 

 the component parts will show up in the record, the 

 system is considered to be in operating condition if 



-tETEP-ODVNE i3Q pa* POWER 



050LLATOR AMPLIFIER [ATTENUATOR 



IMPEDANCE 



WATCHING 



COIL 



SYSTEM TRANSMITTING CIRCUIT 



PlER CIRCUITS 



IMPEDANCE 



MATCHING 



RESISTANCE PAD 



(.20 DB LOSS) 



DB OIN ADJUSTMENT 



Figure 28. Arrangement for overall system check. Signal 

 levels are given in decibels versus 10" 1 " watt. 



the recorder curve is satisfactory in magnitude and 

 flatness. 



The test systems are completely overhauled after 

 a year's use. All tubes are tested; patch cords are 

 checked for continuity and leakage; coil and attenua- 

 tor characteristics are measured. Transmission and 

 leakage characteristics of all lines are checked. Poor 

 leakage-to-ground in coaxial lines may result from 

 foreign particles, and, if so, is usually corrected by 

 discharging a 500-/d condenser at 200 volts over the 

 path. After reassembly, individual circuits are 

 checked and adjusted, if necessary. 



Mechanical. It has been found advisable to estab- 

 lish certain routines in the maintenance of the me- 

 chanical equipment. Important groups of these are 

 the following: 



1. All electric contacts including plugs and jacks 

 are cleaned at regular intervals to forestall troubles 

 arising from dirt and corrosion. Rough or damaged 

 surfaces are refinished or replaced. 



2. The piers are examined periodically for settling 

 by placing levels both on and across the rails, which 

 were designed with bolts for adjustment. 



3. The test equipment, especially that used out- 

 doors, is protected from the weather as much as pos- 

 sible. This includes covering when not in use, keeping 

 pier booths and windows closed, and draining or 

 providing underwater storage for free-flooding de- 

 vices. 



4. All mechanical equipment, such as chain hoists, 

 rigging fixtures, etc., is regularly inspected, lubri- 

 cated, and stored under shelter during the winter. 



6.2.2 Practical Calibration Procedure 



CoiMPARisoN Tests 



A large part of the calibration work done by 

 USRL, particularly in the range from 15 c to 150 kc. 



