means of checking the level applied to the recording head of the Mirrophone, 

 In the case of the Caltron recorder, the eye sensitivity can be adjusted., 

 Access to this adjustment is obtained by removing the front panel of the 

 amplifier section, The sensitivity contx-oi for the eye will be found directly 

 beneath the For best indication the eye should just close at 



the peak of a signal — regardless of the possibility that the 3-kc tone 

 pulse may cause the eye to overlap In the case of the 3-kc tone pulsej, 

 quantity not quality is the determinant „ Note that on the W, Eo Mirrophpne 

 there is no eye sensitivity adjustment, . 



90 On the same panel with the tuning-eye indicator, just beneath the tube, 

 a jack receptacle -will be found marked „ This jack is across the 

 grid circuit of the output tube of the magnetic tape recorder amplifier to 

 make signal level measurements possible at this point., Only a high I mpedance 

 Instrument shou ld _be_pl agg ed into this ,'jacks if a low impedance instrument is 

 used the Mirrophone will fail to record properly,, 



91o Continuous operation of the magnetic tape recorders will cause the pole 

 pieces in the recording head to become dull as a result of constant friction 

 against the tape. If they become extremely dull, the tape signal-to-noise 

 ratio as well as the high-frequency response will be considerably decreased. 

 For this reason,, the second Mirrophone is not put into operation until a 

 signal arrives, 



92,. When the station is first installed, the operator should record pro- 

 gressive frequencies at intervals between 20 cycles and 5,000 cycles on the 

 tape by connecting an audio signal generator into the Mirrophone input, 

 maintaining the same recording level for each frequency, and keeping a log 

 of all control settings , This test is important., After recording these 

 frequencies, connect an output meter across the Mirrophone output, switch 

 the Mirrophone to , and note on the output meter the level for each 

 frequency. These values can be plotted against frequency on logarithmic 

 graph paper if desired, and the curve retained for future reference, 



93, This response test should be repeated every two weeks, using the same 

 levels as in the original test. Compare the values obtained with the original 

 values. They will probably be somewhat lower than the original, giving an 

 indication of the condition of the pole pieces. If the response at 3-kc 

 has dropped at least 10 db, the pole pieces are dull and should be replaced. 

 The U, S, Navy Underwater Sound Laboratory at" New London, Connecticut „ should 

 be notified, whereupon a new recording head will be shipped. Upon receipt 



of this, remove the head containing the- dull pole pieces and return it to 

 USN USL, The pole pieces will appear flat. They are however ground to a 

 araall critical angle, which the pole piece makes with the tape. 



Tape Repair and Upkeep 



94, The most vulnerable part of the magnetic tape recorder is the tape 



22 



