Nagra PORTABLE ANALOGUE AUDIO TAPE RECORDER Instruction Manual - page 40
7.0 CALIBRATION AND CARE
HEIGHT OF THE NEOPILOT HEAD
The middle head of the Nagra 4.2 is used to record and playback the pilot signal. Its azimuth is not
critical, but its height should be correct. The cam of this head does not vary the angle, but only its
height. Before adjusting the azimuth, check and adjust (if need be) this head. Adjust the height so
that the tape is exactly in the middle of the head. The eye is sufficiently accurate for this adjustment.
AZIMUTH ADJUSTMENT OF THE PLAYBACK HEAD.
Playback a Standard Tape at 7½ ips. Place the "Line and Phones" switch in the "Direct" position.
The modulometer will thus indicate the playback level. Check that the microphone potentiometers
are in their extreme anti-clockwise position, and adjust the "Line and Playback" potentiometer to
give a convenient playback level. Standard Tapes are generally recorded at between -10 and -20
dB, so that, in general, it will be necessary to put this potentiometer in the maximum position, or
nearly so, to have a playback level of about -10 dB.
Introduce the key into the pinion of the playback head and find the maximum playback level. After
this, look to left and right for the points where the signal level is lowered by 1 to 2 dB, and place the
pinion midway between these points.
A correct adjustment corresponds equally to a stable playback. Errors due to "sabring" of the tape
are hardly perceptible at the top of the curve, and only become important on the sides.
Once the playback head has been correctly adjusted for azimuth it should not be touched again.
AZIMUTH ADJUSTMENT OF THE RECORDING HEAD.
Two methods are possible:
1. Standard Method
To use this method, it is necessary to have an audio frequency generator, which can give 1,
3, 10 and 15 kHz and an AF voltmeter or an oscilloscope. It is possible to use a second
Nagra as a voltmeter, by introducing the output signal of the recorder to be adjusted into the
line input of the second one, and using its modulometer as a voltmeter.
Procedure:
a.) Introduce a signal from the generator into the line input of the Nagra, and adjust the level
to -15 dB on the modulometer.
b.) Connect the voltmeter to the line output. The signal to be measured will be approximately
0.8 V R.M.S. The "Line and Phone" switch should be put into the position "Tape".
c.) Thread up a tape and record. The played back signal should be observed on the
voltmeter.
d.) Start with 1 kHz and increase the frequency. When the playback signal starts to be
reduced in level by several dBs, adjust the record head. It should be possible to arrive at
15 kHz, always looking for the midpoint between two of equal attenuation.
2. The "Rectangular 1000" method
Whilst the classic method necessitates bulky instruments, the "Rectangular 1000" method,
equally precise, can be carried out with the aid of the reference generator and good quality
headphones. The ear serves in this case as detector.