Nagra PORTABLE ANALOGUE AUDIO TAPE RECORDER Instruction Manual - page 36

MAGNETIC HEADS 

There are four tape heads on the NAGRA 4.2, these being ERASE, RECORD, PILOT and 

PLAYBACK (see "CONTROLS at the front of this manual for their location). 

Contact between the magnetic head and the tape must be perfect. Some tapes leave deposits on 

the heads. Fortunately, the deposits are clearly visible. A dirty playback head gives a muffled 

sound, lacking high frequencies. If the high frequencies come and go rapidly (one to ten times per 

second), the azimuth needs adjusting. A dirty recording head will record at a low level, and the 

sound will be distorted. Under the same conditions an erase head will give poor erasing. To remove 

the deposit, it is necessary to soften it with the aid of a solvent. The simplest method is to take 

some absorbent cotton or a rag, soaked in alcohol, or even water and to lightly rub. Chlorinated 

solvents, such as trichlorethylene, should be used with care, as it is possible they may lightly attack 

the resins used in the construction of the heads. 

ADJUSTEMENT OF HEAD AZIMUTH 

Theory 

The recording and the playback of a magnetic tape is a function of gaps in the recording and 

playback heads. These gaps should make a certain angle, arbitrary in itself, with the tape, but which 

should be the same for recording and for playback. Any angular difference between the recording 

and playback heads will produce a loss of level. This phenomenon becomes more important as the 

wave length (that is to say the ratio between the tape speed and the recorded signal frequency) 

becomes short. 

Practically, a faulty azimuth gives muffled recordings without high frequencies. To ensure the 

interchangeability of tapes, the azimuth angle has been standardized: the angle  between the gap 

and the tape should be 90

?

. Special recorders, whose heads have been optically aligned, have 

been constructed in order to produce Standard Tapes whose purpose is to permit the adjustment of 

the azimuth on ordinary tape recorders. It is to be noted that if a recorder has been used for 

recording with a badly adjusted azimuth, it is perfectly possible to save the recordings: it is sufficient 

to adjust the playback head in consequence. This can be done by means of the ear, by orienting 

the playback head to obtain the sound, which is richest in high frequencies. This method is evidently 

also applicable to tapes, which have been deformed by a faulty spooling, or by climatic conditions. If 

the tape is "sabred", or curved the notion of azimuth becomes delicate, and depends upon the 

relative positions of the heads and the guides. Parenthetically, it is these problems, which limit the 

use of very low speeds, below 7½ ips, because it is difficult to ensure a sufficiently precise azimuth 

unless only a narrow track is used. In effect, the error tolerance of the azimuth increases as the 

width of the track decreases. 

In reducing the width of the track, so is the signal-to-noise ratio reduced. Thus the solution of low 

speed and narrow track is, above all, used nowadays for amateur machines. 

Variation of the High Frequency Level with Azimuth Error 

When the azimuth is very slowly adjusted past the optimum position, the high frequencies are 

reduced at first very slowly, then their attenuation is accelerated the  further the adjustment is made 

from the correct point. A curve, which represents this attenuation as a function of the angle of error, 

has a rounded summit, and sides more and more steep. 

This is important, because if the azimuth is adjusted simply in looking for a maximum, it is quite 

possible that instead of being at the summit of the curve-that is to say at the optimum point-it could 

be at one side or the other. If the errors of the playback head and the recording head are additive, 

the tape could be recorded out of tolerance. The recording head is adjusted by referring to the 

playback head. Thus the azimuth of the recording head has the sum of the errors: that which is 

produced in adjusting the recording head, and the playback head. 

It is therefore important that the azimuths should be adjusted as closely as possible to the top of the 

curve. This is possible by looking for two points about the maximum, which correspond to a certain 

attenuation of the high frequencies, and then adjusting to the midpoint.