Nagra PORTABLE ANALOGUE AUDIO TAPE RECORDER Instruction Manual - page 41
In effect, the human ear cannot appreciate the absolute value of a high frequency sinusoidal
signal, but, on the other hand, it can determine the harmonic content of a rectangular signal.
In practice, this becomes a judgment of a tone and to render it as "hard" as possible.
Procedure
a.) Thread a tape onto the Nagra whose playback head has already been adjusted
b.) Press the button REF. GEN.
c.) Record and listen to the signal played back in the headphones ("Line and Phone" switch
on the position "Tape").
d.) Adjust the azimuth of the recording head to obtain a sound as rich in harmonics as
possible. The sound produced in the headphones should be the same in both positions
of the "Line and Phones" switch. Look for the two points on either side of the optimum,
which give a sound equally muffled, and adjust for the midpoint between them.
6.2. MAINTENANCE OF THE MOTOR COLLECTOR
The motor of the Nagra 4.2 has a collector in a special alloy, which is perfectly resistant to
corrosion. This ensures good operation, even under aggressive climates. The motor brushes are of
a graphite silver composition, and their pressure on the collector is ensured by gold plated beryllium
bronze springs.
The brushes wear by friction on the collector and the product of their wear forms a self-lubricating
layer called patina, necessary for the correct operation of the motor. The thickness of this patina
depends on the specific pressure of the brushes. If this is too low, the operation will be noisy
(squeaking). If it is too high, the insulation between the segments of the collector may become
bridged over, resulting in an increase of current consumption.
Remedy for Motor Noise
It should be noted that by motor noise, the normal noise of the motor due to the ball bearings and
the friction of the brushes on the collector segments should be discounted only a squeaking
produced by the vibration of the brushes should be taken into account. Experience has shown that
this can happen when the pressure of a brush falls below 12 grams. The remedy consists of an
increase of the pressure by stretching the brush springs. The normal value is 25 gr ±5 gr.
Metalization of the Collector
During the running in period, it is possible that the brushes have not taken on the exact form of the
collector. Local specific pressure can produce a very thick patina, which can short circuit the
segments of the collector. In the worst case, the “SPEED & POWER” indicator on the front panel
will warn the operator. It is recommended to check occasionally (every 100 hours of use) the no-
load current of the motor. To do this, place the meter in the "MOT" position. The meter then
measures the current taken by the motor. There is no special scale for this measurement, but the
lower middle scale graduated 0 to 2 V can be used arbitrarily. With no load, that is to say, without a
tape and with the pinch wheel just separated from the capstan, the needle should indicate between
0.2 and 0.3 V. If the needle indicates higher than 0.4 V, the collector probably requires cleaning.
Cleaning of the Collector
Open the Nagra and pull off the motor shielding (being careful not to deform it). Switch on the motor
and clean the collector by rubbing with a rag or absorbent cotton soaked in a solvent. The insulation
between the segments of the collector should be the same colour as that on the top of the collector.
If the conditions are extremely bad, a very fine abrasive polishing cloth may be used, but it is
essential not to use powdered abrasive, which could possibly cause damage within the motor itself.