Connect an audio generator across the resistor and adjust output at 1 kc/s to give 70
mV at the cathode follower. The frequency response obtained should coincide with
the graph shown on the back of the respective circuit diagram.
Where a considerable deviance exists, checks should be made on the voltage values
obtained within the circuit with special regard to the negative feedback loop which is
frequency conscious.
3.1.5 Bias adjustment
The procedure is described for one channel only. It is to be carried out in both
channels for achieving identical performance.
Connect a VTVM with a bias filter (described in 3.1.1.) in circuit to the output of the
playback amplifier. Press the record button, leaving the record level control closed,
and tune the filter for minimum indication on the VTVM. This external filter will not
be required on REVOX G-36 recorders already equipped with bias-traps. It must be
observed, however, that the residual bias remains below the level specified under
3.1.1.
Where a filter is not available, the overall performance can not be tested while
recording, as bias leakage masks the audio signal. It will be necessary to take all
readings after first recording, rewinding and then playing the tape again.
Any alteration of the bias current changes the remanent tape flux and consequently
effects output level, frequency response and distortion. As no two heads can be made
to perform exactly alike, there is little value in measuring the bias current alone and
directly. In adjusting the bias current, all of these variables are taken into
consideration and the near optimum value has to be found first before the frequency
response tests can be commenced with.
The recommended procedure is to employ a long playing tape of compatible quality
and record a 1 kc/s signal approx. 3 db below full modulation at the 3 3/4 ips tape
speed. The bias should be adjusted with P 10 and P 11 (fig. 14) for channels I and II
respectively, for maximum audio output.
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