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Adjustment of Variable Resistors
R 40 Automatic Recording Level Control
Adjustment
Connect an audio-frequency generator across the contacts 1 and 2 of the socket marked £ Q
and feed a signal of 1,000 cps and 10 mV. Then connect an audio-frequency vacuum tube volt-
meter across the contacts 3 and 2 of the socket marked T J . Set the switch of the automatic
recording level control at its "On" position and depress the recording key and the start key.
Adjust R 40 in such a manner that the vacuum tube voltmeter reads 5 volts. Thereafter check
the automatic recording level control.
Check
In order to check the automatic recording level control feed a signal of 3 mV into the circuit
described above, then increase it to 200 mV (35 db). The resulting variation of the VTVM reading
must not exceed + 1.5 db.
When the voltage is decreased from 200 mV to 3 mV, the time constant must amount to approx-
imately 3 minutes, i. e. the output voltage of 5 volts must only be attained approximately 3 minutes
after the decrease.
R 36 Recording Level Indication
Connect the audio-frequency generator across the contacts 1 and 2 of the socket marked £ Q
and feed a signal of 1,000 cps and 10 mV.
Connect an audio-frequency VTVM across the contacts 3 and 2 of the socket marked "D". Depress
the recording key and the start key, and switch off the automatic recording level control. Adjust
the "Level" control for a VTVM reading of 5 volts.
Thereafter adjust R 36 in such a manner that the pointer of the VU meter stands on the mark
between the green and the red bands of the scale.
R 38 Bias
The ultrasonic bias has some influence on the responce curve of the recorder in the range
between 10 and 16 kc/sec. The voltage given below is an average. Final adjustment of the bias
must only be effected after the frequency response curve has been checked.
Connect an audio-frequency VTVM through a voltage divider (100 kiloohms + 1 kiloohm) to point
A according to Fig. 1. Then depress the recording key and the start key.
Adjust R38 for a VTVM reading of 240 millivolts.
R 44 Push-Pull Output Stage
Connect a milliammeter with an impedance of 5 ohms and a range of 60 mA to point C according
to the circuit diagram. Depress the start key, set the "Level" control at zero and adjust R 44 for
a current of 22 mA.
If a milliammeter of the proper type is not at hand, this adjustment can be made with the aid of
an oscilloscope. For this purpose feed a signal of 1,000 cps and approximately 1 mV across the
terminals of the recording/playback head and connect the oscilloscope in parallel with the loud-
speaker (the loudspeaker must not be switched off). Turn up the "Level" control one third and
adjust the potentiometer R 44 so that the oscilloscope indicates a maximum of distortion. Then
turn back R 44 to a point at which the distortion is just eliminated. Leave this setting of R 44
undisturbed.
Conversion for 60-cps-Mains
In order to convert the recorder from 50 cps to 60 cps or vice versa, the motor pulley must be
exchanged.
Hinging Out the Amplifier (see Fig. 2)
In order to hinge out the amplifier, screw off the bottom plate, remove the screw E and release
the pin F by pushing down the angular locking member.
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