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237209
231814
245894
238890
214234
235176
243394
Fig. 7.2.
Set the machine for recording. Connect an audio
generator to the pick-up input and adjust the fre-
quency to 400 Hz. Set input level LINE knob for 0 dB
reading on the indicator. Reduce the generator out-
put voltage by 30 dB and record at several frequen-
cies within the specified range, ensuring that the
signal level is same at all frequencies.
Rewind and play back from the PREAMP output the
400 Hz signal which is to be taken as a 0 dB refe-
rence. Then check that the signal levels at the other
frequencies lie within the tolerance range.
If the signal amplitudes in the treble range are too
high or too low, increase or decrease the bias. See
6.5.1.
6.8 Distortion Check
Using a new tape, record a 400 Hz signal at 0 dB
reading on the record level meter.
Check that the distortion is 3-5 °/o. If there is too
much distortion, reduce the record current. See para-
graph 6.6.1. If the distortion is less than 3 °/o, the
record current can be increased.
6.9 Output Amplifier
Connect an audio generator set at 1000 Hz to the
PICK UP socket and an oscilloscope and a VTVM in
parallel with a 4 ohm 15 W resistor to EXT SPKR out-
put. Set VOLUME to maximum, function selector to
AMP and speaker selector to EXT. Turn bass- and
treble controls to their centre position.
6.9.1 Output Symmetry
Increase the input signal level until clipping of the
waveform can be observed on the oscilloscope. Ad-
just R127 (Balance Adj.) for equal clipping of positive
and negative peaks. Reduce the input signal below
the clipping level. The output voltage should then be
6.3 V (10 W in 4 ohm).
6.9.2 Quiescent Current
Connect a mA-meter in series with R133 in the out-
put amplifier and set VOLUME to minimum. Adjust
R129 (Current Adj.) for a meter reading of 30 mA.
Alternatively adjust for a voltage drop of 16 mV
across R133.
7.0 Special Versions
7.1 Model 1 5 SL
Model 15 SL differ from the standard Version by an
extra playback amplifier that enables simultaneous
playback of the two tracks.
In order to prevent accidental erasure of master pro-
gramme on upper track, when the track selector is
in STUDENT position (recording on lower, playback
from upper track), the track selector is locked when
the function selector is in PLAYBACK. The interlock
is released by momentarily setting the function switch
to AMPLIFIER.
7.1.1 Playback Amplifier
The amplifier is assembled on a separate board and
comprises transistors Q301—Q305 with associated
components. Playback equalization is obtained with
frequency dependent feedback from Q303 to Q301.
Playback volume is adjustable with R12.
The amplifier is via the track selector supplied with
signals from the head half that is not used for record-
ing (free head). Signals from the output of power
amplifier and additional amplifier are mixed in resis-
tors R322 and R323 and fed to pin 4 in the DIN-
socket MIC.
7.1.2 Mechanical Interlock
Fig. 7.1 shows the track selector disc 237209 when
locked in position STUDENT.
The function selector arm 245894 is in position
PLAYBACK, and the locking arm is engaged with
track selector disc by means of spring 238890.
Fig. 7.2 shows the situation when the locking arm has
been released. When the function selector is set to
AMPLIFIER the locking arm is pushed away and the
catch on the locking arm is disengaged from the
track selector disc. The track selector is now free to
be set to any position.
237209
231814
245894
238890
214234
235176
243394
Fig. 7.1.
29
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