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CH2
CH1
EQUALIZING
AMPLIFIER
5.4.2 Equalizing Network in Playback Mode.
A simplified circuit diagram for the equalizing ampli-
fier in playback mode is shown in fig. 5.3. The nega-
tive feedback circuit from emitter of Q203 to emitter
of Q201 gives the playback frequency curves shown
in fig. 5.2b. The capacitor C210 determines the play-
back curve in the range 70-300 Hz. The series
resonance circuit L201, C204, R208, R209 deter-
mines the frequency response in the upper frequency
range. The components mentioned above refer to
71/2 ips. See chapter 8.1 on modifications for Low
Noise tape. The equalizer switch is linked with the
speed selector, and correct equalizing at the various
speeds is obtained automatically when the speed is
changed. At 33/4 ips, a speed correction is obtained
by means of resistors R239 and R212 and capacitor
C205. At 17/8 ips, speed correction is obtained by
R239, R212, R214, C206 and C210. The output signal
is fed via C213 to PLAYBACK BOOSTER.
5.5 PLAYBACK BOOSTER
The playback booster is a part of the preamplifier
and consists of transistor Q204 with associated com-
ponents, which gives a flat frequency response from
30-20 000 Hz. The amplification is 6 times, giving a
signal level of 750 mV at the PREAMP output during
playback of a tape recorded at normal level.
5.6 INDICATOR AMPLIFIER
The indicator amplifier comprises transistor Q205
with associated components. Signal is applied to the
base of the transistor from the emitter of Q203, via
C216 and the adjustment potentiometer R225. The
indicator is connected in series with the emitter re-
sistor for the transistor. The transistor is biased so
as to conduct during positive halfcycles of the signal
only. Capacitor C218 will charge rapidly while Q205
conducts. If the input level decreases, the transistor
will cut off, and C218 will discharge through 8228 at
a relatively slow rate.
5.7 OSCILLATOR
Transistors 0206 and 0207 form a push-pull oscillator
identified in the main, circuit diagram, as ERASE
OSCILLATOR, which generates erase and bias vol-
tage at a frequency of 85.5 kHz. The frequency is
mainly determined by the erase head and the induc-
tance of T201 in parallel with the adjustment capa-
citor C221. The purpose of C229 is to protect the
transistors Q206 and Q207, when shorting the bias
head for measurement of record current.
5.8 OUTPUT AMPLIFIER
The output amplifier is identified in the main circuit
diagram as 10 WATT POWER AMPLIFIER and com-
prises transistors 0104 through Q109 with associated
components. The signal is applied via potentiometer
VOLUME and is amplified in Q104 and Q105. The
balance point for Q105 can be set by means of R127.
The quiescent current in the phase-shifter transistors
0106 and Q107 is set by means of R129.
5.9 POWER SUPPLY AND VOLTAGE REGULATOR
The mains voltage is applied to the primary of the
mains transformer via the ON/OFF switch. The DC-
voltage is fed from the fullwave rectifier to the voltage
regulator comprising transistors Q110 through Q112
with the zener diode D101 as voltage reference.
The motor is connected on the primary side of the
transformer via a microswitch which is operated by
means of the tape motion lever when tape is inserted
in the tape path. The combination R10, C3 reduces
noise from the microswitch.
5.10 HEADS AND TRACK SELECTOR
The four track models of series 14 and 15 have a
track selector that enables recording and playback
on upper or lower track while the vacant head half is
connected directly to the output FREE HEAD.
On two track models, recording and playback always
take place on upper track while FREE HEAD is con-
nected to lower head half. The connection is indi-
cated within dotted lines in the lower left corner of
the main circuit diagram. Simplified diagrams of the
head connection for the three positions of the track
selector are given below.
5.10.1 Position DUO
This position can not be used in record mode. In
playback, the two head halves are connected in
series as shown in fig. 5.4. Prerecorded stereo tape
can then be played back in mono. Furthermore, the
position DUO enables playback of commentary from
one track and background music from the other
track.
Fig. 5.4 Playback DUO.
25
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