Headphone Amplif Ier - Dual C 839 RC Service Manual

Functional description AF section
Playback
The voltage from the magnetic head (approximately 3001/V when
playing back the DIN reference level) is amplified by low-noise
transistor T 4010. L 4002 in the base circuit serves as an RF trap.
During operation with fade-edit, no stray RF signals should
reach the playback amplifier. The playback amplifier is formed
by IC 4003. Equalisation is performed by negative feedback net-
work R 4088, C 4034, R 4049 and C 4033 constant at 70 ps.
When switch S 5 is in position 1 or 2, R 4087 and C 4035 are
connected via diode D 4010. The time-constant increases by
50 so that 120ps are now available.
The playback level when playing back the Dolby reference level
is set for each magnetic head with the aid of trimmers R 4052
and R 4054. This setting ensures that the Dolby processor func-
tions perfectly. The Dolby unit is a self-aligned component.
L 4201 (for filtering the high frequency) and L 4202 (alignment
to 19 kHz) must not be adjusted. (This is carried out at the
factory). The output signal (Stp. 3) is supplied to the output
jacks, the display amplifier and headphone amplifier via relay
contact. The maximum output level of 580 mV may be reduced
by means of trimmer R 4035.
Muting circuit
In order to prevent starting and switching noises from reaching
the output jacks, the device is provided with two separate muting
circuits. When the unit is switched on, capacitor C 4052 ensures
that transistor T 4006 remains blocked until the voltage is in-
creased. The reed relay does not respond.
1.) When the unit is in normal condition, a high signal is applied
to the output of the muting circuit. This signal is applied to
the base of T 4007 which drives, blocking T 4006. The relay
is unable to operate. Simultaneously, the muting signal is
applied to the base of the limiter transistor T 4003 via D 4020
and R 4055. Transistor T 4003 drives, applying the input
signal to 0 V. When the capstan motor is started either anti-
clockwise or clockwise, the high pulses are switched to the
NAND gart (IC 4006) by the motor electronic system. The
output of the muting circuit goes to low after a time-lag of
approximately 1 second (C 4026 or C 4063), thereby releasing
the muting function. When the motor is started or the direc-
tion is reversed, the muting circuit remains operational until a
constant speed is reached. The RF generator is also blocked
by the muting signal (high) (Stp. A 7).
2.) If switch S 1 is in record position (sections 8 and 9), transistor
T 4008 becomes conductive in the forward direction and 0 V
is applied to the DIN output via R 4039.
Recording
Three different sources are available for recording (Mic, Line,
Dm ). The Mic-signal is amplified by low-noise transistor T 4000
and is supplied to IC 4002 for the purposes of impedance con-
version. The DIN signal (current supply complying with DIN
45 511; 0.1 - 2 MA) is amplified by IC 4000 and supplies ex-
cel lent noise values.
The Line-signal reaches the input selector switch S 4 via IC 4001
(impedance transformer). Depending upon the switch position,
the input signals reach the mixing amplifier formed by T 4001
and T 4002 via modulation regulators R 4020 (Line) and R 4008
(Mic). Transistor T 4013 serves to filter the +12 V. From the
mixing stage, the AF signal is supplied to the limiter (T 4003 and
T 4004) via R 4026. When the limiter is switched on, the level is
limited at 30 mV.
The signal reaches the input of the Dolby unit (section 9) via the
record/playback switch S 1. In position "h" switch S 7 (MPX/
Dolby) connects +12 V to point 1, thereby rendering the Dolby
circuit inactive. In position "g" (S 7), the MPX filter is switched
on (suppression of pilot frequency). C 4025 is coupled to 0 V via
the bilateral switch (13). Transistor T 4005 becomes biased in
the forward direction and the bilateral switch (5) opens, isolating
the connection between points 8 and 7 on the Dolby unit. The
LED display D 4301 is activated by switch S 7.
With the Dolby circuit, the AF signal branches to two outputs.
The signal which has not been influenced by the Dolby processor
is available at point 5 for monitoring purposes. lt is supplied to
the Line-output, the display amplifier and the headphone ampli-
fier via switch S 1. The dolbyised signal is supplied to the record
amplifier via the output (point 3). Transistor T 4014 is blocked.
lf, however, switch S 1 is positioned to play position, T 4014
becomes biased in the forward direction, biasing the record signal
with 0 V. The record level for each magnetic head is set separately
with trimmers R 4062 and R 4064. The record amplifier is con-
stituted by IC 4004. L 4001 is aligned at the factory (resonating
at 18.5 kHz) and must not be adjusted. With the aid of switch
S 5 (tape selector), equalisation is matched to the particular type
of tape by transistor T 4011 and sensitivity is matched by T 4012.
The record signal is supplied to the magnetic head via the track
selector switch S 2 after having previously been set to RF voltage
minimum via the RF wave trap with L 4000.
RF generator
The RF generator is a push-pull oscillator with a frequency of
approximately 105 kHz. The RF for the erased heads is tapped
via L 4001 and biasing for the particular magnetic head is set by
means of R 4082 and R 4084. The oscillator is influenced via
switch S 5 and transistor T 4102 in such a way that, depending
on the position of the tape selector switch S 5, the optimum
operating point is achieved. A high signal at the base of T 4104
effectively biases transistor T 4103 in the reverse direction,
blocking the generator. If a high signal is applied to the input E 2
(record mode), transistor T 4105 become biased in the forward
direction, the base of T 4104 becomes low, T 4104 becomes
biased in the forward direction, switching on the generator. A
high signal at the output E 1 (music cassette) effectively biases
T 4104 in the reverse direction, having the same effect as the
high muting signal via imput A 7. Capacitor C 4104 in the base
circuit of T 4103 ensures that the oscillator starts up smoothly
and with no click.
Fade Edit
Low is applied to the base of T 4104 via input D by switches S 9
and S 10. The transistor becomes biased in the forward direction,
releasing the generator.
When capacitor C 4301 discharges via switch S 10 to the base of
T 4103, this transistor slowly becomes biased in the forward
direction and the generator slowly starts to operate (fade-out
procedure).
Display and limiter
Both during record and playback, the AF signal reaches the dis-
play amplifier formed by IC 4100. In order to allow for increased
saturation at high frequencies, frequency correction is carried
out by means of S 5. Boosting by 400 Hz at 10 kHz results in
approximately 10 dB. The display is comprised of a 12-stage
LED chain, driven by IC 4301. The control signal for IC 4301 is
tapped at the output of the display amplifier and the level is set
to 0 dB during playback of the reference level with R 4127. The
control signal is rectified by D 4316, C 4302 ensures damped
return (approximately 0.5 s) of the LED display. The -20 dB
mark is set by means of R 4303. The inertialess display may be
switched oft by switch S 11.
Zero of the limiter operating range is set by means of R 4121.
The control signal is amplified by transistor T 4106 and is suppli-
ed to the base of T 4004 via switch S 6 (limiter). T 4004, in turn,
drives transistor T 4003 which acts as a differential resistance,
forming a voltage divider together with R 4026. The settling
time (approximately 3 - 5 s) of the limiter after lt has responded
is formed by means of C 4115. Limiter transistor T 4003 is used
to mute the input signal. This is performed by applying a high sig-
nal to the base when the following functions are operative:switch
S 1 in position (play) and in the case of all muting functions.
Headphone amplif ier
The unit is provided with a separate headphone amplifier( IC4101).
Headphones with an impedance of between 4 and 2000 ohms
may be connected. An incorporated volume control facility for
each channel (R 4128) permits the volume to be matched inde-
pendently of the output signal.