Dual C 810 - Functional Description of Electronic Section; Playback Operation Description; Recording Operation Description; Record Level Indicator Function

Functional
description
C
810
electronic
section
{n
this
unit,
all
active
functions
necessary
for
record
and
play-
back
except
the
HF
oscillator,
tape-type
selection
and
DIN
input
amplifier
are
combined
in
two
integrated
circuits
per
channel.
Playback
The voltage induced in the magnetic head (approximately 300 UV
during playback of the DIN reference level) is fed to the playback
amplifier input (PIN 27). One part of the output voltage (PIN 25)
reaches the inverting input (PIN 26) via the degenerative feedback
network R 106, R 109 and C 112, thereby determining amplifi-
cation and corresponding equalization. By altering the magnetic
head resonance by means of capacitors C 101 — C 104, the fre-
quency response in the treble range may be corrected. Mid-range
may be altered slightly by means of R 107 or R 108. Playback
equalization is switched from 120 Ms to 70 Us by means of
T 101. The playback Jevel is set using VR 102 to 580 mV at the
output jack by playing back the Dolby reference level. This is
necessary if the Dolby circuit is to function perfectly. The
playback signal from VR 102 now reaches the electronic record/
playback switching circuit via PIN 4 and is fed to the Dolby
processor IC 102 via PIN 5. If the Dolby selector switch S 2 — 1
is switched off, the signal in the Dolby circuit is amplified by
approximately 26 dB. If S 2 — 1 is switched on, in the case of a
low-value signal, amplification is carried out dependent upon
frequency and level.
From output (PIN 7) of the Dolby circuit, the signal reaches the
LINE output via S 1 — 4, also reaching PIN 10 and PIN 19 from
IC 101. PIN- 19 is the input to the headphone amplifier, PIN 10
the input to the DIN output amplifier. The headphone signal is
amplified to a value of approximately 1.6 V and then fed to the
headphone jack via PIN 18. The DIN output signal is only fed
via PIN 9 to outputs 3 and 5 of the DIN jack during playback so
as to eliminate input crosstalk during recording via DIN connec-
tion leads. The DIN output amplifier is arranged as an impedance
transformer and is not characterized by voltage amplification.
{n order to prevent switching noise reaching the output jacks, the
output signal is muted by means of switch S 6 when the start key
is not depressed. Once the start key is depressed, the signal
through T 109 or T 110 at the input to the Dolby circuit remains
muted until the capstan reaches the appropriate speed so as to
eliminate audible ‘high-level howling’.
Recording
Three different sources are available for recording, all of them
being fed to the record level regulator P 101 by means of S3 —1.
The line input leads directly to the input selector switch while
the signa! at the DIN input is amplified by means of T 401. This
amplification stage is designed in such a way that very satisfactory
signal to noise ratios are achieved at energization Jevels in ac-
cordance with DIN 45511. Thesignal at the Mic input is amplified
by approximately 43 dB via PIN 23 in the IC 101. The signal is
then fed from PIN 21 to the input selector switch,
Having passed through the record level regulator P 101, the
signal is fed to the electronic record/playback switch PIN 4, and,
via PIN 5, thereafter taking the identical path to the Dolby
circuit as previously described in the section '’playback’’. Between
PIN 2 and PIN 6 of the Dolby integrated circuits, a 19 kHz
(MPX) filter may be switched in as desired using S 2 — 2 in order
to suppress any residual pilot frequencies when recording from
radio. Such residual pilot frequencies could impair effective
functioning of the Dolby processor.
Unlike playback, the signal (PIN 3), unaffected by the Dolby
process, reaches the outputs and switch S 2 — 1 viaS 1 — 4. If
S 2 — 1 is actuated, the record signal at PIN 7 is “dolbyized"’.
The recording amplifier is likewise located in IC 101. The recording
current is set using VR 104 before the input (PIN 11) as is level
matching for the various types of tape by T 103 or T 104. The
record equalization degenerative feedback network is arranged
between PIN 13 (output) and PIN 12. Equalization for Cr- or
FeCr tapes is switched via T 102 and T 103.
The record signal is fed to the magnetic head via wave trap L102,
magnetic bias being provided by VR 106. The frequency of the
HEF oscillator (T 105, T 106) is approximately 105 kHz. The
operating point for FeCr- or Fe tapes is set by VR 107 and
VR 108.
Record level indicator
During both record and playback, the output signal reaches the
meter amplifier, likewise located in IC 101 (PIN 17). The DC
voltage resulting at the output (PIN 15) is then fed to the
moving-coil instruments of the record level indicator. Calibration
is by means of VR 103. In order to allow for reduced modulability
of tapes at high frequencies, frequency response correction is
carried out by R 125 and C 124. Boosting by 400 Hz at 10 kHz
amounts to approximately 10 dB. Indicator return is attenuated
by means of C 123 in order to transform the record level indicator
into a sort of “semi-peak-value indicator”.
Limit switch
The limit switch serves to switch the unit off whenever a cassette
has reached the end of the tape. Furthermore, the unit must be
switched off if the tape is not being properly taken up, e. g.
owing to too great friction in the cassette.
If the ring magnet no longer rotates, pulses cease to occur at
the output of IC 301. If the pause function is not depressed,
IC 302 then triggers the stop function via T 306 by means of
the pull magnet PL.
Power section
The power section supplies the electronic system with a negative
and a positive operating voltage of approximately 11.5 V in each
case which is regulated via T 107 or T 108. The motor is supplied
with a non-regulated voltage of approximately 13 V as a regulator
circuit is located within the motor.