Dual CR 1730 Serviceanleitung Seite 11

CR 1710: AF section
Phono (cinch, DIN)
Tape 1 and Tape 2 (cinch, DIN)
In order to guarantee an input impedance of 470 kOhm, there is ап im-
Tone controls
Mono
Volume, balance, loudness
Output stage
The transistors TR 201 and TR 202 perform the muting function (see
muting). The output amplifier is a non-ferrous complementary output
stage with an AF output having neither DC voltage nor electrolytic capa-
citors. Via the deferential amplifier TR 203 and TR 204, the AF signal
is routed to the transistors TR 205 and TR 206. The zero signal current
is stabilized via TR 207 and TR 208. The complementary driver tran-
sistors (TR 209, TR 210, TR 211 and TR 212) amplify the AF signal
and drive the power output transistors TR 213, TR 214, TR 215 and
TR 216, which are also complementary. The output signal is routed to
the loudspeaker connections via the fuse FU 201 and the switching
contacts of the headphone socket. The loudspeaker outputs are deacti-
vated by connecting a plug to the headphone socket.
Power supply unit
CR 1730: AF section
Phono (cinch, DIN)
Refer to the details on the phono input of the CR 1710.
Tape 1, tape 2, aux (cinch, DIN)
In order to guarantee an input impedance of 470 kOhm, an impedance
converter (TR 05) is located after the input selector switch (S 01-1 to
S 01-6). In addition, tape 1 has an impedance converter (TR 07) for the
monitor function. When the front sockets of tape 2 are occupied, the
sockets for tape 2 on the rear are deactivated.
Monitor
Tone controls
High and low filters
The filters have a drop of 12 dB per octave; response frequency ofthe
low filter (S 201-1) at 50 Hz and of the high filter (S 201-2) at 6.5 kHz.
Mono
The left and right channels are merged by the mono switch (S 201-3). A
level surge caused by the addition is prevented with resistors R 433 and
R 434. Via R 135 and D 105, operation voltage is applied to pin 16 of
IC 102 and the stereo decoder switches to MONO.
Volume, balance, loudness
Output stage
The transistors TR 201 and TR 202 perform the muting function (see
muting). The output amplifier is a non-ferrous complementary output
stage having an AF output without DC voltage and without electrolytic
capacitors. Via the differential amplifier DR 203, the AF signal is routed
to the transistor TR 207. The zero signal current is stabilized via TR 205
and adjusted with VR 201. The complementary driver transistors
(TR 209, TR 211) amplify the AF signal and drive the power output
transistors TR 213 and TR 215, which are also complementary. The
output signal is routed to the loudspeaker output and the headphone
connection via the switching contacts of the relay and the "speaker"
selector switch S 502. If the "speaker" selector switch is in position 1
and 2", the connected pairs of speakers are series connected in each
channel.
Speaker protection circuit
The task of the protective circuit is to prevent a voltage from reaching
the speaker output should a defect occur. This is achieved by TR 221
becoming reverse biased and relay RY 201 becoming de-energized, as a
result of which the switching contacts interrupt the AF path. Under nor-
mal operating conditions, TR 221 in the protective circuit is the only
transistor which becomes conductive.
If, in the event of a defect, a positive or negative DC voltage is applied
to the loudspeaker output in the area of the power transistors TR 213 —
TR 216, D 207 detects the positive DC voltage. TR 220 becomes con-
ductive, the base of TR 222 becomes low and thus TR 222 becomes
negative biased. In the case of a negative DC voltage, the diodes D 208
and D 209 become conductive, TR 221 becomes negative biased and
the relay is de-energized. t
If there is a short-circuit at the speaker output, the voltage drop over
R 231 (TR 230) increases, transistor TR 217 becomes conductive, the
base voltage at TR 219 drops and thus this transistor becomes forward
biased. Via D 207, the positive DC voltage is switched through T R 220,
TR 221 becomes negative biased and the relay is de-energized.
Switch-on delay: the relay is not energized until both speaker outputs
reliably conduct 0 V DC voltage.
Power supply unit
The power supply is designed for supply voltages of 230 V and 115 V
(AC voltage). To convert the voltage, the fuses (FU 501 and FU 502)
must be changed on the power supply board in accordance with the data
printed оп the board. The secondary side of the mains transformer has
two windings: one main winding to generate the operating voltage for
the output amplifier, the preamplifier, the impedance converter, the
filter circuit and the equalizer preamplifier. The other winding is for
the RF section and the bias voltage of the fluorescent display. The
tuning voltage of the capacitance diodes is also generated by the main
winding and rectified via diodes D 502 and D 503. The FM tuning vol-
tage is stabilized with ІС 502, which compensates for temperature drifts.