Revox A77 Service Manual

Playback amplifier cards each contain one playback pre- )
amplifier and one drive amplifier. The playback head
is d.c. coupled to the input stage ( Q 801 ). D.c. feed-
back from the emitter voltage divider of stage ( Q 802 )
goes over the playback head to base ( Q 801 ) and stabi-
lizes its quiescent point. The feedback current is less than
1 µA which avoids magnetization of the playback head.
AF- components of the feedback are shunted through
C 803. Its capacitive resistance becomes effective at very
low frequencies ( less than 20 Hz ) so that these frequencies
are fed back. Playback equalization is switchable NAB -
IEC ( CCIR ). The equalization circuit is in the feedback
branch collector Q 802 / emitter Q 801. The RC- member
R 808 / C 805 / takes care of the lows equalization I 3180
µsec 1.
Basic time constants of equalization are obtained by short-
ing the partial resistors of the circuit. For :
7.5 ¡ Ps NAB ( 50 µsec ) 3 - 4 - 5
IEC ( 70 µsec ) 3 - 4
3.75 ips NAB / IEC ( 90 µsec ) 4 - 5
At the pre- stage exit is a 120 kHz tank circuit for the
bias frequency and trimpot P 801 for setting the level
of the bus voltage. The signal is conducted over the before-
and - after - tape switch, volume control, and playback
mode switch ( channel selector ) to the drive amplifier.
The drive amplifier is three- stage d.c. coupled. It ampli-
fies the signal to the level of the terminals OUTPUT,
headphones, as well as for the modulation of the plug-in
power amplifiers. D.c. feedback, emitter voltage divider
Q 804 / base Q 803 stabilizes the quiescent points of all
three stages. Balance regulation is accomplished with poten-
tiometers P 303, P 304 I coupled I in feedback branch
emitter Q 805 / emitter Q 803.
In STOP position, when the pressure lever is at rest, out-
put of the playback pre- amplifier ( 13 ) is shorted through
contact S 5 I switch board 1.077.435 I.
5.7. Power Amplifier ( 1.077.850 )
CH I
CH II
Fig. 5.7-40
The power amplifier cards have separate rectifiers and
filters for each channel. The power transformer has two
separate windings ( 32 Vac ) for the power amplifiers.
The entire amplifier is d.c. coupled and works with a single-
ended push-pull output stage. The loudspeaker is, practi-
cally free of d.c., in the diagonal of a bridge formed by
Q 905 and Q 906 as well as the electrolytic capacitors
C 907 and C 908. Q 901 and Q 902 operate as voltage
amplifiers.
Series connected D 901, R 911, D 902 are used to keep
the no-load current of the output transistors constant,
independent of temperature. By strong d.c. depending feed-
back I 100 % I, collector Q 906 emitter Q 901, the zero
point of the loudspeaker output is held constant. AF- am-
plification is set by the voltage ratio R 905 to R 906.
5.3. Power Supply
( Diagram Tape Drive 1.077.100, No. 1.077.540)
I Removal, per 3.9. I
Both lines of the power supply can be switched off ( Micro-
Switches ). In addition there are two break- contacts which
disconnect upon removal of the housing. The sectional
tape core power transformer has five windings for follo-
wing units :
Capstan and reel motors:
Relay control :
Amplifier supply :
Output amplifier :
Output amplifier :
Fig. 5.8-41
130 Vac
22 Vac
24 Vac
(150 Vac) with 4 taps
( supply voltage 27 Vdc)
( supply voltage 21 Vdc,
regulated )
32 Vac ( CH 1 )
32 Vac ( CH 2 )
21

Revox A77 Specifications

General

Brand	Revox
Model	A77
Category	Tape Deck
Language	English