Revox A77 Service Manual

If the instrument is in the STOP mode, relay A is without
current and contacts a 12 and a 11 are in rest position ( fig.
5.9 — 45 ). If relay A is energized I PLAY mode I, it will
hold over self- holding contact a 8/12. Contact a 7/11 closes.
A charging current to capacitor C 111 flows over the relay.
This energizes relay C and switches the right-hand reel
motor to 105 V over contact c 5/9 ( self- holding relay C is
interrupted by contact a 12 ). The charging current falls
with increasing charge on C 111 and relay C drops off.
When switching back to STOP mode, contact a 7/11 opens
and capacitor C 111 discharges over the windings of brake
and roller solenoids.
The following table summarizes motor voltages for the drive
modes.
Mode
Motor Volt
Rewind Motor
o
ages in Vac
Winding Motor
o
PLAWREC
55-6 = 49
42-6 = 36 55 42
>>
21
105
Table 5.9-46
99
15
5.9.1. Photoelectric End- of- Tape Switch
The working current loop of the relays is closed over the
self- holding contacts, the STOP button and the collector-
emitter path of switching transistor Q 112. Triggering the
end- of- tape switch ( Schmitt- trigger ) does the same thing as
depressing the STOP button.
As soon as light falls on photoelement R 155, its resistance
drops and base voltage at Q 111 rises. Collector voltage
at Q 111 drops and cuts off 0 112.
+ 27 V
TO RELAYS A, B AND C
SELF- HOLDING CONTACTS
R118 R120 e R119
Fig. 5.9.-47
The process is accelerated by the falling emitter voltage
on R 119 ( feedback ), so that a pulse- type transcient occurs
despite the gliding resistance change of R 155 ( sensitivity
inertia ).
The trigger conditions light - dark, generate the following
voltages at collector Q 111: light, 1 V; dark about 22 V.
To check the photoresistor, for light and dark values, it
must be disconnected from the circuit. Connections FH1 /
F H2 ( green ) on switch board ( fig. 3.8 — 11 ) have to be
pulled for this. The resistances are: light, less than 2 kOhm;
dark more than 20 kOhm.
I Don't solder at the soldering terminal of the photoresistor,
it is heat sensitive I.
5.10. Speed 7iegulation, Capstan Motor ( 7.077.725 )
I Removal per 3.9.)
The electronically regulated
uses a method unknown so
is why this technology of
rather detailed. Checks and
electronics are done per 6.2.
tape drive of REVOX A77
far in tape recording. That
regulation is described here
adjustment of the regulation
0 0 L
(
o
cii
Fig. 5.10-48
The capstan motor speed is electronically measured and
continously regulated. The driving element is a robust and
unpretentious asynchronous motor, whose homogeneous
outer rotor generates a minimum of torque pulses, reflected
in scant wow or flutter.
To measure the speed, a 120- teeth high- precision gear is
cut into the outside surface of the rotor. They are induc-
tively counted by a special pick-up head. The speed is then
transformed into a proportional frequency which is anal-
yzed for momentary value in an electric circuit. Any devi-
ation from the nominal, 1600 Hz for 7.5 ips and 800 Hz
for 3.75 ips, is detected as to magnitude and sign, and
changes the voltage applied to the motor.
The actually regulating network ( fig. 5.10-49 ) consists
of a bridge rectifier in series with the supply, whose diagonal
is loaded more or less by a power transistor. A power
transistor with nearly constant base current doesn't show
a genuine resistance characteristic ( I — V ), for it acts
as current control element.
23

Revox A77 Specifications

General

Brand	Revox
Model	A77
Category	Tape Deck
Language	English