Revox A77 Service Manual

Recording is equalized per NAB standard. Lows equaliza-
tion ( 3180 , usec ) is accomplished by feedback through
series- connected C 504 and trinnpotentiometers P 501 and
P 502. Highs equalization can be set separately for both
speeds; P 502 for 7.5ips, P501 for 3.75ips.
Trimpot P 503 serves for voltage balance. The driver ampli-
fiers are laid out for high overmodulation stability. Tran-
sistor Q 505 supplies the necessary collector d.c. to Q 504
for modulation.
Q 505 represents high impedance for AF which furnishes
the entire output a.c. for modulating the record head.
Modulation reserve is - better than 15 dB. A tank circuit
with a resonant frequency of 38 kHz causes current depen-
ding feedback in the emitter circuit of the driver stage
( L 502 / C 514 ) for pilot carrier remainders which could
interfere with the erase frequency. Between the decoupling
point and record head is another tank circuit for the bias
of 120 kHz ( L 501 / C 517 ). The decoupling point is
shorted by the record relay during playback.
Amplification of the VU- meter stage can be calibrated on
P 504.
To protect the VU- meter indicators against RF I ( radio
frequency interference ), resulting in misindication, the
stage is decoupled for RF with C 508.
5.4. Oscillator ( 1.077.710)
Fig. 5.4-36
The push-pull collector circuit guarantees minimum odd
harmonics. Symmetry of sinusoidal waves is assured and
interfering d.c. components can't arise. Erase and bias of
120 kHz is high enough to prevent their interfering. The
erase frequency is symmetrically decoupled. In mono- ope-
ration, load is held constant by the compensation coil
( para 5.5. ) which proves advantageous for voltage and
frequency stability.
Bias current can be set with trimpots P 707 to P 710 separa-
tely for CH 1 and CH 2 as well as for both tape speeds.
20
5.5. Record Relay ( 1.077.715 )
Fig. 5.5-37
Purpose of the record relay with an electronic switch is
to turn on the oscillator " softly ". To get an integral
curve of the switching voltage, Q 601 I switching tran-
sistor is delayed by an RC - network I R 603, C 602 I.
REL 601
3-6
C602
R603
Fig. 5.5.-38
R601 Q601 OV
C 602 is discharged at the start and the base of Q 601
is at OV. If contacts 2/3, 5/6 of record relay REL 601
close, bias voltage rises with the charge on C 602 till satu-
ration. When contacts 2/3, 5/6 open, C 602 discharges
over R 602.
The head compensation coil has two winding halves acting
as dummy load for the inactive half of the erase head
during mono- operation. The compensation coil balances
out inductive strays of the erase head.
Resistor R 604 corrects Q - factor and assures minimum
voltage jump between monaural and binaural operation
( exchange of erase head may require resetting ).
5.6. Playback Amplifier ( 1.077.720 )
o
In
II
Fig. 5.6-39
CH II CHI
u
u
u

Revox A77 Specifications

General

Brand	Revox
Model	A77
Category	Tape Deck
Language	English