Revox A77 Service Manual

6. ELECTRICAL ADJUSTMENTS 6.2.3. Tacho Adjustment with Cathode- Ray Oscilloscope
AND MEASUREMENTS
Measurement Instruments and Aids
— Demagnetizing coil
— AF- millivolt meter or VTVM ( vacuum tube voltmeter )
— AF - generator
— Distortionfactor meter
— Oscilloscope or digital counter
— Wow and flutter meter
— Test Tapes ( Reference Tape )
Test tapes per NAB ( 1965 ), and DIN Heimton 7.5 ips
( 1966 ) and 3.75 ips ( 1967 ) are equalized alike for respec-
tive speeds.
7.5 ips = 3180 µsec, 50 µsec test tape DIN 45513 / 19 H
( from AG FA or BASF ) or NAB 7.5 ips.
3.75 ips = 3180 µsec, 90 µsec test tape DIN 45513 / 9
( from AG FA or BASF ) or NAB 3.75 ips
(90 µsec).
— Adjustment Tape 4 - track ( . 25 " )
for height adjustment of the magnetic head on 4- track
instruments.
All AF and RF measurements are to be taken with
shielded cables.
6.1. Demagnetizing
— Magnetic heads and tape guides are to be demagnetized
before any measurement or adjustment with running
tape.
Strong d.c. - magnetization disimproves signal-to-noise
ratio and frequency response.
It can also permanently damage the test tapes.
6.2. Tape Speed ( Regulator Card 7.077.725 )
6.2.1. Pick - up Head - Adjustment
The air- gap between pick-up head and rotor shall be . 3 -
.4 mm ( . 012" - . 016" ). Adjustment is correct when a 35 -
50 mV tachometer signal voltage is measured I at 3.75 ips I
on connections blu E2 and brn El of the regulator card.
6.2.2. Tacho Adjustment
Exact adjustment of the discriminator, respectively of the
capstan motor speed, can be accomplished only with a
digital counter.
— Connect digital counter to blu E2, brn El.
— Switch recorder to PLAY ( with tape in place ).
— For 7.5 ips, the frequency must be 1600 to 1601 Hz.
Measurement time 10 sec. minimum. Apply corrections
to core of discriminator coil T 201.
— For 3.75 ips, the frequency must be 800 to 801 Hz
I 7.5 ips adjustment has to be made first ! ) - Apply
corrections to trimpot P 201.
If need be, adjustments can be made with an oscilloscope
referenced to the 50 Hz network frequency test point D
( diagram 1.077.725 I there is ( closed to OV) an a.c. signal
of 100 Hz of the following shape :
The superposed frequency comes from the regulating sys-
tem and is 1600 Hz for 7.5 ips, 800 Hz respectively for
3.75 ips.
Measurement is accomplished in the PLAY mode with
tape in place. The superposed frequency must remain stan-
ding with respect to the base frequency ( at 50 Hz network
frequency ). ( Adjustment procedure per 6.2.2. )
6.3. Playback Adjustment
Preparation :
Check regulated supply voltage ( 21 V ). Apply correc-
tions to P 106 I Power Supply Card 1.077.540 1.
Check of Balance Adjustment :
— Connect AF- generator to AUX - input.
— Parallel the signal voltage on both inputs by bridging
the solder terminals of the AUX cinch- connectors with
alligator clamp.
— Set controls per fig. 6.3-52
OREEMEI. •
MONITOR RECORD
DA•X
Fig. 6.3-52
14•1
— Supply about 30 mV / 1000 Hz.
— Measure signal voltages at the OUTPUT ( 2 V approx ).
Both outputs shall have same signal voltages. Apply
correction with the balance control for voltage diffe-
rences at less than 3 dB at the outputs. Check the ampli-
fier channels if the voltage differences are larger.
— Don't readjust the balance control. .
26

Revox A77 Specifications

General

Brand	Revox
Model	A77
Category	Tape Deck
Language	English