Revox A77 Service Manual

8. MODIFICATIONS
8.1. Rewind
For bad rewind with 18 cm ( 7" ) reels :
Replace resistor R 125 ( 820 Ohm ) by resistor 1.2 kOhm,
9 Watt ( drive control 1.077.370 ).
8.2. Switch Noise
The following change can improve the situation where
operation of the STOP button out of RECORD and PLAY
modes causes clicks :
Ground connection brn ( fig. 3.13 — 18 ) is unsoldered from
the printed side and resoldered to the ground conductor
at playback switch S5 ( switch board 1.077.435 ).
Another improvement results from replacing the diode
D 601 ( record relay card ) by an RC- network: C = 50 pF /
30 V, R = 100 Ohm / 1/8 W ( series connected, observe
polarity ).
8.3. RFI
Interference from strong radio an TV transmitters can be
removed by the following measures :
Solder 1500 - 5000 pF ceramic capacitors from ground
connection shield ( shielded cable ) to chassis ( tubular
rivet of connector fastening ) at the disturbed inputs ( see
example C 154 OUTPUT terminal ).
Powerful UHF television transmitters may be picked up by
the playback head cables. If an interfering buzz occurs, the
following change should be effected in the playback
amplifier:
Connect a capacitor of 100 pF ( maximum 220 pF) from
base to emitter of Q 803 ( similar to C 803 on Q 801).
8.4. Transfer Play Sensitivity
Resistor R 316 and R 321 are 10 kOhm in recorders of
lot 1. Substitution with 3.3 kOhm resistors ups recording
sensitivity ( gain 10 dB approx).
8.5. Replacement of Capstan Motor
Starting with Serial Nos. S15500 and G16150 a new type of
Capstan- Motor was installed which operates on a 20 V
higher supply voltage and therefore requires a modified
power- transformer. The voltage appearing at point E8 blu
(speed control board 2.077.725) now measures 150 V.
When using an old motor in conjunction with a new
transformer, a type 40318 transistor must be used in
position Q 209 on the speed control board.
8.6.
Capstan Motor with Sleeve Bearings
Modification effective from serial Nr. S 60483 and G 78003
for illustration see parkslist page E 10.
The capstan shaft extends through the full length of the
motor and is supported by two sintered- bronze bearings.
The lower bearing is designed to restrict the axial move-
ment of the shaft by means of a plastic coated thrust
(low- friction) washer. All bearings are lubricated for their
life and will normaly not require any service.
Capstan shaft and low- friction washer may be replaced in
the field.
If a bronze bearing has become defective, the motor must
be sent to the nearest factory service- center. ( For disas-
sembly instructions see section 3.7.)
8.6.1.
Removal of Capstan Shaft and low- friction Thrust Washer
Removal of capstan shaft and low- friction washer is possible
while the motor remains installed in the recorder.
- Carefully remove shaft lock A by pulling it sideways
with a pair of angled pliers. DANGER! Do not
attend to pry the lock loose with a screw- driver etc.
because it will fly off quite forcefully. Slide off the
rotor portion of the capstan motor.
- After removal of the tape guide between the play-
back head and the capstan shaft, the shaft can be
pulled up and out of the motor. ( Take care not to
disturb the setting of the adjusting nut on the tape
guide.)
- To remove the low- friction washer take out the
circlip B with the aid of special pliers. The bearing
parts can now be disassembled in the following
order: Springwasher E, low- friction washer F and
thrust bearing G.
- The capstan shaft may now be removed by pulling
it downwards and out of the motor.
- When re- assembling the lower bearing, apply one
drop of lsoflex PDP65 oil to the low- friction washer.
When handling shaft and bearing, extreme care must be
exercised, so as to avoid damaging them and to prevent any
dust particles from entering the precision bearings. ( runout
accuracy 1/1000 mm).
35

Revox A77 Specifications

General

Wow and Flutter	0.08% (7 1⁄2 ips)
Frequency Response	30Hz to 20kHz (7 1⁄2 ips)
Signal to Noise Ratio	60dB
Type	Reel-to-Reel Tape Deck
Track System	4-track, 2-channel stereo/monaural
Reel Size	Up to 10.5 inch reel
Motor	2 x reel, 1 x capstan
Inputs	Mic, Line
Outputs	Line
Tape Speed	3 3⁄4 and 7 1⁄2 ips