Revox G-36 Service manual

Contents

1 General 3
2 Mechanical Description 3

2.1 Mechanical checks and adjustments 4

2.1.1 Tape path 5

2.1.2 Brakes 5

2.1.3 Tape tension 7

2.1.4 Pinch roller 8

2.1.5 End of tape switch 8

2.1.6 Wow and flutter 9

2.1.7 Tape speed 10

2.2 Routine maintenance 10

2.2.1 Cleaning 10

2.2.2 Lubrication 10

2.2.3 Servicing of solenoids 11

2.2.4 Servicing of push button assembly 11

2.3 Removal of capstan assembly 11

3 Electronic Description 13

3.1 Circuit checks and adjustments 13

3.1.1 Measuring instruments 13

3.1.2 De-magnetising 14

3.1.3 Head alignment 14

3.1.4 Playback amplifier 16

3.1.5 Bias adjustment 17

3.1.6 Measurement of overall frequency response 18

3.1.7 VU-Meter adjustment 19

3.1.8 Adjustment of head shielding 20

3.1.9 Channel to channel crosstalk 20

3.2 Tuning the bias traps 21

4 Troubleshooting 22

4.1 Mechanical troubleshooting 22

4.2 Electronic troubleshooting 23
5 Technical Data 24
6 Circuit Diagram 26
7 SPARE PARTS LIST - ISSUE IV 39

1 General

The REVOX Series 36 has been constantly improved and kept up to date, since it was
first introduced in 1956. However, the present G still bears the same basic layout and
conception as did the original A recorder.

Compared with the preceding models, the following major innovations have been
incorporated in the REVOX G 36 recorder : Hysteresis-Synchronous Capstan motor,
VU-meters for record level control and a tape-tension switch to permit the use of all
reels from 7 inch up to 10 1/2 inch diameter.

The G 36 model is available in both 2 and 4 track versions. Identification is made by
the appropriate number being stamped on the head mounting plate, and on the upper
side of the lower chassis.

The recorder consists of an upper and lower chassis. These are joined at the rear by
means of the back panel which carries the input and output sockets, mains voltage
adjuster and fuse-holder, and at the front by means of the loudspeaker panel
mountings. The upper chassis carries the tape transport mechanism and the lower
chassis bears the power supply unit and the electronics.

All Audio Connections are led to standard phono sockets. To interconnect the
REVOX recorder with other Audio Equipment, a selection of adapter cables is
available.

2 Mechanical Description

The machine employs three motors. Two are identical and perform the wind and
tension functions. The third motor is large and drives the capstan. It is of the pole
switching variety and speed change is therefore effected electrically.

The wind motors are designed to provide pulse free torque. Back tension is applied
by electrical counter torque during the record and playback functions as well as
during wind.

The Capstan Motor is of the Hysteresis-Synchronous type, incorporating a special
design feature to prevent hunting. By switching to either the 6 or 12 pole stator
winding, the tape speed of 7 1/2 or 3 3/4 ips may be selected. The capstan drive, the
pinch roller assembly and the headblock with the tape guides are carried on one die­cast frame. This ensures accurately parallel and permanent alignment of all major
transport elements. The coupling between motor and flywheel consists of special
silicone-rubber strips, with the whole assembly acting as a mechanical filter. A direct
slip free drive is thus achieved whilst maintaining negligible wow and flutter.

The pinch roller is held against the capstan during record and replay functions with
considerable pressure. This necessitates the use of a capstan bearing designed
specifically to withstand considerable side pressure for long periods. The pinch roller

arm carries the shield for the playback-head and the tape-lift mechanism for the
rewind function.

The brakes are mechanical in nature but are electrically operated. This is achieved by
the action of the brake solenoid which pulls the brakes off during all operating
functions. When the stop button is pressed however, or when the current fails for any
reason, the solenoid is de-energized, and the brakes automatically come on, with the
higher braking force always on the trailing spindle.

All functions are controlled by the push buttons. Arcing damage and noise is
prevented by the use of diodes and RC networks. A sensing lever on the RH tape
guide (3 in fig. 1 and 8) operates a switch (SE on schematic) which, in series with the
stop button, provides a current path to a solenoid incorporated in the push button unit.
This has the effect of clearing any selected function either by depressing the stop
button or operation of the end of tape switch. On recorders up to serial No. 36 500,
the function of the end of tape switch is delayed by about two seconds to avoid
tripping during the start mode. On machines with higher serial numbers (recognizable
by the end-of-tape switch being made of gold-plated wire with NC contact
configuration) this time delay (relais) has been utilized to supply increased operating
voltage to the wind motors, thus providing the required starting torque. (Inset
schematic B and diagram 7.736.001-002:6 refer.)

The tape guide pins to the left and right hand side of the loading slot assist in
achieving uniform tape tension due to the change in friction with varying wrap-around
from large to small spooling diameters.

A three digit tape counter is driven from the take-up turntable by a rubber belt. The
translucent resetting knob of the counter is lit by a small pilot bulb which acts as a
mains indicator.

A remote control facility is provided

Removal of the shorting link from the socket on the rear panel and the insertion of the
correct accessory plug, lead and switch, enables the recorder to be started or stopped
in either the record or playback function as selected. Operation of the remote control
switch has the effect of de-energizing both the capstan and brake solenoids.

ATTENTION If no remote control is used, the dummy plug must be inserted into the
socket, otherwise the recorder will not start.

2.1 Mechanical checks and adjustments

It is advisable to use a full and an empty 10 1/2 inch spool when carrying out
adjustments. For the majority of tape transport checks, it is essential to keep the
plastic top cover in position. For other mechanical adjustments the top and head
covers should be removed. To remove the plastic top plate, pull off the grey control
knobs, the transparent selector discs and the plastic cover over the pinch-roller arm.
Further undo the mounting screws (1 in fig. 2) of both turntables, of the head cover
and the mounting lugs (2 in fig. 1) of the pinch-roller cover. When reassembling, take

great care in tightening the turntable screws uniformly to prevent them from
wobbling. For best results, they may have to be rotated by 120°.

2.1.1 Tape path

The tape path level is determined by the left and right hand tape guides (3 in fig. 1)
and by a small guide (4) positioned between the playback head and the capstan.
We do not recommend that these guides should be interfered with in any way.
Adjustment to the spool carrier height can of course be made to enable the tape to be
wound centrally between the spool flanges. This is brought about by sliding shim­washers of varying thickness onto the spooling motor shafts after removal of the
bakelite brake drums. To pull a brake drum off the shaft, screws should be inserted
into the three tapped holes (1 in fig. 2) so as to serve as “handles”. Care should be
taken to prevent damage of the brake bands at this stage. Where a brake drum has to
be removed it is recommended that the brake band be removed first.

2.1.2 Brakes

The layout of the brake system is shown in fig. 2. The mounting bracket (5 in fig. 2)
on the rear end (stationary) of the brake band must be held tight against the vertical
pin on the chassis by sufficient tension of the leaf-spring on the bracket.

It is important to check that the brake band (6) is running flat on the brake lining and
not biting on the upper or lower edge. Adjust alignment of brake band by slightly
pivoting it in the rivets if required.

Correct tension for spring 7 is obtained when 160-220 grams applied as a tangent
force to a 10 cm diam (spool and tape) causes the spool to move. See fig. 3.

Brake tension may be altered by resetting the brake return spring (7) into any of
the three holes on the end-brackets (5 and 9). The brake release solenoid must cause
sufficient movement (approx. 3 mm) of the front-end bracket (9) to fully free the
brake band from the lining.

Depress the “stop” and “play” buttons simultaneously to energize the brake release
solenoid. If the movement is incorrect, loosen the nuts (10) and re-position the
solenoid. The front-end brackets of the brake bands may be bent to achieve
simultaneous release of both brakes.

The brakes should be kept clean and dry. Do not use any oil or grease on the linings.

2.1.3 Tape tension

Back tension is applied by the combination of reverse direction torque from the feed
motor and the degree of wrap around on the LH guide pin. Wear on the mechanical
parts of the recorder will not alter the amount of tape tension. Any effects that could
be attributed to inaccurate tape tension may be caused by:

a. Faulty adjustment of the brake band
b. Electrical fault in feed motor
c. Electrical fault in phase shift-condenser
(C 72 pos. 22 in fig. 5 and 6).

2.1.4 Pinch roller

The pressure of the pinch roller can be measured by a gauge fixed to the pinch roller
arm as close as possible to the spindle of the pinch roller (fig. 4). A constant tone tape
should be played and the pinch roller withdrawn by the gauge until the note is heard
to change. A reading in the region of 1.7 kg should be obtained. Adjustment to this
value can usually be obtained by turning nut no 13 in fig. 5. Should this not provide
the necessary amount of correction it is advisable to check the position of the solenoid
no 11 in fig. 5 which will be correct when there is approx. 0.5 mm clearance at point
A (fig. 5) with the solenoid energized. It is essential that the solenoid’s-slug fully
bottoms in the energized position and when properly adjusted, there should be no
movement of the slug when lifting the pinch roller off the capstan. Correct setting can
be obtained by loosening the nuts (12 in fig. 6) and moving the solenoid in the
required direction.

2.1.5 End of tape switch

A. Photoelectric end-of-tape-switch

Recorders with serial numbers from 58 000 onwards are equipped with a light
sensitive auto shut-off device. It consists of a light source (Osram 3644), the
photoresistor ORP 62 and a printed circuit section containing two transistors (see fig.

1).

The photoconductive cell is housed in the tape guide pin on the right hand side of the
recorder. It has a dark resistance of greater than 100 kΩ and this value drops to below
3300 Ω under illumination. The associated current amplifier energizes the push
button release solenoid when light reaches the photoconductor. For normal operation
of the recorder the sensing element is at its high dark resistance and both transistors
are biased into cut-off. Illumination of the photo-resistor changes the bias condition
on T1 thereby raising its collector current. A voltage drop develops across the
solenoid winding and T2 begins to conduct. Since T2 opens a current path parallel to
the photoconductor, positive feed back action sets in which causes the collector
current of T1 to rise quickly into saturation.

To de-energize the solenoid and to restore the nonconducting condition, the supply
voltage has to be interrupted.

B. Mechanical end-of-tape switch.

This switch and its associate operating levers must be thoroughly clean to function
correctly. On G-36 recorders up to serial number 36 500, the tape sensing lever
operates a snap-action switch (SE schematic A) which is closed with tape tension
applied. The right position of the switch assembly is essential for correct operation to
be obtained. This adjustment is carried out by slackening the fixing screws and
moving the switch bodily until the snap-action switch closes when the sensing lever
still protrudes by 0.5 to 1 mm from the outer diameter of the tape guide.

On recorders with serial number 36 500 and up, the sensing lever operates a gold­plated wire contact. This switch opens when tape tension is applied (see BE in
schematic Band 7.736.001-002). Adjustment should be carried out analogue to the
above specifications for the snap-action switch by bending the long wire loop. In the
resting position sufficient contact pressure should be available to make the short wire
loop move beyond the point of contact by approx. 0.5 mm. This is achieved by
bending the contact wires while operating the tape sensing lever by hand.

The end-of-tape switch and its operating lever are then properly adjusted when the
switch remains open (or closed on older models) for any movement of the sensing
lever inside the tape-guide post. Accidental tripping due to sticky splices etc. will
thus be avoided.

2.1.6 Wow and flutter

Accurate and useful measurement of wow and flutter can only be made with an
appropriate instrument. The recorder is calibrated using the EMT Model 420.
Possible causes of flutter may well lie with the capstan, capstan motor. Wow can
usually be seen as associated with the pinch roller speed and in some cases a faulty
pinch roller may be the cause, and in others too much back tension or insufficient
pinch roller pressure. For all wow and flutter investigations the transport mechanism
must be completely clean.

2.1.7 Tape speed

Tape speeds can be checked by running a marked, measured length of tape through
the recorder. For a 100 sec. run 950cm would be required at 3 3/4 ips and 1905 cm at
7 1/2 ips. The difference in running time in seconds will be the speed variance as a
percentage. Some variance can be expected with temperature increase but this should
lie within the quoted tolerance. In very cold weather it may be noted that the recorder
runs accurately at 3 3/4 ips but slows down when switched to 7 1/2 ips. This is due to
drag from the cold grease in the capstan bearing, especially when new. After a short
time of operation at 3 3/4 the machine should be capable of running at 7 1/2 ips.

When detecting any deviations from the figures quoted, an accurate reading of the
mains frequency should be taken first before investigating pinch-roller pressure,
brakes etc. On a 50 cycle power line, a drop to 49.5 cycles will make the recorder run
slow by 1% and this should be borne in mind when checking equipment with a
specified accuracy of ± 0.3 percent.

2.2 Routine maintenance

2.2.1 Cleaning

From time to time the working parts of the recorder which come in contact with the
tape should be thoroughly cleaned. Cleaning of the head faces is particularly
important, especially in the case of four track heads where seemingly invisible
particles can often have an adverse effect on performance. On no account must any
deposits be scraped off with metallic tools. Loose tape dust, may be brushed off. For
cleaning of the heads and capstan shaft, use a cotton-swab soaked with methylated
spirits. (Avoid any solvents from coming into contact with the plastic parts of the
recorder).

2.2.2 Lubrication

The capstan motor, the capstan bearing and the wind motors are equipped with
bearings of sintered material. Each bearing has an adequate supply of lubricant which
should last for the life of the bearing. Should the replacement of a motor bearing
become necessary, return the unit to the nearest REVOX Repair Station. On wind
motors with ball bearings (Series I and II) it is advisable to replace the ball bearings

once their supply of lubricant has been used up. Felt linings are to be saturated with
Teresso 43 (Esso).

If signs of wear become visible on the capstan shaft, the whole bearing plate complete
with shaft should be replaced (see sect 2.3). Lubrication of the capstan bearings is not
anticipated. However, where this can not be avoided, Teresso 43 (Esso) only must be
used.

2.2.3 Servicing of solenoids

When it appears that a solenoid is tending to stick or function erratically, it should be
dismantled and the slug and housing thoroughly cleaned with methylated spirits.
When both parts are properly clean and dry, molybdenum grease may be rubbed into
the working surfaces, which must finally be wiped dry before re-assembly.

2.2.4 Servicing of push button assembly

Conventional switch cleaners are not recommended for the high current spring
contacts used in the REVOX, and where cleaning is necessary, the contacts should be
polished clean. The interlocking bars and push-button shafts may require occasional
greasing with molybdenum grease. Where a part of the assembly needs replacement,
unscrew the four fixing screws, bend the pushbuttons to the right and ease the unit to
a better working position. For better access to the switch assembly, the housing of the
pinch roller solenoid should be removed by undoing the M 4 nuts (pos. 12 in fig. 6).

2.3 Removal of capstan assembly

This may become necessary where the recorder is to be used on a mains frequency
which differs from that for which the recorder was manufactured, or when replacing
the capstan assembly becomes necessary.

Having removed the chassis, as described in sect. 2.1. from the cabinet and the
loudspeaker and panel, separate upper and lower chassis sections by undoing screws
no 16 shown in fig. 6. Lift upper chassis without exerting excessive force, if necessary
loosen the fixing screws of the selector switches (18 in fig. 6) to gain more freedom of
movement.

Unscrew the capstan mounting nuts (19 in fig. 6) and remove the assembly
downwards and forwards out of the chassis. The fixing screws 20 in fig. 7 on the
flexible coupling are to be removed through the large holes in the capstan bearing
plate. The motor mounting screws (21 fig. 7) should now be removed using
REVOX’s special 8 mm nut driver, after which the flywheel assembly can be
separated from the motor.

3 Electronic Description

Access to the majority of the electronics of. the recorder can be obtained by removing
the base of the recorder. The REVOX G-36 has two identical record- and replay­channels. The common bias and erase Oscillator can be switched onto either one or
both channels, thus allowing stereophonic recordings, two-track sound with sound and
monophonic recordings. Each channel is equipped with pre-amplifier stages to accept
low-level signals from linear high-impedance sources. (Step-up transformers are
required where low-impedance dynamic microphones are being used.) For
monitoring the signal level, the recorder is equipped with two VU-meters and
associated matching amplifiers. The built in power amplifier drives a 21 cm Ø
loudspeaker and an appropriate switching arrangement permits listening to either one

- or both channels, before and after tape.

3.1 Circuit checks and adjustments

3.1.1 Measuring instruments

The following items are essential:

a) 10 000 Ω/V DC Multi range meter
b) VTVM with min. sensitivity of 3 mV full scale
c) Low distortion audio generator
d) Appropriate calibration tape (use “REVOX Bezugsband” or “Bezugsband” 19 DIN
45513" both of which have a frequency response corresponding to the G-36
equalization characteristic)

Where a test tape with response characteristics of the recorder under examination is
not available, other test tapes may be used and a set of correction figures for the
individual frequencies can be obtained from the following equation:

Ґ

and Ґ2 designate the time constants of the equalization characteristics.

1

Other desirable but not essential instruments include a Distortion Factor Meter and an
Oscilloscope.

Direct record and replay figures can only be taken when a suitable bias filter is
connected between the output of the recorder and the measuring instrument.

An external filter will not be necessary for those REVOX G-36 Recorders which are
already equipped with the bias-suppression circuit (printed circuit with L3, L4, C 48,
C76, C77, C78, R112 and R113 on amplifier chassis to the left of P1, see fig. 14 and

schematic G-36) provided that the residual RF-signal is at least 35 db below peak
record level (3 % distortion).

A wow- and flutter meter may be required in rare instances only.

3.1.2 De-magnetising

From time to time the ferrous parts in contact with the tape may become magnetised.
When this occurs, especially with head magnetisation, an increase in background
noise will be evident and in serious instances, partial erasure of the tape may come
about. De-magnetisation should be carried out with the appropriate instrument at
frequent intervals and care should be taken at all times to ensure that magnetised tools
do not come in contact with the head assembly. Permanent damage may be caused to
a calibration tape by magnetised sound heads, as they will tend to partially erase the
high frequencies on the tape. They may also have some adverse effects on the overall
performance of the recorder.

3.1.3 Head alignment

Head adjustment should only be necessary when a head requires replacement or
where the setting has been interfered with.

Five adjustment screws are provided I their functions are as follows:

The centre screw no 22 must be undone to remove the head or to adjust the head
around the vertical axis. Adjust screw no 23 at the rear of the head for height, which
will be correct when the upper brass spacer of the heads is divided by the upper tape
edge. Front screw no 23 should then be adjusted to bring the head face vertical with
the tape. Azimuth adjustment must be carried out by turning nut no 24.

Azimuth adjustment of playback head

Good high frequency response is possible only when the gaps of the record- and
playback heads are parallel to each other. Where recordings are to be exchanged for
replaying on other machines, it is furthermore essential that the head gaps are
accurately aligned to an angle of 90° against the direction of tape travel.

Correct adjustment is achieved by proceeding as follows:

- connect VTVM to the output of one channel

- run alignment tape and adjust nut 24 until a maximum reading is obtained from the
VTVM (observe the instructions included with the tape)

When correctly aligned, a sharp maximum will be indicated and the signal level must
drop when turning nut 24 in either direction.

Azimuth adjustment of record head

It is essential that the playback head has been aligned as outlined above.

- record 12 kc/s on good quality blank tape

- with the output coupled to a VTVM nut 24 of the record head should be
adjusted to give maximum output from the tape.

It is of little significance which of the two channels is used for carrying out the azi­muth adjustment, however, it is advisable to check the performance of the other
channel also.

Head adjustment by the phase check method

The azimuth position of the head gap on stereo tape recorders influences not only the
signal level at high frequencies, it also effects the phase relation between channels.
The criterion of minimum phase deviation provides a more accurate indication of
correct gap angle than the adjustment for maximum output level.

To avoid a phase angle error of 90°, the azimuth should first get aligned by adjusting
for level maximum.

A simple method for adjusting phase can be seen from the diagram fig. 9. Both
channels are connected in parallel and azimuth adjustment is carried out at a test
frequency of 10 kc/s to give maximum output from the tape. Where a double beam
oscilloscope is available this should be employed so that traces from both channels
can be observed simultaneously. Accurate azimuth alignment will permit phase to be
maintained between channels at all frequencies, when correctly aligned a change of
frequency will not affect the locking of phase between channels.

To adjust the record head ensure that playback head is correct. Using the playback
channel as a measuring reference, a signal of 10 kc/s should be recorded and the
record head adjusted to give maximum playback output. Again the phase check
method may be employed and this will have the effect of phasing the machine from
input to output via tape at all frequencies.

Adjustment of four track heads

This is a highly delicate operation as with a track separation of only 3/16", slight
inaccuracies in head alignment cause a track overlap and lead to the very annoying
“dead channel” cross-talk.

Where a four track recorder is under examination, it is advisable to obtain the
REVOX four track alignment tape.

Relevant instructions are included with the tape.

3.1.4 Playback amplifier

Before any tests are made on the playback section of the recorder, it is essential to
ensure that the sound heads are scrupulously clean; minute particles may cause false
readings by partially lifting the tape off the heads.

Connect a VTVM to the cathode follower output of each channel in turn. A reference
signal recorded at 32 millimaxwell per 1 mm of tape width should give an output of
approximately 3 db below that specified in the technical data (refer to 5.). The
frequency response section of the calibration tape is normally recorded 20 db below
this level for measuring the performance characteristics of the playback channels.

A response of + 2/-3 db referred to the level at 1 kc/s is acceptable. Where
unsatisfactory figures are obtained it is advisable to replace the playback head as
outlined in 3.1.3. and take new readings. Should this not bring about the desired
improvement, the playback pre-amplifier should be examined. Remove the earth
connection of the playback head and insert a 10Ω resistor in series (fig.10.)