Revox A-700 User Manual

1975 Revox A700

Revox A700

Test Report (Studio Sound and Broadcast Engineering, October 1975)

FOR MANY YEARS the name 'Revox' has been associated with high
quality recorders in the professional/semi-professional and better­endowed domestic applications. For some reason Revox have seen
little competition in the field; while others have designed recorders to
compete with Revox, their end products have either been too
expensive or lacking in the sound construction and good long-term
performance that is associated with the Revox trademark.

While I do not know the numbers of Revox type 77s sold,
considerable confidence in the sales of the new type A700 is
confirmed by the fact that the pricing of the machine is based on 100

000 units and that the development of the specialised integrated
circuits has probably cost £20 000, let alone the other development costs. With Revox working on such a
large-scale production, it is of course difficult for newcomers to compete in this rather specialised market,
and with a few exceptions the following review shows that the type A700 is indeed a first-class recorder.

Tape transport

The basis of the tape transport is a flanged alloy die-casting, which is machined for the attachment of the
three motors and the head block, itself a separate die casting. The remainder of the unit is in the form of a
cadmium-plated steel chassis to which is attached the electronic parts and the heavy mains transformer
which is, of course, better kept off the main die-casting.

Constant tension winding is obtained by servo control of the two outer-rotor type spooling motors, with the
resulting capability of handling either 267 mm NAB spools or cine centred spools; however, NAB adaptors
are not supplied with the recorder. Tension control signals are derived from tension arms adjacent to each
spool, the tension arms being spring-loaded with their position sensed by a variable transformer which
electrically controls the spooling motor torque. Proceeding towards the head block, there are two large
diameter idlers, the right-hand one driving the tape timer which is calibrated in minutes and seconds at a
tape speed of 19 cm/s, and is also used to detect tape motion—again using an rf variable transformer
technique.

The head block casting forms a plug-in unit with space for four heads, the normal complement being the
erase, record and replay heads. Azimuth adjustment is really sound, with very positive head location. The
tape guidance has been given great thought; there are four adjustable spring-loaded guides on the head­block. I am, however, a little worried about the wear properties of the guides, which appear to be un-plated
brass.

As with all respectable recorders, a flutter roller is included to reduce friction noise (scrape
flutter), but I was surprised to find that a fixed guide is included between the flutter roller and the record and
replay heads. The capstan motor is a special servo motor which is phase locked to a crystal oscillator,
offering constant speed irrespective of the incoming mains frequency at the three tape speeds of 38 cm/s, 19
cm/s and 9.5 cm/s. However, when it is required to operate at different speeds an external frequency may be
used as a reference. The capstan is of large diameter, thus easing mechanical tolerances. The pinch roller

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1975 Revox A700

operates in the trailing position by a solenoid-operated arm, which also manipulates the hinged replay head
hum screen.

All the tape transport drive functions are electrically interlocked, and tape tension is even controlled in the
start and stop modes with a resulting smooth operation without any tendency to throw loops or other
untoward habits. It is quite possible to go directly from fast wind to record without disaster—the tape stops
from fast wind, is lowered on to the heads and proceeds in the record mode!

However, not all is perfect; one shortcoming is that, in spite of the fact that tape motion is detected and the
tape end is optically detected, when the tape comes to a finish the recorder takes a considerable time to stop
the take-up spool with the result that tape can fly all over the place. This defect is caused by too much inertia
and too little friction around the tape motion-sensing roller. A further matter is that, in my opinion, the fast
wind modes are too fast, such that even 3M type 207 with its matt back does not give a very clean wind on
the take-up spool.

All function controls are illuminated push buttons, the current function(s) showing a white light in the case of
non-record functions and a red light in the case of record functions. The three tape speed selection buttons
are only illuminated when the selected tape speed has been attained, providing a safety factor when
changing speed. Other than the normal tape movement buttons, there is a non-locking 'pause' button which
operates in any mode, including fast wind and two further control modes. One of these does a fast rewind
when pressed and reverts to play when released—very useful for editing; the other button provides an 'auto’
function whereby the recorder optically senses a spliced 'window' at the ends of the tape. When the end is
reached, it rewinds and reverts to its original play or record mode at the beginning of the tape.

Access to the heads for editing is quite good, and certainly there is no problem in gaining proper access for
cleaning and de-gaussing. However, it is not possible to listen in the fast wind modes where the tape is
removed from the heads by two solenoid-operated guides. Rock and roll operation is possible in the stop
mode.

Electronics

All the electronic components are mounted on printed boards, most of which are identified with component
references as an ease to servicing. The general arrangement is that small boards plug into a mother board,
which contains a number of components; however, access to these and other components is excellent and
the many integrated circuits plug into sockets as a further aid to servicing. The overall standard of
construction is excellent and the power supplies, which can be operated from all the common mains voltages
and frequencies, are protected by some eight identified fuses. One small irritation is that the mains lead is
not of the plug-in type; however, plenty of storage space is provided for it and its plug.

On the record side comprehensive mixing facilities are provided, selection of stereo inputs being by means
of two input selector switches, each of which has two (left and right) slider-type level controls. One selector
has positions for microphones of high or low sensitivity, RIAA phono, radio or auxiliary inputs, while the
second selector provides for further microphones of high or low sensitivity, multiplay/ echo, an auxiliary input
and an off position.

The microphone inputs are balanced 6.35 mm jack sockets on the front panel and, with the exception of the
radio input, which is a five-pole DIN socket; the remaining rear panel inputs are phono sockets. A further
slider control is provided for setting master level, recording on either or both of the half tracks which are
selected by illuminated pushbuttons.

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1975 Revox A700

Two VU meters are provided for monitoring level in the before or after tape conditions and are also fitted with
peak indicator lights which have a relatively fast response. I am, however, surprised that Revox have not
gone to peak type meters.

The entire replay section is switched to before or after tape by the same switch as the level meters, and can
also be switched to the four modes of operation—stereo, left, right or mono. However, of the two available
line outputs, one is before the mode switch and the other after. These line output phono sockets and the DIN
type record/play socket are at fixed level and occur before the tone control circuits which affect the remaining
outputs.

These remaining outputs comprise a power amplifier output in the form of a DIN socket and two headphone
outputs in the form of stereo jack sockets on the front panel; one of these disenables the power amplifier
output when the phones are inserted. These outputs are controlled by a slider gain control and are affected
by the bass and treble controls which take the form of rotary switches.

In addition to the audio facilities, the power amplifier output supplies a 20V dc line for switching the mains in
the auxiliary power amplifiers made by Revox. As this socket is a standard 180° DIN socket it follows that
some caution is required not to attach other equipment to the 20V dc line. Further sockets are provided for
remote control, and for external speed control; a dummy socket being fitted to the review machine for the
future addition of slide synchronisation.

Perhaps unfortunately, there is no provision for the addition of noise reduction systems, but such an addition
should not be difficult as a customer modification.

Replay performance

Checking of the replay equalisation to the NAB standard was accomplished by means of DIN calibration
tapes manufactured by BASF which are, themselves, subject to a tolerance of

1 dB at high frequencies. It

is therefore most encouraging to report that the Revox performance was within

0.8 dB from 40 Hz to 18
kHz at a tape speed of 38 cm/s and 1.2dB from 40 Hz to 18 kHz at 19 cm/s or from 40 Hz to 16 kHz at 9.5
cm/s. Clearly the machine had been most carefully aligned, and this was confirmed by checking azimuth in
terms of relative phase between tracks.

The signal-to-noise performance in terms of reference level to noise is also excellent as is to be seen from
the following figures which show the performance of the replay amplifier system without tape but with all
motors running (see table 1).

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