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Revox A77

The Old Workhorse

Article from Home & Studio Recording, April 1985

We take a close look at this studio stalwart in all its various forms and point out what to look for when buying one second hand.

The Revox A77 name was once synonymous with the words 'stereo master recorder'. For most studios in the early seventies, it played such a role—and very successfully too. Although overshadowed these days by its younger brother, the B77, it still gives sterling service in many home studio set-ups. Model A77 recorders are widely available secondhand and with their rock-solid reputation for reliability, even today, make an ideal first mastering machine. With this in mind we asked long-time A77 owner John de Jong to emphasise the important points one should look for when buying a used model.

Introduced in 1967, the Revox A77 quickly established itself as probably the most popular tape machine in its price bracket with worldwide sales of over 300,000. Perhaps best described as a high quality domestic machine, the A77 soon found its way into many professional situations where its reliability and electrical specification made it a useful secondary machine to have around. It was superseded in 1977 by its younger brother, the B77, which has now become an industry standard in this class of machine. It's interesting to note that, at least on paper, the A77 has an identical performance specification to the B77 - the major differences being in the packaging and metering which make the B77 an easier machine to use. But more of that later.

During the ten or so years that it was on the market, the A77 evolved from a Mark 1 through to Mark 4; early machines were characterised by a silver control panel and clear plastic knobs, whereas the Mark 4 version had a dark grey control panel with grey and chrome knobs. Table 1 summarises the distinguishing features of the four basic variants. With one or two exceptions, most of the evolutionary changes were cosmetic with only a minor redesign of the 'works' - notably the capstan motor and drive circuit, and the tension arm and head block assembly. In addition, a number of standard options were available; two or four track operation, with or without carrying case, and so on. The Mark 3 and 4 models were also available as high speed (7½/15 ips) machines.

Although the A77 is no longer in production it shouldn't be thought of as simply a bit of tape machine history! Many are still in regular 'home' service and the continued availability of spares and number of service outlets means that the Revox A77 will occupy a corner of the secondhand market for many years to come.


The A77 Mark 4 we've chosen to look at is an unpretentious machine; rather dull to look at, perhaps, especially when compared to some of its racier Japanese counterparts, but "all that's gold doesn't glitter" and aesthetics aside, it is solid, well constructed, and - as the years have shown - built to last.

The control panel is divided into two sections with monitoring controls on the left below the tape transport controls, and recording controls on the right alongside the meters and the power/speed select switch. Between these two sections are three quarter-inch jack sockets for headphones and microphone inputs one and two, the latter paralleled with phono sockets on the recessed connector panel at the top of the machine. Monitoring is controlled with the two dual-concentric controls: the inner pots are output Volume and Balance controls while the outer rings provide switching between Source and Tape signals and Mono monitoring of either (or both) channels or Stereo output.

Over on the record side, two pushbuttons on either side of the meters select each track to 'record' and underneath these are a further two dual-concentric controls providing control over input level and switching between five sources: 'HI' or 'LO' impedance mic, 'RADIO' (low-level DIN input), 'AUX' (line input), and 'I-II' which allows one track to be bounced across to the other for 'sound-on-sound' effects. The only real complaint I have about the control panel layout is the position of the three jack sockets; it's all too easy to plug a microphone into the Phones output which can't do microphones much good!

The connector panel at the top of the machine carries all the inputs and outputs (apart from headphones) on either phono or DIN standard sockets. The exceptions to this are the non-conformist mains input connector and the remote control connector, both of which are of dubious parentage! (It's a fairly simple job if you're technically-minded to replace the existing power connector assembly with a standard IEC mains connector: although probably not recommended by Revox it does help if you're always losing power leads!) All tape transport control functions can be switched remotely via this rather curious multipin connector - note that a blanking plug must be fitted when a remote control is not being used otherwise the front panel tape transport controls will not work.

Although the A77 is harder to get into compared with professional tape machines that are designed to be regularly lined up when in service, once you are inside you are rewarded with a logical mechanical layout with plug-in circuit boards and well labelled line-up trimmers. Any tape machine should be regularly serviced for best results; if you have test equipment and some technical knowledge at your disposal the A77 is not difficult to set up correctly, although I would definitely recommend that you invest in a service manual before attacking it with a screwdriver!

As an aside for the technically-minded, note that the level setting procedure in the manual (based on finding the level at which distortion commences) is no longer valid because tape has improved significantly in the last few years: the normal line-up level for 0VU is 257nW/m.

Practical Life

To give you a feel for how the A77 behaves in use, let me now look at some of the operational peculiarities and hazards that I've noticed during several years of ownership.

Tape Transport

No complaints at all about tape handling as long as you remember that there's no sophisticated control logic as found on more recent machines. All this means in prctice is that you have to be a little more patient when going from fast wind to 'play'.

The first thing that strikes you when loading a tape is that it's not particularly easy, especially with 10½ inch spools! There are two reasons for this: firstly 10½ inch spools only just fit on and things are a bit cramped near the headblock assembly, and secondly, the tape path is below the level of the control panel which means it's harder to thread through, which brings me to my next point...


The heads have a plastic cover which is easily removed (perhaps too easily because it often gets lost!), giving reasonable access to the heads for cleaning and editing purposes. Access is limited though because of the raised control panel which also makes editing a bit tedious. It is possible to mark an edit point on tape at the playback head, but if you are doing a lot of editing I have found that it is best to mark the head at the right hand tape guide and then have a line scribed on the editing block corresponding to this distance from the playback head. This arrangement makes it easier to mark the tape as well as keeping the playback head clean from chinagraph pencil marks. While we're on the subject of editing, note that there is a switch to turn off the reel motors - great for dumping larger chunks of tape.

You may come across some machines which have been modified to make editing easier that feature a raised motor and headblock assembly and a restyled one-piece top deck. This is one of the few modifications which when carried out properly has factory approval, the only likely degradation on these machines being a slight worsening of the noise performance due to mis-alignment of screening components.


I found this to be the most confusing part of the tape machine and I am comforted by the fact that other people have had similar problems! There are two areas of concern: (a) what are the meters actually looking at and (b) how do you interpret the reading? The following pointers may help: (a) The meters are permanently connected to the output of the record amplifiers. This means that they always read what is going in to the record amps, and this depends on two sets of switches - the Record and Source selector switches. With the Record switches (either side of the VU meters) de-selected, signal from the input selector switch is routed to the opposite record amp; this means that meter 1 reads input 2 and meter 2 reads input 1. When you initialise the Record switches, however, the meters are interchanged and read as normal. To sum up, the meters always look at their respective inputs unless the record switches are deselected, in which case they are reversed. Not confused yet, I hope!

The reason for this seemingly illogical system is to allow various 'sound-on-sound' effects to be implemented; with only one Record button selected, it's possible to record a mix of the two inputs on that channel only, or to record a mix of one input plus 'bounced' output from the other channel. A little confusing at first, but a versatile system which in practice is not difficult to use.

Note that the only way that you can meter the output from tape is to select 'I-II' and 'II-I' on the input switches, although as before, with the Record switches de-selected (off), the meters will be reversed.

(b) The so-called 'VU' meters do not conform to VU standard and give little more than a good idea of the average signal levels present. They should not be trusted and experience will show the best way to use them. If you are recording from, say, a mixing console with good output meters, the best way to work is to set your levels using a line-up tone and record using the console's own meters (this assumes, of course, that the A77 has been set up correctly and that the meters have been calibrated for a pure-tone input).

Input Headroom

After the input selector switch, the first thing that a signal sees as it hums its way into the A77 is an amplifier; for the input level control is after this front-end stage. What this means is that you can't put in what the recorder considers to be a large signal, and then use the level control to back it off to a reasonable level. If you do this, you are in danger of overloading that first amplifier with the result that you get a nasty cracking noise on signal peaks. It's best to work with the input level control set above the halfway point, both from the point of view of input headroom and noise. The A77 is designed to work with 'domestic' line-level signals of around -10dBu (250mV), so if you are working at higher levels it's advisable to attenuate the signal by 10dB or so by using a resistive 'pad' before the input.

Output Noise

Similarly, to maximise the output signal-to-noise ratio it's best to work with the output level control wide open and attenuate the signal elsewhere if necessary. Particularly in mid positions of this pot, there is noticeable pick-up of the whine from the capstan motor drive circuit. This may be a fault on my machine but nevertheless the above rule applies.

Buying Secondhand

If you buy an A77 today it will obviously be secondhand as they stopped making them in 1977! Here are a few points to bear in mind before you buy:


These are still readily available at many service outlets throughout the UK so there should be no problem in maintaining a machine or bringing it up to specification if it's been damaged or mistreated. As I pointed out earlier, many of the circuits are plug-in cards and servicing and lineup is easy compared to most other tape machines. Some parts and repairs are more expensive than others, obviously, so it's worth getting a quote if you notice a major fault; two things in particular worth checking for are head wear and capstan motor wear.

Head Wear

Apart from the general appearance of the machine, the condition of the heads will give some indication as to how well the machine has been looked after and how much use it has received. Excessive head wear is characterised by a drooping high frequency response and a head with a flattened face and a widened pole-gap. Some wear is to be expected, however, and as long as the heads are not worn unevenly or excessively, there may be many hours of useful life still left in them.

Capstan Motor

The capstan motor should run evenly with only a slight whine being audible - any rumbling or coarseness will indicate trouble. A new motor (circa £100) will be required if the recorder is a pre-Mark 4 model, otherwise an exchange motor may be possible. If you have the chance, record a 1kHz tone on both tracks and listen for any pitch (wow) or level fluctuations on playback. If so, this will indicate excessive wear, especially if the frequency of 'wow' is related to the motor drive frequency, otherwise it will be a good indication that the machine is only worn mechanically.

A Warning!

There are a number of non-approved modifications to the standard A77, some of which are more successful than others. Here are a few to watch out for: The most common non-standard modification is the conversion of a low-speed machine to high-speed simply by fitting a larger diameter capstan. Although this may work well in the short-term, the larger capstan causes tape misalignment with attendant long-term wear problems. It can be recognised by a stick-on label below the speed select switch (often in German to make it look more realistic) and by the fact that the capstan shaft will be offset in its mounting hole towards the tape. If it's offset the other way you're OK.

Other things to watch out for which I'll just mention are sync playback (ie. from the record head), non-standard vari-speeds and high speed Mark 3 and 4 versions fitted with Dolby B (none were produced). With so many A77s around you're bound to come across a few that have been tinkered with; they may be good value for money and work perfectly well for the job you've got in mind, but it's worth checking that what you are buying is what it pretends to be.


Many of the operational difficulties that I've outlined in this report are easily overcome with a little patience and care and should not colour your opinion of this otherwise excellent tape machine. It's interesting to note that many of these limitations were overcome when the Revox B77 was introduced, although the performance specification remained essentially the same. In terms of performance, the A77 is better than many other machines in this price bracket and, if you can come to terms with its operational quirks, represents good value for money at a secondhand price of around £200-£300.

With the number of bona fide service agents around and the ready availability of spares, the Revox is easily maintained, and if you look after it carefully, it'll probably still be devoted to duty in ten years time!

My thanks to F.W.O. Bauch Ltd for Information contained in this article. They may be contacted for further information on Revox tape machines and for spares and service (F.W.O. Bauch Ltd, (Contact Details)).

Table 1: the main features of the four 'marks' of Revox A77 tape machine.

(1967-late 1968)
Brushed aluminium front panel and shiny top cover. Grey control knobs with clear plastic skirts - legend printed on panel. Grey transport pushbuttons, VU meters only illuminated when in record mode. None.
(late 1968-1971)
As Mk1. Modified tape tension arm (now sprung instead of fixed). Tape guide between tension arm and erase head replaced with roller-bearing version (was fixed).
Control knobs as above but front panel now grey. Spool turntables now silver finish. Permanently illuminated VUs. Now available as high speed version. Dolby B available on low speed only. Later MkIII fitted with improved heads giving longer life (recognisable as only one screw used to hold head to baseplate instead of two).
Grey conductive finish on spool turntables (to reduce static) and crackle finish top cover. Control knobs, rotary switches and transport controls now silver finish with scale markings on grey skirt. Apart from cosmetic changes later machines had quieter tape counter and capstan motor drive circuit using ICs instead of discrete components, giving greater varispeed variation (±80% instead of ±7%).

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A Powerful Combination?

Publisher: Home & Studio Recording - Music Maker Publications (UK), Future Publishing.

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Home & Studio Recording - Apr 1985

Donated & scanned by: Mike Gorman

Feature by John de Jong

Previous article in this issue:

> C-ducing Cymbals

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> A Powerful Combination?

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