Revox A77 restoration

The Revox company introduced the A77 tape deck in 1968 replacing the vacuum tube based G36 deck (which is believed by some to be the best sounding machine ever made for tape playback). A few years ago I bought, at a very low price, a MK II version of the A77 that had been involved in a fire. I began completely disassembling the deck, taking everything off of the frame and even unfastening the frame itself. The beauty of the A77 is that it is completely modular, with a few basic tools it can be taken apart. My goal was to bring this deck back to as close as possible its original condition, part by part.
Here is a photo of the basic, three part frame with the three motors reattached. You can see the AC transformer in the center, the power supply board to its left. The speed control circuit board is below the transformer



  • Kenneth
    Two stoopid questions:
    1) I'm confused by the way that you worded it, but was the G36 or the A77 considered by some to be the best sounding machine ever made for tape playback?
    2) Was your purchase and refit of the Advent Dolby noise reduction unit prompted by this machine?
  • Sorry for the confusion, the wording was not clear. The G36 (called a 736 in England) was the last vacuum tube based consumer deck Revox produced. So, when the A77 was introduced, a solid state design, it was compared to the earlier G36. Most felt the A77 was the superior in terms of making a recording, but many felt the G36 sounded better when playing an already recorded tape. The G36's reputation for playback capability has remained high and is a sought after machine. Some of this performance difference might be attributed to the nature of the playback heads used in vacuum tube versus solid state decks. In tube decks the heads are usually high impedance compared to heads used in solid state machines. They also tend to be higher in output and as a result require less gain for playback circuit designers to make.
    One of the goals is to have a playback deck with direct connection to a De Havilland 222 tape preamp instead of using the original playback circuit. I'm not sure how I will do this on the A77, I'm hoping for inspiration along the way.
  • EndersShadow
    EndersShadow Posts: 17,502
    Ken it’s a privilege to watch you bring things back to life.
    "....not everything that can be counted counts, and not everything that counts can be counted." William Bruce Cameron, Informal Sociology: A Casual Introduction to Sociological Thinking (1963)
  • mhardy6647
    mhardy6647 Posts: 32,032
    edited January 2021

    The G36 :)

    Pete Townshend had -- a few -- of 'em, it appears, BITD. :)


    The A77 is a very respectable machine, though -- even if it doesn't have quite the cachet of its predecessors or its successors.

    @KennethSwauger -- is your A77 rehab a warm-up to tackle a big-boy Studer deck? ;) I hope so!

    Good luck & have fun! We're all* living vicariously through your rehabs! :) Keep the posts & photos coming! They're grrrrrreat! :)

    * I.e., those of us who are into such things... :#

  • Here's a photo of the speed control circuit board. This circuit uses the output of a tachometer placed close to the outer rotor of the capstan motor (the brownish colored object on the right) to regulate the accuracy of the motor. I replaced most of the transistors and the Faston connectors and some capacitors. There is another version of the circuit board which uses two integrated circuits. This change was introduced in the MK IV version.

  • Ah, the Studer A80 would be a wonderful deck to have, that's for sure. I'm about 15 years too late for that, however. A friend bought one for $800.00 some time ago. That amount would probably buy a playback head these days. Maybe not. Reconditioned units sell for $45,000.00 and have a waiting list to fill.

    By shear coincidence this story about the writers top ten open real decks choice is on Hi Fi News & Record reviews website.
  • mhardy6647
    mhardy6647 Posts: 32,032
    That's a really weird list.
    I've got three of them (which should tell you how weird it is)... and I used to have a TC-350... have a TASCAM not all that different than the one in the article... and an RT-909 (as opposed to an RT-707).
    really weird list.


  • There is a wiring harness that allows the various parts of the transport section to connect to each other. This includes the three motor run capacitors and the control circuit board to the power supply and the capstan speed controller board. I used a Dremel with a small wire wheel to clean all the Faston connectors. In hindsight I should have positioned this harness before I installed the two circuit boards. I had to drop down the capstan control board in order to bring the harness under the AC transformer.

  • Here's a photo of the harness with the three motor run capacitors at one end. Sorry for the dark image.

  • Here's a photo of the capstan motor just before reassembly. I sent the axis to Jack Clark of JM Tech Arts to have the end surfaced for better tape contact. I replaced the ball bearing and cleaned everything thoroughly. You can see the two belleville spring washers which preload the bearing allowing the deck to be used either vertically or horizontally.

  • honestaquarian
    honestaquarian Posts: 3,021
    edited January 2021
    I think he "horizontally" paid jack Clark to do everything ;):p:D

    Who threw the canned air?!?!?
  • Here's a photo of the capstan motor put back together. The capstan axis is a very important component in getting low speed errors (wow and flutter) and needs to be carefully handled. The run-out is given as better than 1/1000 mm which equals 40 micro-inch according to the Revox service manual. Now to work on cleaning up the magnetic shields that go in front of and below the motor to reduce hum pickup by the heads and the electronics.0d5k8061npba.jpg
  • Here's a photo of the lower magnetic shield that goes around the capstan motor. I put a piece of masking tape over part of the metal to show before/after of polishing.

  • This is a photo of the finished transport control circuit board. This component has the relays that control all the transport functions, such as play, fast forward, rewind and record engage. The brake solenoid and the solenoid that activates the capstan roller are engaged or disengaged according to the logic of which function is needed. There's also a portion of the board that applies a slightly higher level of voltage to the take up motor to prevent tape bounce when first beginning play then the voltage drops down to normal. There are two transistors that are part of the end of tape shut off system. I replaced just about everything on this board, all of the higher power resistors, the transistors, capacitors and even the relays themselves are new.sigpbayzy52i.jpg
  • Here's another photo of the board from a different angle.

  • wye0v293gam7.jpg

    This shows the two transistors, I used a Teflon mounting bracket to match what Revox did in later versions of this board.
  • This photo shows the capstan motor's metal shield and the transport control board mounted on the frame. I still have to connect some Faston connectors.byewhnhp1m6d.jpg
  • Here's a photo of the capstan roller's solenoid in the paint shop. It was fairly crusty when I started, so I had to sand off the rust and oxidation and work at the housing with Flitz metal polish.

  • [Deleted User]
    [Deleted User] Posts: 7,658
    edited February 2021
    Thanks for your kind comments, tonyp063. I think I started soldering along time ago on Heathkit, Dynaco and Harmon Kardon kits. To avoid ineffectual soldering joints I first polish the component leads with Flitz to get off any oxidation, then after soldering I check the continuity from the lead to the next component on the circuit board it gets attached to. I've gotten so I can tell from the meter's beep if it's a good connection or not. Then I trim off the excess leads.
    Post edited by [Deleted User] on
  • Here's a photo of the finished capstan roller solenoid.

  • mhardy6647
    mhardy6647 Posts: 32,032
    Wow. I seriously doubt it looked that good new.
  • Thanks, Mark, I get a little carried away with these projects. Here's a photo of the brake solenoid which seems very similar to the one used on the capstan roller. I believe there is a slight difference in the DC resistance of the two coils.

  • [Deleted User]
    [Deleted User] Posts: 7,658
    edited February 2021

    The copper washers on the mounting screws adds a nice touch.
  • mhardy6647
    mhardy6647 Posts: 32,032
    ...I get a little carried away with these projects...
    You've taken up the right hobby! ;)