Originally Web posted 9 July 1998.
Content last modified
Monday, 3 May 2021
External links last verified Monday, 3 May 2021, other than obsoletemedia.com links, because the site is too ornery for my tastes.
This material was originally written for an audience of experienced audio electronic repair technicians and broadcast engineers. Due to the many emails i have received from folks outside these narrow categories, i have decided to elaborate on some of the answers, and add some very common popular questions and answers, originally omitted because the intended audience already knew about them.
Thanks for visiting!
Gerry Gibbs’ submission to the June 1994 Radio Guide got me thinking… i checked the Comments pages at work in the A-77 manual, and here are:
Revox A-77 Repair Tips
Sonic Purity, former stereo repair technician
Transport functions don’t stay latched - Goes to Stop unless buttons are held
- Check for missing Remote Control “dummy” plug. There should be a 90° DIN plug (10 pin) inserted into the remote control DIN socket. If it is missing, the above symptoms occur, since the machine’s relay logic thinks that the Stop button on the remote control is being continuously held down.
The original dummy plug jumpers pins 1 & 2 with wire, so that the machine can operate normally without an attached remote control. You need to either have a dummy plug inserted, a remote control attached, or somehow have pins 1 & 2 jumped if your plug is missing, and you are not using a remote control.
Thanks to the research of one correspondent, we now all know that you can find more information on the dummy plug pinout and what to do at the Revox A-77 Remote Control Plug Schematic page, part of Obsolete Media’s Tape Deck Remote Control Plugs subsite. Which site, as of 3 May 2021, is forcing Javashit on everyone to even look at it, so i’m no longer link validity checking it. Be abused there if you wish, at your own risk.
- (rare) Broken wire in actual, existing, connected remote control and/or bad/dirty switch contacts on Stop button of either or both machine and remote control (if present).
Transport fails to stop automatically when the tape runs out or breaks, otherwise works OK
- Check for missing or burned out auto-shutoff lamp. It is located in the headblock area. From memory (voltage may be wrong) it is a 24V mini-wedge base lamp, which pulls straight out/pushes straight into the socket. Europeans may have an easier time finding replacements than folks in other regions, since these lamps were also commonly used in Tandbergs and other European products of the era. I have never personally seen the light sensor nor support electronics fail.
Machine spills tape stopping from fast winding - Brakes don’t work well
Yes. This is a known design problem with these machines. I usually recommend the following:
- Learn to “rock the reels”. This is basically using your brain to do what more modern machines do automatically. Or to look at it another way, newer machines “rock the reels” for you automatically when you press Stop. Most broadcasters and many recording studio folks who learned on pre-mid-1970s equipment learned to “rock the reels”, and you can too. Our example will assume that you are currently in Rewind mode, and want to Stop elegantly. Reverse the directions to stop from Fast Forward.
- Press Fast Forward (i.e., the opposite direction)
- If you did nothing more, the reels would slow down, momentarily stop, then slowly build up speed in the opposite fast wind direction (Fast Forward in our example). Wait until the reels momentarily stop, then…
- Press Stop at that moment, when the reels are momentarily still
“Rocking the reels” is much gentler on tape (much of which is old and brittle now) than just slamming the machine into Stop. This method can also be used on many (not all) older Ampex, Scully, TEAC, Sony, etc. relay-logic machines. Do NOT try this on machines which do this themselves in their logic (soft-touch Tandbergs and 1980s and newer machines [all brands], for example). It is also likely to cause more problems than it solves on machines where the transport is controlled by a mechanical knob, gearshift, or similar item, instead of electrical pushbuttons.
If you really want to improve the brakes:
- Remove the top gray panel and clean the brake drum and band surfaces with your favorite solvent.
- Experiment with different settings of the springs which pull the ends of the brake bands towards each other, setting band tension. No one in our shop ever found a reliable way to adjust these, so anything goes… just don’t bother writing me if you make things worse!
Capstan has no torque/won’t turn at all, funky audio, assorted weird problems
Thanks to Stereophile Mark, who has offered his help with Revox A77 problems (thanks, Mark!).
- Ensure +21V supply is at the correct voltage. If not, clean or replace P106 and readjust.
- P106 is 2.5kΩ .15W 20%. Any 2kΩ trimpot that physically fits and meets or exceeds these ratings should work just fine.
- Everyone should clean ALL pots, or at least all pots they adjust for reliability and minimal noise. The author recommends suitable Caig products, esp. the spray(s).
Capstan won’t turn at all, or runs at wrong speed (fast or slow)
- Start by ensuring that the +21V supply is at the correct voltage per the above entry.
- Replace bridge rectifier D205 (BY123). I used to recommend testing it, which you may do if you know how. I have since decided it is easier and more reliable to merely replace it with a good-quality bridge rectifier of the same or higher ratings, since some failures will only show up in-circuit under load and not in conventional diode tests. BY123 is showing up on the WWW as 200V 1A and 25A surge. We used generic 400V 1A or 3A (usually the latter) bridge rectifiers where i used to work, with excellent results.
- Replace Q209 (40318), the large TO-3 cased transistor on the heatsink. This is an especially good bet if the speed does not change at all when switched between low and high. This too can be tested, though i no longer recommend it, for the reasons cited above.
- Since the capstan motor is servo-driven, there are no 50/60 Hz frequency adjustments on the A77 (nor other Revox machines of the same age or newer), as there are on some other contemporary reel-to-reel brands.
Both of the above components carry the full capstan motor current and must therefore dissipate heat and thereby are most likely to fail. If the above suggestions do not cure the speed problem(s), keep reading, esp. the capacitors section. Replacing one or both of these components should have no effect on the previously-correct (we hope!) speed adjustments.
Excessive wow & flutter esp. near the end of a reel
- Check/adjust pinch roller pressure.
Won’t drop out of Play when FFWD engaged
- Check for Play relay not dropping. Test hold-in voltage: must be ≈1V, otherwise your relay is too good at staying on, and must be replaced with one of more modest abilities.
Defeat the A.C. power interlock easily with two 8-32 x 3/4" or 4 X 18mm machine screws. Actually any metal pins around this size should work.
Skews tape at capstan/pinch roller interface
- Check capstan shaft for grooving. Studer/Revox wanted about US$121 for a new shaft (around 1990; who knows today?), but your friendly local machine shop can most likely remove just enough material to eliminate the grooves (and thereby the skew) for only US$12.50 or so. Simply readjust both speeds, and enjoy!
Solenoid pulling problems
- According to several sources, chrome-plated solenoid plungers can lose their magnetic properties and therefore pulling power. Replace with the improved black/gray type (apparently under the same part #s) to cure.
Discriminating against flaky capacitors
- Although devastating with humans, with capacitors it is often beneficial to discriminate on the basis of color & “ethnicity” (Home audio repair techs know all about Blue Sanyos & White Elnas and their propensity to leak, for example). For continued reliability, replace all Gray round hard plastic (ADE?) electrolytic capacitors in A-77s (especially early models). Popular locations include Record Amp C507/509, P.B. Amp C813, Input Amp C405/425, and Speed Servo C210/211. They like to open up.
- The author’s favorite source of fresh electrolytic capacitors for jobs such as this is Digi-Key.
Pinch roller engages capstan in fast wind modesThanks to Jean M. of Belgium
- Check for a shorted D123, which isolates the pinch roller solenoid from the brake solenoid. The easiest way to test this is to perform a diode test, measuring between the orange lead of the pinch roller solenoid and the blue lead of the brake solenoid (cathode = orange lead, anode = blue lead). Be sure the replacement diode is a rectifier type which can handle the full solenoid current for extended periods of time.
Identifying A77 “Mk.” versionsThanks to Tom M., though errors are mine
- I do not have solid documentation on this, and welcome further details from (other) Revox experts:
- Brake drum fabric on the brake band, not the brake drum
- Silver lower panel (like Mk. I) and brake drum fabric on the brake drum
- Dark gray lower panel
- Knobs are different
Between 1999 and the present, i have received several requests for service information and/or recommendations for places to take Revox equipment, especially A77 decks, for service. Since many of these email queries have had several points in common, i decided it would save all of us time for me to answer these semi-FAQs here.
- Q1. Can you supply me with parts and/or manuals for Revox products?
- A1a. No. I have only the manual and parts for an A710 of my own, presently not working, which i hope to someday fix up. I used to work at a stereo repair shop, Resistance Repair, in Berkeley, California, U.S.A. At the time, they had only one manual per model, and a limited supply of the most commonly used Revox parts (since Revox parts have long been VERY expensive, at least in the U.S.). Note that the vast majority of electronic components (diodes, transistors, resistors, capacitors, etc.) are standard items which either themselves or generic substitutes can be obtained from your favorite electronic parts sources. My favorites are the mail order companies Digi-Key, Mouser, Newark, and others)
- A1b. Do the usual WWW searching for manuals. Links keep going dead, so i no longer include them in this article.
- A1c. Your ship has come in! The Mother Lode has arrived! Thanks to the extreme generosity of Goreski at B.A.S.E./Analog Rules!, there are now free downloads of complete Revox manuals!
Important: Goreski, like me on this site, went to a lot of time and effort to scan, process, post, and host these manuals, which otherwise would be hard to find and/or cost some serious money… and he is offering them for free. The least you can do is send him a Thank You message… better would be to offer a small payment to help offset his ongoing hosting costs and reward him for his generosity!—there’s a tip jar right on the download page.
- Q2. Who do you recommend for Revox service?
- A2a. In the United States of America and Canada:
JM Technical Arts
313 Rembrandt Dr.
Old Hickory, TN 37138
Contact: Jack Clark, 615-754-8323 (voice), 615-754-8314 (fax)
- A2b. If you know or strongly suspect that you have an electronic/electrical problem rather than a mechanical problem:
Bay Area Studio Engineering
B.A.S.E. does fine work… the issue, as for nearly everyone else, is that Revox-specific parts have been scarce for a long, long time now. Electronic and electrical parts, on the other hand, tend to be more generic and fully satisfactory replacements remain available.
San Francisco, California
- A2c. Elsewhere in the world: as of 2021, i no longer have any idea. The current Revox site doesn’t get into servicing their ancient stuff, just waxing poetic about it.
- Q3. Can you answer my Revox-related question?
- A3. Maybe. So far, there have been few enough of them (thankfully) that i have actually been able to attempt an answer or two. Please keep in mind that no one is paying me to maintain this website, nor answer email, and that i must earn a living in other ways, to the same degree as most of you. You are welcome to ask me a question as long as you understand the following:
- I may not have the answer you seek.
- I may not reply to your query. I may be too busy, preoccupied, depressed, or have other reasons for not responding. My response time normally varies from very fast (same day) to very slow (three weeks or more), unpredictably. Please do not take a non-response as a personal affront!
- My advice, if any, is offered as-is, with no warranty expressed nor implied. It is worth all you paid for it, or less .
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