目次
A Revox B77 that has sat unused for twenty years rarely fails in one single place. The brake bands have gone hard and glazed. The counter belt has turned to tar and taken the gear with it. The pinch roller has hardened to the point where it polishes the tape instead of gripping it. Each of these is a small job on its own. Together they are the whole distance between a machine that sits on a shelf and a machine that records.
This page treats the tape transport as one system rather than as a list of separate repairs. It begins with the mechanism as a whole, then works through the three parts that fail most often, in roughly the order you will meet them when a machine comes in.
Part numbers quoted here can be looked up in the Revox B77 and A77 parts reference, which holds the complete exploded view and every capacitor position in searchable tables.
One point before you start. Nothing in the tape path should be forced. Every assembly on this machine comes apart in a defined order, and if something resists, you have almost always missed a screw or a circlip rather than found a seized part.
Overhauling the tape transport
We are archiving the knowledge and information necessary for repairing the Revox B77.
The information will be updated continuously, and we hope to complete it as a single resource with everyone's help.
Disassembly procedures, parts destinations, and consideration of domestic alternatives.
We also aim to systematize test points, waveforms, calibration values, and failure examples with photos and tables.
All images and information quoted from the web are noted, but if there are copyright issues, please advise.
Front panel structure
Image source: Revox B77 official manual
Head structure
Image source: Revox B77 official manual
Capstan motor control board
Forum Notes
The capstan motor control board has trimmer potentiometers for speed adjustment.
There was one case where the wiper lost contact with the carbon track, causing the motor to stop.
The B77 Capstan Speed Control board (1.177.325/326/327) has at least two trimmers for speed adjustment (the location and designation vary slightly depending on the serial number and board version). Their functions are generally as follows.
- SLOW (LOW/19 cm/s) adjustment trimmer: sets the reference speed for 19 cm/s (calibrated with a frequency reference tape).
- FAST (HIGH/38 cm/s) adjustment trimmer: sets the reference speed for 38 cm/s.
Approximate physical location
Two small trimmers are arranged near the top or upper-right side of the board (on Mk-I they are often blue or white single-turn types; on Mk-II/327 they may be sealed types).
The silkscreen marking is one of “FAST / SLOW”, “HI / LO”, or “38 / 19” (varies by version).
They are often located near the ICs (NE555 or comparator ICs) when viewed from connector P1 (the 8-pin connector that carries tacho/speed/AC input).
Some versions have an additional trimmer for loop gain/start correction (one extra) besides the speed trimmers.
Its designation may be “Servo”, “Gain”, “Start”, etc.
First, identify the two “38/19” trimmers.
Can a “wiper losing contact and stopping” occur?
If the wiper goes open-circuit (infinite resistance) or makes poor contact,
- the reference voltage is lost, resulting in a reference frequency of zero → the servo cannot operate, causing a stop or loss of speed,
- conversely, some units may run at an extremely high speed (depending on the board constants).
Quick Diagnosis and Temporary Recovery
- Before applying power, record the position with a photograph.
- Measure the total resistance (end-to-end) of the trimmer with a DMM, and note the actual wiper position (end-to-wiper).
- With the power off, rotate the wiper back and forth between the ends several times to break through any oxide layer → return it to the original position.
- If this improves the situation, consider replacing it with a sealed type (Bourns, Copal, etc.). Replacing a single-turn type with a multi-turn sealed type (10-22 turns) is effective in preventing recurrence.
- If it does not recover: the wiper may be broken or internally cracked → replace.
Replacement Tips (Safe and Quick)
- Read the actual measured value of the trimmer (e.g., 10 kΩ, 22 kΩ) and replace it with a sealed type of the same value.
- If the pin pitch does not match, mount it using short jumper wires.
- After replacement, set the initial position to the center, then proceed with calibration.
R14
R14 is a trimmer potentiometer (10 kΩ) for the speed reference on the B77’s “Capstan Speed Control PCB (1.177.325/326/327)”, used to fine-tune the capstan rotation speed reference.
On forums, there are multiple reports of “R14 (speed adjust) failure causing stoppage or instability”, with cases where replacement resolved the issue. diyAudio
- On the component side, near the top or right side, around the ICs (NE555/comparator circuit), a small slotted trimmer is often R14 (some versions have R14 printed on the silkscreen).
- The trimmer that is connected to the timing network (R/C) of the NE555, with wiring returning from the vicinity of connector P1 (the 8-pin with TACHO±/SPEED/0V…), is the one in question.
- Version differences: Early Mk-I (1.177.325-00, etc.) may have a single-trimmer configuration, where this is R14 (high-speed reference), and the low speed is derived from a fixed ratio. Later/derived versions (1.177.325-81/326/327) may have separate SLOW/FAST trimmers; in that case, the high-speed one is R14, and the low-speed one has a different designation (e.g., R…). *Follow the actual silkscreen on the unit.
- Multiple service notes and replacement kits mention “R14 = 10 kΩ” (the “Preset pot upgrade kit” correspondence table also includes the trimmers on the Speed Control board). pinkfishmedia.net+1
Reference: On Tapeheads/DIYaudio, there are reports that even after replacing R14, other causes (tacho wiring, rectifier/small electrolytics, output TO-3, oxidation other than trimmers) can also be present and may not resolve the issue. tapeheads.net+1
Quick Checks and Actions (Identifying and Replacing R14)
- Look for “R14” on the board silkscreen (if found, it is confirmed). If not, measure the end-to-end and wiper resistance of the trimmer in the above location and check if the total resistance is approximately 10 kΩ.
- With the power off, rotate the wiper back and forth between the ends several times to scrape off any contact film → return to the original position.
- If the speed is unstable or stops, replace with a sealed cermet/multi-turn 10 kΩ trimmer (10-25 turns). If the pin pitch differs, extend with short leads.
- After replacement, allow a 10-minute warm-up, then perform speed calibration in the order of 38 cm/s (FAST) → 19 cm/s (SLOW), repeating 2-3 times. Use a 3 kHz reference tape and a frequency counter (or a high-resolution tuner) to adjust.
- Also, be sure to check for solder cracks on P1 and the polarity of the tacho wires (if these are floating or reversed, adjusting R14 will not fix the problem). vintage-radio.net
Parts List
*Manually transcribed from the service manual
Counter Repair
It is rare for the counter itself to fail, and even if it does, replacements can be secured.
Most cases where the counter does not move are due to a failed rubber belt or gear. (When purchased second-hand, these parts are often missing altogether.)
These can be bought as a set on eBay, but the cost-performance is poor, so it is preferable to source them domestically.
For the rubber belt section, the specifications are: inner diameter 88 mm, inner circumference 276 mm, cross-section 1.2 mm square.
Image credit: nagravox
Referring to models like the A700 makes it easier to understand: some models require this type of gear, while others, like the B77, do not.
This gear is available from nagravox for $25.
On eBay, similar prices are seen for sets including the rubber belt, but it is said that a 3D-printed substitute is possible, and some eBay sellers offer items made with a 3D printer.
Brake Lining
First, I want to mention that I rarely use the stop button during fast winding in either direction. If there is a problem with the brakes, the tape will spill or break. I have had many machines pass through here in the last month, and I do not rely on memory to know whether the brakes have been serviced. After thoroughly cleaning the brake bands with acetone, I use a small steel wire brush to scrub the entire drum, removing buildup and fluffing the lining. After reassembly, I test the difference when rotating the reel table in the normal and opposite directions. There should be a significant change in the measurements between the two directions, and the left reel should behave opposite to the right reel. Next, I adjust the spring position on the brake arm to match that difference. If the spring has fatigued slightly, as a final fine adjustment, I bend the brake arm as shown. This is the limit of how much it should be bent. Two points to note: bend the upper arm only after it has passed the stop pin. Also, ensure the lower arm does not touch the stop pin at rest. Adjustment of the lower arm is possible with only a slight bend.
Active as a Revox rebuilder since 1975: Mark Gardiner
STUDER Brake Bands for Professional Tape Recorders
The brake system
The B77 brake is not purely mechanical; it is a hybrid of electrical control and mechanics.
The basic operation is simple: a spring keeps the brake constantly ON (pressed), and energizing an electromagnetic solenoid releases it.
Furthermore, the logic board combines the take-up motor torque control (electronic brake) and tape tension detection to create an optimal braking sequence for each mode: stop, play, fast forward, and rewind.
Typical Troubles and Initial Diagnosis
To build a three-machine B77 setup at Kuon R&D, we are restoring the remaining unit.
This third unit has a completely broken brake control and needs repair.
As mentioned, the brake control is a hybrid, so isolating the problem is paramount.
Understanding Part Names
B77 Brake Mechanism Disassembly and Repair Procedure
Removing the brake mechanism is relatively simple compared to electrolytic capacitors or other boards; just remove the screws.
The plate section also has three screws, which require a precision screwdriver.
As of October 2025, the plate itself costs just under 20,000 yen on eBay.
Colorful variations made in China are also available, so creating your own unique unit might be a nice idea.
The structure itself is simple, switching the drive section on and off electrically for control.
There are parts that need adjustment: the left and right rings. The braking force changes depending on how tightly these are fastened.
The tightness depends on the position where the spring is attached.
A guideline is to achieve the same deceleration feel with the same force.
Hook the spring into the flange holes (multiple are provided) to minimize the difference in initial tension between left and right.
In the end, the brake on this third B77 for our academy was stuck because something was jammed and fixed in place.
Since it was age-related deterioration, we cleaned it with alcohol, applied sewing machine oil and 556, and after moving it for a while, the brake started working.
However, compared to a normally functioning unit, it still seems a bit stiff, so we will add the adjustment method for this later.
Tape counter belt and gears
~ Fix it for less than 1/10 the price of eBay. Make full use of Japanese industrial standards ~
Does the counter on your Revox B77 not turn?
When you open it up, do you find the yellow belt crumbled to pieces, or hardened and cracked?
Searching for a dedicated belt on overseas sites reveals astonishingly high prices.
But there is nothing to fear. This is "Monozukuri powerhouse Japan".
Generic industrial parts available at home centers or MonotaRO can be used as substitutes.
*Note: Currently being updated we are considering restoring the gear with a 3D printer.
We will announce it here once the update is complete.
Current Analysis: The True Nature of the Yellow Belt
The "yellow toothed belt" in the photo below.
This is an original timing belt (toothed belt).
The area above this line is displayed for free.
- Role: Transmits the rotation of the right reel motor to the counter without slipping.
- Reason the original is toothed: To prevent slipping even during sudden fast-forward and rewind actions, eliminating count errors.
However, unless you need second-by-second logging for strict professional recording, a "non-slip rubber belt (O-ring)" is a perfectly adequate substitute.
Many B77 users around the world have switched to O-rings due to their easy availability.
If second-by-second logging is required, a digital conversion is necessary.
Here is a specific purchase link for the "O-ring S-60", which has the most proven track record as a Revox B77 tape counter belt and is inexpensively available in Japan.
Determining the Size of the Yellow Gear (Short Belt)
① Reading the Caliper Value
- Measured value (diameter): 42.0 mm (the "0" mark on the caliper aligns with the "42mm" position on the main scale)
② Calculating the Circumference
Calculate the length (circumference) when the belt is stretched out straight.
Diameter x 3.14 = circumferencecopy
42mm×3.14=131.88mm42mm×3.14=131.88mmcopy
③ Target Values to Search For
When substituting with a rubber belt (O-ring), choose a size "slightly smaller" than the measured diameter (42mm) and stretch it during installation to create grip.
Therefore, the sizes to look for are as follows.
- Target inner diameter (ID): 38mm to 40mm *Choose 2 to 4mm smaller than the measured 42mm
- Target thickness (cross-section): 1.5mm *Based on the thinness of the original belt and the depth of the pulley groove, the S standard (1.5mm) is optimal
Based on the above values (inner diameter 38-40mm), we identified a standard product (JIS standard) available in Japan.
Please use the following keywords and codes.
Optimal Solution: O-ring S-40
For a measured diameter of 42mm, use a ring with an inner diameter of 39.5mm.
This provides appropriate tension and is the best-fitting size.
- Standard: JIS B 2401 S standard (for fixing, cylindrical surfaces)
- Part number: NOK O-ring S-40
- Specifications: Inner diameter: 39.5 mm, Thickness: 1.5 mm
Search Data (for Readers)
When readers search on MonotaRO, Amazon, or Google themselves, please copy and paste the following information.
- Search term: O-ring S-40 or NOK S-40
- MonotaRO order code: 07671814 (NOK O-ring S series, Class 1 A S40)
- Measured diameter: 42mm
- Compatible O-ring: S-40 (inner diameter 39.5mm)
This is correct. Next, based on this calculation, we will also determine the values for the other "long black belt".
[Note]
The original yellow belt often hardens and becomes brittle, or conversely stretches out, due to aging.
The yellow belt in the first photo (caliper value approx. 42mm) may be severely deformed or a part from a different location.
For the B77 counter, the size of the black belt in the fourth photo (caliper value approx. 64mm) is correct (that length is necessary given the distance between the reel table and counter), so "S-60 (inner diameter 59.5mm)" will fit perfectly in length.
As long as you clean the dirt remaining in the groove of the original yellow belt, it will move surprisingly smoothly.
Currently, using this O-ring as a substitute seems to be the mainstream worldwide.
Black Belt (Longer One)
① Reading the Caliper Value (True Value)
- Main scale: Exceeds the number "6" (60mm).
- Graduation: The "0" on the vernier (secondary scale) aligns with the "fourth" mark after "6".
- Measured value (diameter): 64.0 mm
② Calculating the Circumference
Diameter x 3.14 = circumferencecopy
64.0mm×3.14=200.96mm64.0mm×3.14=200.96mmcopy
This circumference of "approximately 201mm" is a physically very consistent value for the distance from the right reel motor shaft to the counter pulley on the Revox B77.
Correct Compatible Part Based on This Value
For rubber belts, it is standard practice to choose a size "about 4mm to 5mm smaller" than the measured value (64mm, which may be stretched) to provide tension.
Therefore, the target sizes to look for are as follows.
- Target inner diameter (ID): 59mm to 60mm
- Target thickness (cross-section): 1.5mm to 2.0mm
Search Data and Summary
Now that you know the sizes of the two belts, you can control the counter movement.
There is no need to buy original parts.
The following two "O-rings" can perfectly substitute.
① Internal short belt (replaces the yellow gear)
- Measured diameter: 42.0 mm
- Compatible part: NOK O-ring S-40 (inner diameter 39.5mm)
② Drive long belt (replaces the black rubber)
- Measured diameter: 64.0 mm
- Compatible part: NOK O-ring S-60 (inner diameter 59.5mm)
Be sure to search by size.
Replacing the pinch roller
Even today, many Revox A77 machines are still in active use. The Revox series continues to have parts available in 2025, allowing for replacement and repair to keep them running. Simple replacements are not overly difficult and can be done at a level comparable to assembling a plastic model. This article will help you thoroughly understand the procedure for replacing the pinch roller on the A77.
When to Replace
The timing for pinch roller replacement must be determined by monitoring the condition of the rubber. Once it starts to become sticky, it is already too late, so early replacement is necessary.
Basics: Standard Pinch Roller Replacement Procedure
Pinch roller replacement requires tools and preparation. Tools include a flathead screwdriver (some models may use Phillips screws) for removing the front panel fixing screws. Also, tweezers or fine needle-nose pliers (for removing retainers and washers), a small flathead screwdriver (for prying open clips), etc. Having multiple tweezers is reassuring. Personally, I find reverse-action tweezers, which I use when building DIY microphones, to be quite convenient.
Pre-Work Preparation
Unplug the deck's power cable and secure a safe workspace. Always place the deck horizontally. Although the A77 is often imagined in vertical use, if space allows, horizontal placement provides overwhelmingly more stable tape movement. Also, to avoid losing small parts, it is advisable to work with a tray or cloth laid out.
Removing the Front Panel
On the A77, to access the pinch roller, the front gray panel (head cover section) must be removed. First, remove the various adjustment knobs below the yellow line. With the deck horizontal, pulling them upward makes removal quite easy. Only the power switch knob can be quite stiff, so carefully remove it upward using multiple flathead screwdrivers without damaging it. Remove the two fixing screws at the bottom of the front panel (indicated by red arrows). Many repair manuals and guides mention two screws, but on my MKIII, a total of four screws needed to be removed. Each screw has a washer, so handle them with care. Dropping a washer inside makes retrieval difficult. The top of the panel is secured by plastic snap pins (fasteners). After removing the screws, gently lift the entire panel upward and carefully release the fasteners one by one.
Pinch Roller Removal Procedure
Once the front panel is removed, it is time to remove the pinch roller itself. Locate the pinch roller: it is the black rubber roller that sits next to the capstan (rotating metal shaft) on the right side when viewed from the front of the deck. The brass-colored part on the right is the capstan shaft, and the black disc on the lower left is the pinch roller. In normal stop mode, the pinch roller is away from the capstan, making replacement easier. Ensure the deck is not in play mode (in play mode, the pinch roller is pressed against the capstan under spring tension).
Removing the retainer: The pinch roller is fixed to the arm along with its shaft. A small retainer (clip or bolt) is used to secure the shaft. According to the A77 service manual, the structure involves "holding the groove on the shaft from above and removing bolt E from the side." In practice, you should see a C-clip-like retainer in the gap between the pinch roller and the arm. This retainer fits into the groove on the shaft, holding it in place. Using needle-nose pliers, grasp the bent end of the retainer and carefully pull it out (be careful, as the clip may spring away). If it is screw-fastened, use a small screwdriver to loosen the side fixing screw and remove it. If the shaft spins freely, hold the groove at the top end of the shaft with a flathead screwdriver while working.
Removing the pinch roller and shaft: Once the retainer is removed, pull the pinch roller and shaft upward together. At the same time, the roller body will come off sideways (toward the capstan). If it is stuck, gently wiggle the roller while pulling. Once the shaft and roller are removed, pay attention to the small washer at the bearing. There may be one thin white plastic washer (shim) each above and below the pinch roller shaft. These are spacers that ensure smooth rotation and prevent play, and they are easily dropped or lost during removal. Carefully retrieve and store any removed washers (they must be returned to their original positions later).
Completion of old pinch roller removal: Inspect the removed pinch roller and shaft, and clean off any dirt. The bearing part (brass bushing in the center of the roller) is an oil-impregnated sintered bearing; if solvent gets on it during cleaning, re-lubrication will be necessary as described later.
Caution
This is an absolute must: when removing the pinch roller, always place a soft cloth under the arm.
The white part is the washer inside, and naturally it is sandwiched on both sides. The lower part in particular is prone to falling, so be careful.
Comparing Old and New Pinch Rollers and Handling Precautions
Let's compare the removed old pinch roller with the new one. There are differences in appearance and feel between aged parts and new ones. In the case of hardening deterioration, the rubber hardens and may barely give when pressed with a finger. A new roller has appropriate elasticity and slight flexibility, denting slightly when pressed. A hardened roller cannot provide sufficient friction, causing tape slippage, uneven rotation, and wow and flutter. Additionally, a roller with surface wear may show glossiness or steps and unevenness from tape travel. In extreme cases, the surface may melt and become sticky or develop cracks. In particular, green-colored rollers are known to soften with age and become sticky, almost "melting." Using a sticky roller can cause the tape to get tangled around the roller, leading to a major disaster, so when purchasing a used unit, checking the pinch roller condition first is one approach. The surface of a new roller has a matte texture and a uniform cylindrical shape. The new roller fits properly with the shaft and rotates smoothly when clean.
Handling precautions: Be careful not to get oil or dirt on the rubber part of the new pinch roller. Oils from hands or tools can adversely affect tape travel, so wear clean gloves or wash your hands before handling. Also, avoid excessive use of alcohol-based solvents. In fact, there have been cases where overzealous cleaning with commercial cleaners caused the rubber to deteriorate and develop surface irregularities. If cleaning, lightly wipe with a cloth moistened with a small amount of anhydrous ethanol. After wiping with solvent, allow it to dry completely before use.
Installing the New Pinch Roller
Now install the new pinch roller. The procedure is basically the reverse of removal, but pay attention to key points during reassembly.
Cleaning and lubricating the bearing: Wipe the shaft and bearing (brass bushing in the center of the roller) of the old pinch roller with a clean cloth to remove dirt. If there is deteriorated old grease or dirt, lightly clean with anhydrous alcohol. Then, apply a very small amount of machine oil to the bearing for lubrication. Sintered oil-impregnated bearings are infused with oil, but if oil is removed during cleaning, a light application of oil will ensure smooth rotation. It is also good to lightly coat the shaft with an oil film (wipe off excess oil with a tissue).
Arranging the washers: Return the thin washers (shims) removed earlier to their original positions. First, place one lower washer on the shaft hole on the pinch roller arm side, then pass the new pinch roller onto the shaft and set it in place. Next, fit the other washer onto the upper side of the roller (where the shaft protrudes). The washers are very thin and easily displaced, so make sure they are correctly positioned above and below the roller.
Mounting the shaft and roller: With the lower washer in place, insert the shaft through the pinch roller from above. While passing the shaft through the hole, slide the roller into position from the capstan side. When the shaft is fully inserted, the side groove (for the clip) will appear outside the arm hole.
Securing the Retainer
Attach the retainer (clip or fixing bolt) to the groove on the shaft. If using a clip, press it firmly into the groove with your fingers or tweezers to secure it. If using a bolt, align it with the shaft groove, insert, and tighten appropriately. Ensure the retainer is securely attached and the shaft does not float up or down. As a precaution, pinch the pinch roller with your fingers and gently move it up/down and left/right to check for play and smooth rotation. Fine-tune the retainer position if necessary.
Reattaching the Front Panel
Once the pinch roller replacement is complete, reattach the front panel as it was. Align the upper snap pins with their holes and press gently until you hear a "click." After confirming all three pins (left, right, center) are properly seated, tighten the two lower fixing screws to secure the panel. Finally, return each control knob and pitch control knob (if equipped) to their correct positions.
Basic operation test: Power on, load a tape, and enter play mode to observe whether the pinch roller presses normally against the capstan. Immediately after replacement, the roller surface is not fully broken in, so run a few reels of tape and observe. If the tape runs stably without meandering or abnormal noise, the basic replacement is complete.
Checking the effect after replacement: After replacement, improved tape running stability (wow and flutter) can be expected. In fact, there are reports that replacing a deteriorated roller stabilized rotational fluctuation during 19 cm/s (7½ ips) playback from a previous variation of 0.04-0.05% to about 0.03%. Similarly, sound wavering and running noise should be reduced.
Advanced: Alignment Adjustment and Detailed Check Methods
Once the pinch roller itself has been replaced, for advanced users, performing tape path alignment (adjustment) can maximize the deck's original performance. Here, we explain specialized adjustment procedures such as pinch roller pressure adjustment and tape path (running path) checks.
Pinch Roller Pressure Adjustment
Simply replacing with a new roller improves running, but if the pinch roller pressure (the force pressing the tape against the capstan) deviates from the specified value, optimal performance cannot be achieved. The Revox A77 has a mechanism for adjusting this pressure, and readjustment is possible according to the service manual.
Standard pressure specification: According to the official Revox service manual, the pinch roller pressure is adjusted to approximately 1.5 kg (3.3 lbs). This is the value measured at the point where, during tape travel, pulling the pinch roller causes the tape speed to slightly decrease (audible change in volume). Specifically, with the tape running in play mode, apply force to pull the pinch roller slightly away from the capstan, and measure the force just before the tape starts to slip. Adjust so that this value is around 1.5 kg.
Measurement method: A spring scale is used for the actual adjustment.
After the transport runs
A transport that has been through the work above will hold speed, stop without spilling tape and wind evenly in both directions. That is the mechanical half of the machine. The electrical half, which is where a machine of this age is most likely to surprise you, is a separate subject: the electrolytic capacitors have aged whether the machine ran or not, and the mains-side film capacitors are the parts that fail loudly.
Work through the transport first. A machine that will not move tape reliably cannot be aligned, and any electrical measurement you take on it will be measuring the fault in the mechanism rather than the fault in the circuit.