Kuon Revox B77
Restoration Simulator

Before you open the machine

1. Unplug the mains cable. Not just the power switch: the switch, the fuse holder, the voltage selector and the primary of the transformer carry mains voltage whenever the plug is in.
2. Discharge the reservoir capacitors on the power supply board (C1 2200 µF and C2 1000 µF) before touching or measuring anything. Confirm under 1 V with a meter. If you are in a hurry, use a 1 kΩ 5 W resistor across each; never a screwdriver.
3. Oscilloscope ground goes to the 0.0 V terminal of the board you are measuring, never to the chassis, and never to the primary side. On the primary side use a differential probe or an isolation transformer or do not measure at all.
4. A machine that has not been powered for years: bring the +21 V rail up slowly from a current-limited bench supply (0 to 21 V over about a minute, limit 100 mA) and watch the current. Replace any electrolytic that is bulging or leaking before you apply power.
5. Take the reels off. A machine that starts unexpectedly will damage heads and tape.
6. This is an unofficial educational tool by Kuon R&D. Revox is a trademark of Revox GmbH. Values marked calc are calculated from the redrawn schematics, values marked manual are quoted from the service manual, and nothing here has yet been confirmed on a running machine unless marked measured. You work at your own risk.

1 Your machine

The B77 was built in several versions. Choose yours and the tool shows which of the 29 board drawings apply, which capstan motor and timing capacitors you should find, and which parts are pending.

2 The machine in 3D

Drag to turn, wheel to zoom, right-drag to move. Press a part to read what it is and where it leads. The explode slider takes the machine apart in the order you would on the bench: covers, reels, head block, pinch roller, then motors and boards out of the back. This is our own early MKI: black deck, aluminium band with the REVOX badge, transport buttons at the left, CHANNEL I and II meters at the right. Outer dimensions (452 × 414 × 207 mm, reels 538 mm across, reel centres 85 mm from the top) are from the dimension drawing in the service manual; the positions of the controls, heads, capstan and the parts inside come from our own front and rear photographs. Shapes are built from measurements, not scanned, so small parts are simplified; a photogrammetry scan of the machine will replace them one by one.

3 From the symptom to the part

Pick what the machine does. The causes are ordered the way we would check them: unplugged checks first (green), powered measurements second (red). Each cause names the board and the part, and the 3D button lights them up.

4 Procedures, one step at a time

Hardened or sticky rubber is the commonest cause of wow and flutter. Replacement is at the level of a plastic model kit if you respect the washers.

1. UNPLUGGEDUnplug the mains cable. Lay the machine horizontal on a cloth so that nothing dropped is lost.
2. UNPLUGGEDPull the control knobs upward; the power switch knob is stiff, lever it with two flat screwdrivers. Remove the lower panel screws (two, on some machines four, each with a washer) and lift the panel off its snap pins.
3. UNPLUGGEDConfirm the machine is not in PLAY: the roller must be clear of the capstan.
4. UNPLUGGEDPut a soft cloth under the arm. Remove the retainer (C clip or side bolt) from the groove of the roller shaft. Hold the groove at the top of the shaft with a flat screwdriver if it turns.
5. UNPLUGGEDLift roller and shaft out together. Catch the thin plastic washers above and below the roller; they must go back in the same places.
6. UNPLUGGEDClean the shaft and the sintered bronze bearing. If solvent touched the bearing, re-oil it lightly. Do not get oil on the new rubber; wipe with a little anhydrous ethanol only and let it dry.
7. UNPLUGGEDLower washer on, new roller onto the shaft from the capstan side, upper washer, shaft fully home until the groove shows, retainer back in the groove. Check play and free rotation.
8. UNPLUGGEDRefit the panel on its snap pins and screws, then the knobs.
9. MAINS ONPowered measurement. Acknowledge the safety notice at the top to show it.

5 The 29 boards: parts, nets, notes

Every board redrawn from the service manual, with its parts list, the connections between them and the notes we wrote while drawing: what the board does, what to replace, and where the original was unclear. Press a part to see every net it sits on and the parts it shares them with. Three sheets are pending (1.177.317 MKI and MKII, 1.128.040) because the scan could not be read with certainty; we do not publish guesses.

Loading board 11…

6 Capstan speed control on the bench

A numerical model of board 1.177.325 as drawn: tacho sine wave to square wave, divide by two at the slow speed, a fixed-width pulse from the NE555 for every edge, the average compared with a reference, and the difference driving the motor through the MJ411. Because the loop holds the pulse duty constant, the tacho period is proportional to the monostable width, and R14 moves that width. The 625 µs period is from the manual; how far R14 reaches, the motor time constant and how dried electrolytics reduce damping are this model's settings, not measurements.

7 Operating points, signal traces, modes

From our verification guide (Kuon R&D, 2026-08-27): the DC points the manual states, the ones we calculated from the drawings, and the order in which to follow a signal. Basis is marked on every value. Ground is always the 0.0 V terminal of the board, never the chassis.

8 Open data and how to help

We want this to become a resource that B77 owners complete together. The board data behind this page is published as JSON and may be used under CC BY 4.0 with attribution to Kuon R&D; our photographs are free to reproduce with credit. Nothing here is affiliated with Revox GmbH.

• Board data: index.json and /analog-tools/revox-b77/data/boards/NN.json (parts, nets, notes per board, generated from the KiCad drawings).
• Pending: tape drive control 1.177.317 (MKI) and 1.177.317-81 (MKII), remote control unit 1.128.040. A sharp photograph of either board or a cleaner scan of those pages lets us finish them.
• Not yet confirmed on a running machine: every value marked calc. If you measure one, tell us the board, the part or pin, our value, your value and the condition. Photographs help.
• What we will add next: the 29 schematic PDFs for download, a photogrammetry scan of our own machine replacing the parametric 3D parts, and German.
• Corrections and measurements: contact Kuon R&D. Sources: our articles on the transport, the boards, speed and calibration and the parts reference.

Continue

• Kuon Revox B77 Simulator: transport and sound →
• Revox B77 / A77 restoration guide (articles) →
• Speed calibration calculator →
• Record with the Kuon P-86S / X-86S microphones →
• Finish recordings in KUON DAR →