Real to Reel

I was asked to build a controller that would add automatic rewinding to an old Revox A77 reel to reel tape player. The idea was to rewind the tape when it reached the end of a track and then play again, so the machine could be switched on and loop all day.

A Revox A77 reel to reel tape player — The Revox A77.

These are good machines to work with. They have DIN ports for wired controllers, which studios used to trigger recording and playback. I found the wiring for one of those controllers and used it as the base for my circuit. I only needed the play, stop, and rewind switches. I settled on an ESP8266 running MicroPython to toggle a set of relays, and added the capacitors and resistors from the original control circuit.

The player has a neat stop mechanism. A small light shines onto a photodiode with the tape running between them, blocking the light. When the tape runs out, the light hits the sensor and trips the stop button. I reused this: instead of running the sensor to the stop button I ran it to the ESP8266, and instead of letting the tape leave the spool I replaced the start and end with transparent leader tape, which the light shines through. That gave the controller a signal each time the tape reached an end and it was time to rewind or play.

The control circuit wired up on a breadboard — Testing the circuit on a breadboard.

Once the breadboard version worked I had a go at a PCB. I had not done much board work before, and there are things I would improve next time, such as the trace widths, where I just used the Eagle defaults. The board had a simple job: somewhere to mount the large capacitor, the resistors and diodes for the control circuit, the ESP8266, and a set of screw terminals out to the relay board. I also left space for a step down power supply module so I could tap the player's mains for the ESP8266.

The PCB layout for the controller — The controller PCB.

I had the boards etched by Aisler. There were three players to control, so I assembled three.

Three assembled controller boards — Three assembled boards.

The inside of the Revox A77 — Inside the A77. These are a pleasure to take apart, everything slots together, parts desolder cleanly, and the metal framing is sturdy.

I spliced into the player's circuit, routing the relays onto the cables for the DIN controller port.

The relays spliced into the player's controller cabling — Spliced into the DIN controller cabling.

The workbench with the players and tools — The workbench.

There is a video of the player running. The PCB files and MicroPython scripts are in the repository.