Repairing a Revox A700 Reel to Reel Tape Recorder

Some time ago I was asked if I could repair a Revox A700 reel to reel tape machine. The tape would not rewind, forcing the customer to play to the end of the tape, swap the reels over and then fast forward so that the tape could be rewound. This was inconvenient for them.

Initial Investigation

After opening the machine there was a light oil everywhere, which I traced back to one of the a.c. electrolytic motor caps leaking. This had to be cleaned up, and the capacitor changed, which involved a lot of mechanical stripping down, plenty of cleaning solution, and a good check of all the connectors before the investigation proper could start. On any equipment over about 10 years old, leaking electrolytic caps can be quite common. A lot of problems can be fixed by changing these out, but the hard part is finding out where to get the parts from.

The first thing was to find out what the tape machine would and would not do. Yes it would not rewind, but it would play, and it would fast forward, and it did recognise the clear tape header to stop. So I tried to see what would happen without a tape. Both drive motors on the reels operated in the correct direction on play, fast forward and rewind. Note: These tape machines have the motors directly driven, no belts here, and they work in opposite directions to keep the tape under a constant tension.

Problem Identified

I spent some time looking at the drive circuitry on the left hand (rewind) spool motor, and could not see anything obviously wrong. However, I noticed that on power up the right hand spool was gently rotating – with a tape in, it was stationary, which is why I hadn’t noticed it before. Time for some inverse logic I thought, so I started to look at the drive circuitry on that side, and sure enough that was where the fault was. One of the semiconductors was starting to break down, and current was leaking through, driving the motor to turn. When the rewind was being applied, the right hand motor was pulling stronger than the left hand, hence no rewind.

Testing it out after repair

With that replaced, it was now time for a final test. I get to repair a lot of audio equipment, and replace a lot of speakers in professional speaker cabinets, and I must confess that a lot of them do not sound very good to me – even so called professional studio monitors. So, when I plugged in the headphones and listened, I was expecting to hear something similar to an 1980’s stereo cassette deck. Boy was I in for a shock. To say that the quality of the sound blew me away was an understatement. The dynamic range was incredible. I hear so many CD’s where there is so much compression and limiting going on, that to my ears it just sounds flat. To hear this tape machine with your eyes closed, and you could very easily imagine that you were sat in the theatre/hall that it was being performed in. That was a comment from my wife when she listened, and she usually listens with a keen indifference, and normally says “It’s all right I suppose”. Even she had to admit that this quality of sound was in a different league.

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