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Reel motor dead spot problem! What is it?

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NakTech

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Jul 2, 2014
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Hello all,
I have been receiving a few PM on the dead spot problem in Nakamichi decks, how to identify it and so on. Though I replied to them individually, I thought it would be useful to others also.

To start with, the reel motor's function is to wind the tape on to the take up and supply hubs in Play, FF and ReW modes. The motor shaft is attached either to an idler wheel with teeth or to a rubber tire. The idler wheel or tire(whichever is applicable) is in the middle of the take up and supply hubs, it swings to both sides to make contact with one of the hubs at a time, depending on the transport mode (Play, FF or ReW).

Here is a picture of the idler arrangement to get an idea about the system..
http://www.eslabs.com/Idler.JPG

The difference between the FF/RW modes and the play mode is that in play mode the motor runs slow. The torque at this low speed is very low and constant. It is kept low by design in order to isolate this torque from the capstan/pinch roller system which determine the playback speed. Ideally, the playback speed is determined by the capstan only. In play mode the function of the reel motor is only to wind the lose tape which is pulled by the capstan. That is why we are able to control the tape speed by capstan motor independent of the reel motor speed. In single capstan transports this isolation is not perfectly achievable and the torque of the reel motor has some effect on the tape speed as well. Double capstan system was developed to overcome this problem.

Getting back to the idler motor, imagine a cassette playing from the beginning of any side. There is no tape on the takeup hub. The torque is sufficient to keep the tape running. After 5-10 minutes, the take up hub has some amount of tape wound in it and it gets heavier. In this condition, the torque available to wind the tape is less compared to what it was in the beginning, some amount of available torque is utilized to move the additional mass of the tape on the hub, but the tape speed is not affected (supposing that the torque of the capstan is very high relative to the reel motor and hence, the torque variation of the reel motor will not affect tape travel). This is also the reason why the reel motor torque has to be precisely calibrated as per the specifications. If the torque is set more than required, there will be large speed variations from the between the beginning and end of the cassette. Overly set torque will cause the tape to run faster in the start and slower in the end. Though some speed variation is unavoidable in single capstan decks, the deviation is supposed to be within 0.5% as per specifications. If the motor has dead spots, when the brushes pass over those spots it will cut the current supply to the rotor causing a sudden drops in torque , with added load of the tape on the hub, the take up hub stops rotating and the auto stop kicks in. Tape stops.

In FF or ReW modes however, it will not stop because the motor runs fast with much higher torque and the inertia gets the dead spots away from the brushes, energizing the rotor again faster.

So in simple terms, the dead spots causes the deck to stop after some time of play. Usually the first 5-10 minutes play without any problem. After that it stops. Single capstan transports may show gradual speed drop before it stops, this effect is not evident in double capstan models.
 
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