The tape transport of the analog tape recorder
The tape recorder has three motors. The motor for the take-up reel does not pull the tape through the machine, it simply collects it up as it emerges. Some tape recorders have a 'tape dump' switch, which switches off the take-up reel motor. This is used during tape editing so that long sections of material that are probably going to be edited out can be played straight through into a bin on the floor.
The motor for the supply reel is - perhaps surprisingly - energized in the reverse direction, so it is in effect pulling the tape backwards. However, the force this motor provides is carefully controlled so that rather than reversing the motion of the tape, it simply tensions the tape against the heads. Early tape recorders tensioned the tape by means of a pressure pad that held the tape against the heads. However, this caused 'scrape flutter' , which degraded the sound, and it hindered access to the heads for editing.
The degree of tension is important. Less expensive tape recorders have a fixed tension, or perhaps switchable between two reel sizes. Simple mathematics reveals that the tension will change according to the diameter of the reel, which itself of course changes as the amount of tape on the reel changes. Better tape recorders have means of sensing the tension and controlling it so it is always at the optimum value, throughout the reel and whatever size of spool is employed.
The capstan motor lies directly beneath the capstan (not visible), which itself is an extension of the rotor of the capstan motor. The capstan provides the motive power that drives the tape at the correct and constant speed. Close to the capstan is the pinch wheel or pinch roller. When the tape is stopped, fast winding or rewinding, the pinch wheel falls away from the capstan to allow the tape to move freely. In play or record mode however, the pinch wheel presses the tape gently but firmly against the capstan. The capstan is generally made of metal (but sometimes a ceramic material), and the pinch wheel is a rubbery plastic. As the machine ages, the pinch wheel is often found to harden, reducing its ability to hold the tape properly against the capstan. Of course, it can be replaced.
The roller guides are primarily there to ensure that the tape is in the correct place with respect to the heads. However, there are additional functions. One of the rollers will be a tachometer roller, or tach roller, which is used to measure how much tape has passed through the machine, whether in play or fast wind mode. This in turn is used to drive the counter or timer, which gives the position on the tape.
Other guides help smooth out the tension. Particularly when the tape starts, there will be a sudden change of tension that is smoothed out by the tension arms. On cheaper tape recorders, these are simply metal bars. Better machines have rollers.
Another type of roller is often seen close to the record and playback heads. This is the flutter damper roller. As the tape passes the heads, there is a certain amount of friction that causes scrape flutter. This leads to sidebands (bands of extra frequencies) and modulation noise (noise that changes in level as the signal changes). The flutter damper roller lessens these effects.
The three heads are, in order, erase, record and play. The erase head wipes the tape clean of any previous recording. The record head and playback do exactly what their names suggest.
The tape transport handles the tape with a high degree of care. On early tape recorders, the operator had to be sure that the tape had come to a complete stop before changing transport mode. On modern tape recorders, the tape can be fast winding, and when the play button is pressed, the recorder will slow down the tape gently, bring it to a complete stop, then engage play mode automatically.