The Mellotron's basic system is the same on all models.
The model M400 is the one which will described in this page. Because of its simplified mechanism, the M400 should help you to understand more easily the way the instrument works.
The system is basically the same as a tape recorder : under every key, there is a tape and a magnetic head. The keyboard has 35 keys, so there are 35 tapes and 35 magnetic heads. Each tape has 3 tracks, on which 3 different sounds are recorded. A selector switch places the heads on the desired track (A, B, C). With a mediate position of the selector, the heads straddle between 2 tracks - which allows the musician to play two tracks at once : A+B or B+C.
Each recording lasts for 8 seconds. Once the tape has been played in its entirety, you have to release the key to be able to play the tape once more.
When you switch on the instrument, a motor (linked to a belt) puts the capstan (1) in motion. The capstan is constantly turning.
When you press a key (2), the tape (3) comes in contact with the replay head, thanks to a felt pressure pad (4), and with the capstan, thanks to a pinch roller (5) ; thus, the tape can play.
As long as the key is pressed, the tape plays for 8 seconds, and then folds up in a wooden compartment (6).
When the key is released, the tape returns to its initial position, thanks to a spring (7).
After a few hours of playing, a Mellotron keyboard often stops working properly, which causes problems of tape rewind and / or tape playing.
Each key has two screws, aimed at adjusting the height of the pinch roller and the pressure pad, in relation to the tape.
1st screw (on the left) : adjusting of the pad
2nd screw (on the right) : adjusting of the pinch roller
This is the only method (used by Streetly Electronics from the creation of the Mellotron) to adjust your keyboard properly :
Always begin with the pad : while pressing the key, unscrew the screw until there is no sound anymore. Then, release the key, to allow the tape to come back to its initial position. Press the key again, and screw the screw until you have the right sound. Then, give a complete turn of the screw.
Then, you can adjust the pinch roller : while pressing the key, unscrew the screw until there is no sound anymore. Then, release the key, to allow the tape to come back to its initial position. Press the key again, and screw the screw until you have the right playing speed. Then, give two complete turns of the screw.