The Mellotron - how it works
The Mellotron
is a sample replay machine but as it was developed in the early
60's before the invention of solid-state memory, floppy discs or
microprocessors then it used lengths of recording tape to store
each 'sample'. Each note on the keyboard [1] of a Mellotron M400
(although the principle is the same for all models) has a six
foot length of 3/8" wide tape under it that is attached to a
clamp bars at the front and rear of the instrument. Starting at
the rear clamp bar [11] the tape [5] passes over a tape
collection bin [4] that holds the loose loop of tape that is
created during tape replay. Next it passes over the capstan [2],
then the tape head [3], through the tape guides and then through
the sprung roller assembly. The clever bit is the sprung roller
assembly that holds the six foot of tape in an arrangement that,
from the side, looks like an elongated 'W' as the tape goes down
to the bottom roller assembly [6], up to the top roller [10],
then back down to the second roller on the bottom roller assembly
[6] then finally to the front clamp bar [12]. The keys above the
tapes each have a rubber pinch wheel [9] above the capston and a
felt pad [8] above the replay head assembly [3]. When a key is
depressed then the felt pad brings the tape into contact with the
head and the rubber pinch wheel presses the tape onto the
rotating capstan [2]. This causes the tape to be dragged passed
the replay heads at a constant speed (7 1/2 inches per second)
thus replaying it's prerecorded sound. As the tape is pulled
through this causes the bottom roller assembly to be pulled
upwards (shortening the elongated 'W') until the full length of
the tape is played causing the tape to stall. When the key is
released then the spring [7] on the bottom roller assembly pulls
the tape back to the original position and thus quickly 'rewinds'
the tape ready for the next time the note is required. This means
that the attack of the recorded note is reproduced and
contributes greatly to the character of the Mellotron's unique
sound.
The following pictures show the innards of my old M400,
Mellotron owners are excused the next few paragraphs as they will
already know what is inside when you lift the lid. For those of
you who have never seen the insides of a Mellotron the once again
the main features are (starting from the rear):-
- Tape collection bins, one for each note, collects the
tape as the note plays.
- Capstan / fly-wheel, in front of the tape collection bins
is the brass (or stainless steel) capstan. The fly-wheel
is on the right-hand end of the capstan, this helps
maintain a constant speed and also takes the drive (from
a drive belt) from the motor below.
- The rear tape guide comes next, this has to be extremely
accurate as mis-alignment can cause tapes to bunch up and
cause a horrible mess.
- The head-block, in front of the capstan holds the 35 tape
heads (one for each note) that reproduce the sound. This
block can move left or right and is used to select one of
three sounds recorded on each tape.
- The tape frame, in front of the head block holds the
tapes and the 'rewind' spring arrangement that returns
the tape to it's original position when the key is
released. In this picture the tape frame is installed but
the tapes have not been threaded so you can see the rest
of the innards.
Inside an M400 with the keyboard removed and
the tapes still retracted inside the tape frame.
The tape frame is a removable unit that contains the
pre-threaded tapes, the return springs and rollers. This allows
the musician to be able to change the sounds easily (after a bit
of practice). The frames were usually supplied in a flat case,
similar to a cymbal case, to protect them as they must be treated
with some care.
A M400 tape frame showing the return springs
at the bottom (picture courtesy Peter Forrest)
The next picture shows the tapes after they have been
threaded. This is an easy job as all the tapes are attached to a
bar that is held in place by the 2 yellow finger bolts at the
rear of the machine.
Stage 2: The tapes have been threaded
Once the tapes have been threaded the plate lying at the front
is placed over the exposed part of the tapes above the collection
bins and the keyboard is then replaced....
Stage 3: The keyboard is replaced.
Note that with practice this can be done easily within 2
minutes, we used to change tape frames mid set when my machine
used to be regularly gigged.
If you want to find out about routine
maintenance then refer to the seperate page on that subject.