Mellotron – Routine Maintenance

Most second-hand Mellotrons suffer from two things, no manual and lack of routine maintenance. Some more unfortunate machines suffer even worse and are completely ruined by attempts to bodge them.
What follows is the relevant information extracted from the original, often vague, manual (in quote blocks) plus some additional hints from a well-known pair of experts that will keep a reasonable machine working well. This is NOT a complete guide to restoration.

The original owners manual

Original Owners Manual
Original Owners Manual

One day the manual will be re-written but until then………..

I must cover myself and state that I am not liable for any injury to you or your Mellotron if you attempt any of these maintenance procedures as you do them at your own risk and I am not responsible for your actions. They are supplied in good faith and have been used on my own machines for years as by now I am pretty sure of what I am up to but if you are in any doubt at all about what you are doing then DON’T DO IT. Instead seek guidance from your local Mellotron expert.

General Notes:
All procedures refer to the M400 – the machine you are most likely to come across. Do not attempt any of this on another model!
Whenever possible always work on a machine which is switched off and disconnected from the mains. Some adjustment have to be done with the machine running so take care as there are rotating and moving parts.
Remember demagnetise all tools that you use during maintenance.

M400 Technical Specification
For completeness (and a bit of a laugh) here is the official technical spec reproduced exactly as it appears in the manual.
Notes: cps = Hz, dodgy abbreviations are as printed. As for the actual figures there is no information to how they were obtained, so they should be taken with a pinch of salt! Considering the tape speed I would expect a well maintained machine to easily exceed 12kHz.

HEIGHT: 34″ (86cm) WIDTH: 34″ (86cm)
DEPTH: 22″ (56cm) WEIGHT: 122lbs (55kg)

POWER SUPPLY: Transformer tapped at 115V, 220V, 240V
50 or 60 cps. Single phase.
Consumption 75 VA.
OUTPUTS: 1. Unbalanced line output via 2 circuit jack,
2. 3 volts into 5K ohms, via foot swell pedal,
3 volts into 600 ohms without foot swell pedal.
SIGNAL TO NOISE: Better than -55dBM.
TAPE VELOCITY: Normal 7 1/2 i.p.s.
Variable speed range +/- 15%
Reproduce characteristic modified NARBT standard.
TONE CONTROL: 12 dB cut at 10 Khz
REPRODUCE HEADS: Low impedance: 3MH: 35 in total.

Removing the Tape Frame
This section can be skipped by the experienced. However for completeness here is a reminder of the procedure from the manual as it is referred to elsewhere.

1. Remove the cabinet lid by holding from the rear and lifting upwards.
2. Unclip the left and right key end blocks.
3. Release the four RED knurled finger screws which hold the keyboard assembly. Lift upwards clear of the instrument. Place on a clean surface.
4. Remove the aluminium tape storage box lid located behind the capstan bar.
5. Release the two YELLOW knurled finger screws that hold the rear tape clamp bar in position.
6. Lift the bar upwards and toward you until the tapes are in a vertical position just before the front tape guides.
7. Slowly lower the bar into a position behind the front tape guides. Locating pins at each end of the clamp bar should be rested in the points provided – coloured YELLOW.
8. Release the two BLACK knurled finger screws. Lift the complete tape frame slowly upwards until completely clear of the unit.
9. Place in the tape frame box to avoid damage.
Fitting the tape frame is the reverse of the above procedure. All knurled finger screws must be secure to avoid assemblies coming out of adjustment.

Replay Heads
(1) Cleaning
It is recommended that the replay are cleaned at least every 3 -6 months depending on the amount of use of the instrument.
Proceed as follows:-
Expose the heads as if removing the tape frame. Using a smooth, clean cloth with a little Methylated Spirit or Carbon Tetrachloride clean each head methodically and polish immediately with a clean portion of the cloth.
(2) Demagnetising
The heads should be demagnetised once every 12 months or on fitting a replacement head block assembly, when a head block has been tested using a meter or a head has been in contact with a magnetised instrument.
Proceed As Follows:-
(a) Expose the head block as if removing the tape frame.
(b) Release the retaining springs at the four points on the head block.
(c) Disconnect the output lead of the head block.
(d) Using a demagnetiser carefully demagnetise the whole head block taking care not to switch the demagnetiser off in the vicinity of the head block. Most demagnetiser manufacturers give detailed instructions of this operation which should be followed.

IMPORTANT NOTE: If you remove the head block for any reason then before you reconnect the co-ax head block plug it is important that the input to the pre-amp is shorted first to remove any residual charge. The co-ax plug should then be refitted immediately. Also the head block plug should NEVER be disconnected or connected when the instrument is switched on as there is a high risk that the heads will become magnetised and that could partially erase any tapes played subsequently.
By using a modern battery powered demagnetiser that automatically applies a decaying signal to a tape head on demand it is possible to to demagnetise the heads in situ. To do this you remove the tapes completely from the machine and then WITH THE MACHINE SWITCHED OFF demagnetise each head in turn in accordance with the instructions supplied with the demagnetiser. This saves you the hassle of removing the head block.

Keyboard Adjustments
This section describes the parts that most commonly need attention, I do not include track selector and pressure pad adjustments as they are beyond the scope of this simple guide. The ‘factory set’ adjustments are included for information only and should still be OK on most machines. They can be wrong if the previous owner was a cowboy with a loose screwdriver or the instrument has been severely abused (e.g. dropped from a great height). If so, or in any doubt of your ability to do any of this, I advise seeking experienced help from the usual sources.
Note that pressure pad and pinch rollers have to be adjusted with the machine running so care must be exercised not to come in contact with the rotating flywheel on the right of the machine.

1) The depth of movement of the keys is controlled by the aluminium bar supported about 8″ from the front of the keyboard on top of the keys. The height of this bar is adjusted at the factory to give a maximum depth of key movement of 3/8″.
2) KEY TENSION (or touch)
The key tension is determined by the 2BA Nyloc Nuts, retaining the key leaf springs at the rear of the keys. By turning each nut clockwise the key tension will increase and the key will rise. In manufacture, keys are adjusted by the above method until the key rises and just comes into contact with the bars above the keys. When all the keys are just touching the underside of the bar, a further 1/4 turn is applied. This is the standard key adjustment.
Pressure pad adjustment is achieved by turning the 4BA screws protruding from the top of the keys nearest the front of the instrument – hereinafter called the PAD ADJUSTMENT SCREW. Assume that the pad is not pushing the tape into contact with the reproducing head at all. Clockwise movement of the pad adjusting screw will cause the pad to move down under key pressure and with the correct adjustment allow the tape to play. The pad adjustment screw should be turned gently clockwise until sound is just heard – at this point ONE further complete turn should be applied.
This adjustment is effected by the second 4BA screw nearest the rear of the key. Correct adjustment is a full 2 turns beyond stall. To achieve this setting turn the pinch roller adjustment screw anticlockwise until the point is reached where the tape slows down and just stops driving. At this point the screw should be turned 2 full turns clock-wise. This is the correct adjustment.

Well that was the official version from the manual and although it is correct the description is a bit vague. Here is the same procedure described in a clearer way (I hope).
Assuming there are no faults (see below).
PRESSURE PAD – always adjust this first, NEVER the other way round. Whilst pressing the key, turn the screw anti-clockwise (using a demagnetised screwdriver) until the sound disappears. Let go of the key and let the tape return. Then depress the key again and turn the screw slowly clockwise until the full sound is just heard (you made need to play the key more than once). Then give the screw one full turn clockwise.
PINCH ROLLER – Depress the key and turn the screw anti-clockwise until the sound disappears (the tape stalls). Let go of the key and let the tape return. Press the key again and turn the screw clockwise until you just reach full driving speed (you made need to play the key more than once). Give the screw two full turns clockwise after this point.

If a note cannot be made to play correctly using the above procedure then check the following (you may need to obtain experienced help).
The condition and position of the pad arm (may be bent incorrectly)
The condition of the pinch roller (may be stiff on the shaft or twisted out of square)
Tape frame alignment (check with another frame if possible)
Condition of the tapes (may be badly worn or contaminated)
Condition of the keys (may have swollen and now stick)
Machine geometry (out of square internal chassis – definitely a pro job)

Tape Replacement
As before, the manual says:-
Replacing a Tape
A tape may be replaced with the tape frame in or out of the unit. There follows a description of how this is achieved with the tape frame in the unit.
1) Remove cabinet top lid and front panel.
2) Remove key end blocks by lifting upwards.
3) Remove keyboard assembly by releasing four red finger screws and lifting the assembly upwards clear of the unit.
4) Remove the tape storage box lid.
5) Release the tape to be replaced by undoing the appropriate screws on the rear clamp bar.
6) Pull the tape out of it’s position in the storage loops and release the other end of the tape.
The replacement tape will be supplied with it’s start outermost and this should be placed under the clamp on the rear clamp bar.
Proceed As Follows:-
1) Feed the remaining tape on the spool into the tape storage box and remove the tape from the spool.
2) The end of the tape should then be placed under the clamp on the front tape bar.
3) Using service tool ‘K’ push the tape down on either side of the support roller so that it goes in two loops between the separators. Continue pushing the tape down until the tongs are fully down, then leaving them in place and feeling upwards between the two separators, catch the two loops on the first and second fingers. Pull the loops down on the first and second fingers.
4) Take the spring loaded pulley assembly and break open one side to give access to the rollers.
5) Lift the tape loops onto the appropriate rollers and snap shut the assembly.
6) Check that the height of the tape loop is correct, if not adjust from the front clamp bar.
The tape must now be positioned so that it’s start is accurately placed over the re-producing head.
Proceed As Follows:-
1) Replace the tape storage box lid and the keyboard assembly.
2) Switch on the instrument and tap the key in question.
3) If a delayed sound is heard, release the screws on the rear clamp bar and slowly pull the tape through until the attack of the instrument recorded on the tape responds on tapping the key.
4) Check all tracks in a similar manner.
5) Tighten clamp bar screws and replace end blocks and cabinet panels.
Replacing a set of tapes is the same for as for a single tape.

Some comments on the above.

One way I have replaced a single (faulty) tape is to splice the front of the new tape onto the end of the old tape and then use the old tape to pull the new one into place. This has to be done with care to avoid damaging the new tape.
However to replace all the tapes here is the method I use (heavily based on the above).
You will need a demagnetised screwdriver, a piece of wood about 2 feet long and slightly less than 3/8″ wide (you try finding ‘service tool K’), a clean pair of hands (or wear cotton editing gloves) and loads of patience.
With the frame in the machine and front panel, end blocks, keyboard and tape storage bin lid removed…..
1. Remove the current tapes carefully and store on a suitable spool(s). The convention is to store tapes start outermost with the lowest notes last. So remove the tapes starting at the top of the keyboard, attach the tail of the first note to the reel then use a small piece of splicing tape to attach the tail of the next note to the head of the last one until all are dealt with. Then label the reels with details of the contents including the order of the tapes (or live to regret not doing so).
2. Starting with the lowest note clamp the head to the rear clamp bar using only one screw at present as there will be further adjustments to do later. Position the start mark over the playback head.
3. Carefully feed the tape into the tape storage box until you get to the end (usually marked by a punched hole). Cut here and clamp the tail of the tape to the front clamp bar leaving about 9″ spare tape. CAUTION: DO NOT CUT THE TAPE AT THE NEXT START MARK as this has to be positioned over the head.
4. Using substitute service tool ‘K’ (the bit of wood!) push the tape down either side of the support roller to form two loops between the separators. Catch the loops using first and second fingers and pull the two loops down towards the sprung rollers at the bottom.
5. Pull the spring load pulley assembly out from between the separators and carefully split the assembly to gain access to the rollers.
6. Put the two loops onto the rollers (there is only one way this can be done without twists in the tape).
7. Snap the roller assembly back together (without damaging the tape).
8. Adjust the length of the loops by moving the end of the tape attached to the front clamp bar (use one screw at present). The loops should be just starting to tension the return spring. Note that alternate tapes will be clamped in slightly different places as the heads are staggered in the head block, this is normal. After a bit you will get quite good at judging where the correct position is.
Repeat this for the other 34 notes!
9) Finally adjust the start points by replacing the tape collection bin lid and the keyboard so the instrument can be played. By playing the note and changing the position of the tape in the rear clamp bar the correct start point can be found. If the note start is delayed then pull some tape through until the attack sounds correct. If the attack is missing at the start of the note then let a small length of tape back into the machine. Check all the tracks as the start adjustment sometimes has to be a compromise.
Repeat this for the other 34 notes!
When all is well carefully tighten all the clamping bar screws and trim the ends of the tapes to make a neat job of it.
Then give yourself a pat on the back…………

Mellotrons, Synths and electronics in music