Mellotronists

>1. It has a terrible earth loop, appearently. When it is connected to
>an amp, there is a loud hum, even when the mellotron is off (but
>connected to the mains). When the power cord is disconnected, the hum
>goes away. I reckon this is an earth loop, but how on earth should I
>fix it?

SMS or CMC-10 motor controller?  If CMC-10, swap it out.

We ran into this on one machine and lifted the ground (using one of those 
ground lifting plug things on the power cord).  It was probably the house 
wiring, though.

It can also be one of the transistors in the power supply, one of the big 
silver ones visible from the outside (I think only one is used if you have 
an SMS, so you can swap in the other one to see if the noise goes 
away).  If you have an SMS4 motor controller, there's a low noise 
modification available on mellotron.com.

If the ground wire from the preamp to the frame is coily, uncoil it (this 
is mostly for AM radio noise).

Make sure the headblock is not touching the frame at all.  There should be 
nylon rollers under it, and there are these odd w-shaped clips holding the 
headblock to the frame with non-conductive washers---make sure those 
washers are there.

Finally the wiring used from the preamp to the headblock in older machines 
disintegrates and causes nasty noises (and will cause the preamp to erase 
your tapes if you're not careful).  The same wire is used from the power 
supply to the final output.  Replace.  Short the input of the preamp before 
reconnecting the tape heads (and/or demagnetize the heads).

>2. The output seems weak, much less than an electric guitar. There is
>a few things related to this, I think. Firstly, the tone pot wasn't
>working well, so I tried to replace it - now it won't scratch anymore,
>but it works like a volume pot, which I guess isn't normal.

Nope.  Between (1) and this, I wonder if your preamp is in need of help.

Make sure your tapes are upside right (yes, I've seen them installed 
flipped over).  Also beware of that headblock audio cable---that could 
easily be the problem, and it's a quick/easy fix.

Remember to quickly short the input terminals of the preamp before 
reattaching the tape heads!

>Secondly
>the tape heads are covered with something, that alcohol didn't solve.
>Can this affect the output? I think the heads haven't been cleaned
>ever before the unit came to my hands.

The heads shouldn't have anything on them.  I wonder if someone sprayed 
them with something to make the tapes slide easier---this is not needed in 
a proper machine, but someone may be trying something to make the tape flow 
better in an EMI.

>3. The tape rack is from a normal M400. I realized it won't fit like
>it should - when looking the mellotron from the player's view, it
>makes contact with the mainframe(?) on the left. It forces the thing
>to the left, so that even the tape-collect-box is moved (this is kind
>of hard to describe...)

EMI machines were made with some Mellotron-supplied parts but not any of 
the official squaring rigs.  The frame and components will undoubtedly be 
out of square and perhaps at wrong heights, whatever.  Complete 
disassembly/reassembly may be required.  The tape path must be absolutely 
straight and unhindered.  I have also been told that the motor EMI used may 
be a little weak.

>4. The last thing is related to the former, I think - the touch of the
>keyboard was awfull, I changed it a bit and now it is much better -
>but still, it should be a lot better. I can feel the tape "struggling"
>under some of the keys while they are pressed.

Possibly related to (3), plus bad pinch rollers and bad overall keyboard 
adjustment.

>Also, the middle C is much quieter than the other notes. Actually, you
>can barely hear it - I am quite certain it is not the tape, but
>perhaps the tape head that is at fault.

It is the tape head or wiring, the felt on the pad arm, the pad arm being 
bent incorrectly, the pad arm out of adjustment, or the pad arm needing to 
be moved  around (if you remove the key, you will notice that the pad arm 
is tied on by a screw and can be moved around--try finding the sweet spot).

>So there you have it. This is the second working mellotron in Finland
>at this time, so we don't have too much info on the maintenance of
>this instrument. Any information would be greatly appreciated!

Ugh...Yeah, lots to learn, and it's kinda hard to describe everything in an 
e-mail.  It's more of an in-person "here's how you do it" or "have a knack 
for it" thing, plus the wonky tone control and/or preamp may take some 
diagnosis work.  If you're in Europe, it may be worth sending this to 
Mellotron Archives (Sweden) or the Streetly gang (UK) for some work, even 
if you send just the guts (there are a few guys in the States who can have 
a crack at it as well).

I worked on an EMI for a while and had it mostly straightened out---it came 
down to a badly adjusted motor controller and undoubtedly some other things 
I didn't work out.  Someone out here has since taken that machine and fixed 
it back up.  There's also the story on Norm Leete's page 
(http://members.aol.com/tronpage/rebuild.htm).  So these things can be 
reworked and made right---they're mostly there, just a little askew.  :-)

Anyway, maybe I've given you a start.

...kl...
M400 #805 - rumbles and fades
M400 #1037 - speeds up sometimes

** Ken Leonard - Web Table of Contents:  http://www.kleonard.com
 >> CD available! "Improvised Waves: Mellotron/synth improvisations"
** Get Outdoors New England:  http://www.GONewEngland.org