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charel196 wrote:

From:  "J.K.Beresford" <j.k.beresford@s...>
Date:  Fri Mar 21, 2003  4:12 am
Subject:  Re: [Mellotronists] MK II station changing
  I don't necessarily "want" this John(I don't even own a MKII).I
have just heard some of the horror stories about tape foulups from
the shuttle mechanism.Just was curious if anyone ever tried to
improve on the technology yet.              Charles

_________________________________________________________

Greetings All, Having just completed restoration work
on MK1/124, Ican quickly "encapsulate" the cycling
system for youon MK1,MK2,SFX,and M300 models.

Pulse-tape isthe means for stopping the motor during
"inching." Astrong(saturated) signal of 1000 Hz
works in conjunctionwith the SSCU board.

Once the buttonis pressed on front-panel,a "ladder"
network determineswhether to run forward or backward.
This is the timethat tapes are zooming-along at 36 i.p.s.
The keylock isengaged,and indictator light extinguishes.

The circuit then"times-out" from flank-speed and then
starts to "inch".It's looking for the pulse-signal on the tape.
Inching can beadjusted for stepping width and frequency.

As mentioned above,thepulse tape stops the inching.
Keylock movesout of the way,and indicator light again
illuminates.

The rotary "sync"switch is vital as it is a  safeguard
to ripping tapesout of the machine. Eight little brass dots
(1-6, and fwd/backdirection change) are contacted by a
"wiper" duringmovement. Driven by Reynolds or Meccano
chain. The switchmust be kept clean,and all connections
to it. Thereare several BA9 fittings in this circuit,
2-3  eachfor respective keyboard. From age,wear,etc,
they can getloose and potential disaster can occur.

Other weird-scienceis cycling working great from one
direction,andlike crap from the other. Remember , these
machines areapprox.  35-40 years old. Leave nothing to
chance and checkEVERY cable,connection,solder-joint,etc.

"The sum of allfears" in cycling is complete failure of
pulse detectionand tapes do not reverse at extreme ends.
And the keylocknot working,and someone presses a key.
.....CRUNCH!!.....RIP!go the tapes......

A "torture-test"was devised to prove reliability of SSCU's.
Pushing the Station-Selectbutton and releasing the detent
will cause themotors to move at flank speed from 1 to 6,
reverse direction(hopefully)and back from 6 to 1. Indefinitely,
until a buttonis pressed-in again.  Do not attempt this with
your machine!  BTW, I've encountered all the scenarios
mentioned-above. Now I know why my hair turned-grey,
then fell-out!

A modern approachto cycling is the use of encoding stepper
motors,and digitalckts. (simple adj. "one-shot.") And the
drivers to powerthe motor/solenoid. If a "cycling M400"
were to be constructed(& we've discussed this years ago),
I suspect upgradedbits would be in order.

Hope this tech-talkanswers a few questions,folks.
It's not in fulldetail here. The unabridged versions can be
found in MKII/M300manuals.   Cheers, Jerry the  K.