Doug Mauldin wrote:

> On one of these organs an access panel was removed revealing some of
> the inner workings, including the tremulant.  I could see a pneumatic
> device oscillating at a rate of about seven Hertz ("Alpha waves",
> anyone?) and which clearly corresponded to the tremolo heard in the
> melody ranks.  This tremulant would pulse wind to the pipes by
> alternately canceling and drawing whichever of the two melody stops
> was in use at a given time.  The tremulant was turned off by another
> pneumatic which would force the tremulant pneumatic closed.

This description applies usually to only counter-melody tremulants.
Melody tremulants actually supply wind to a separate relay, so the
pneumatics of the notes are alternately "off" and "on". The registers
themselves are unaffected.

> A problem with these book-playing organs is that occasionally
> the book will hang in the keyframe causing an unplanned fermata.

If there is excessive paper-dust in the keyframe the keys can jam,
causing jammed music transport, and ruining the music card by
elongating the holes, so this should be investigated without delay.

Otherwise it can be caused by slack keyframe drive belts.  The majority
of Dutch street organs are fitted with round-section leather belts
running in round-grooved pulleys, not exactly conducive to a sure grip.
Lifting the "bridge", even momentarily may cause the registers to
operate in a random fashion, thus ruining the music.

Regards,
John Page, UK