Lagging or racing are common problems with foot-impelled Pianolas,
as well as Duo-Arts, even when the governor is functioning perfectly.
First ensure that there is sufficient braking on the music roll.
It is a mistake to have virtually none. The motor/governor do need
some substantial damping to pull against. If with a reasonable top
brake your spool box eats rolls, it needs realigning!
Then get the tempo slide position correct before trying to achieve
the proper speed with the governor spring. Many British instruments
have notches filed on the slide wires going into the various boxes to
indicate the correct travel. But the roll should just start to creep
at about tempo 10.
Then by adjusting the spring you should be able to achieve the proper
speed "at the top of the governor curve": if the roll lags tighten the
spring marginally, if it races loosen the spring. If this marginal
adjustment results in the tempo being out by more than 5, reset the
slide just a little and try again. Tempos should remain quite stable
and accurate by this means, whether the Duo-Art expression box spill
valve opens or closes, during fff passages, and with long rolls.
And a protest about the Duo-Art expression box spill valve. Whatever
impression the Service Manual intended to give, this does not simply
relieve the vacuum pump from unnecessary strain when softer playing
levels are required. For that, an Ampico-type sprung spill would have
served better, and this type of "unloader" is indeed found on many
British Duo-Arts before the expression box.
Moved chiefly by the theme accordion, the Duo-Art spill valve finely
modulates the suction supplied to the knife valves (below power 10) and
thus gives many more accompaniment dynamic levels, hence a well-fettled
Duo-Art's amazing subtlety.
This aspect has apparently been ignored by all who have so far tried
to develop electronic analogues of the system. Don't forget that, even
on very late Duo-Arts, Aeolian employed a cross-bleed from theme to
accompaniment to mimic the spill function somewhat!
Wivenhoe, Essex, UK.