I've just returned from a visit to Germany and am catching up with MMD items about the Pistonola.  As Julian Dyer said, he asked me to do something on this mechanism for the Player Piano Group Bulletin some time ago.  I'm putting an article together, but I feel, sitting here with a piston module in my hands, that I must jump in and offer a few observations to be going on with.  There are two key factors to understanding the mechanism's unusual functions.

First, the primary valve is not the pin tip, but the brass cone immediately under the bar connecting it to the primary piston.  This has been missed by several experts I've met who, hoping to 'get things working', have consequently suggested doing unforgivable things to the bottom pin guide which is in fact not in the signal path.  The valve and alloy seat/nipple must be very carefully cleaned of any dirt or corrosion with a suitable solvent to enable the conical valve to seal.  Once this is done they will self-clean.

Second, and uniquely I believe, the system reads the roll at a constant, relatively low vacuum which keeps drag on the paper and thus wind motor size and consumption to a minimum but plays at very high vacuum levels (cf. transistor emitter follower topologies).  The low reading vacuum is initiated, established and replenished as necessary by the primary regulator found in the upper right cheek of the piano.  This is a sensitive diaphragm-operated slide valve which closes when the set vacuum level is attained.  (In the MMD pictures this is incorrectly labelled the wind motor regulator, which actually is located elsewhere.)

But here's the impressively clever bit: the low vacuum is predominantly supplied through any open primary valves by the high(er) vacuum under the secondary valve pistons.  And it is actually low primary vacuum that lifts the secondary valve pistons which have high vacuum above them.  The loose fits of the primary and secondary pistons in their bores provide the two essential bias (bleed) functions, and there are gauze sieves under the primaries to exclude dust.

Each playing piston has a thin cloth washer underneath which acts as a bottom stop buffer and was probably intended to be a bore wiper and dust seal.  These seem to have shrunk a little on my example, but anyway none of the compressed graphite pistons was stuck or badly scored.  I've attached a drawing showing the mechanism enlarged and actual size which I hope will make its operation clear.

There are many other fine construction points which I will detail sometime later.  The main reason the Pistonola was unsuccessful must surely have been that it was in competition with the big boys and was never fitted into a famous name piano.  And it does seem to have suffered a particular assembly problem: paper-faced hard rubber gaskets which were fitted between the valve blocks and cylinder heads.  Over times these exuded as miniature doughnuts into the 3/32" drillings, partially or completely blocking them with obvious results, especially if any heads were over-tightened during manufacture or servicing.

Somewhere I have a review of the Pistonola by a well-known music critic who noted how efficient and responsive it was.  And the Boyd piano models into which Pistonolas were fitted are reasonably well-scaled and well-made instruments.  So don't write them off too soon.

Paddy Handscombe
Wivenhoe, Essex, UK.
Thu, 23 Mar 2000 01:48:00 +0000