On Thu, 27 Aug 2009 13:29:43 +1000, Sylvia Else
wrote:

It has three. Five volts on side. Zero on the other. Middling in the
middle. This presumably reduces its sensitivity to track wear.

Nope. The mechanical wear is all mechanical. Looking at a typical
spec sheet for commodity cermet potentiometers, I'm finding cycle
lifetimes from 100,000 to 1,000,000 cycles.

Someone suggested wirewound. Bad idea as they're only good for
perhaps 10,000 cycles:
http://www.calcentron.com/Pages/clarostat/clarostat_potentiometers.htm

I was looking at sourcing online yesterday. But the pot's in a quite
restricted space, has an anti-rotation tab, and specific requirements on
the position of the flat on the shaft (maybe that's standard). Also, I
was quite unable to get a stable reading on the resistance from the
centre connection to either end, even after soldering wires to it -
presumably because of its wear. Also, it's impractical to make
(presumably voltage therefore) measurements in situe - the pedal
requires too much force, so I'd have to pull it out, and then either put
it back (it works most of the time), or do without for however long it
takes to source a replacement.

I'll assume you tried lubing and cleaning it. If sealed, find a way
to open it. If it's a square pot, drill out the rivets and replace
them with 0-80 screws and nuts (or a piece of brass rod with the ends
smashed to form a rivet.

You're not supplying enough info to find a suitable replacement. End
to end resistance. If the voltage is half at midpoint, it's a linear
taper pot. The rest is mechanical. Anti-rotation tabs are standard.
The flat is usually where the screw goes on a know, which is on the
OPPOSITE side of where the arm would be pointing. If it's what I'm
guessing this is, you should have no problem finding a replacement.
Photo?

If Yamaha had offered me a replacement pedal unit for $lots, I'd have
taken a second look, but for about $25 including postage, I get a
replacement that's guaranteed to fit. Finding a substitute would likely
be more trouble that it was worth.

Well, that's why I have a well stocked junk box, some old devices to
cannibalize, and friends with an equal or greater mess of parts.

If the replacement fails before the piano definitively dies, or I do,
then I may look at retrofitting an optical sensor. I really only need
on/off. The pot's used for "pitch bending" which is not something I use.

Sorry. I assumed it was the damper. I wouldn't imagine the pitch
bending (portamento) control to be so heavily used as to wear out.
Something else is probably going on. It could be a defective pot, but
might also be some other type of failure. If the terminals are
available, you might want to temporarily attach an electrically
similar pot to the leads and see if it works.