Jeff Liebermann wrote:
On Thu, 27 Aug 2009 12:09:15 +1000, Sylvia Else
wrote:

Ummm... thanks for not calling it a rheostat.
Why on Earth would I do that?

Because it really is a rheostat. Officially, if there are three wires
involved, it a potentiometer. If there are only two, it's rheostat.

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


Since my FC-4 has only two connection on the phone jack, it's a
rheostat. However, the internal variable resistors in the non-optical
pedal mechanisms I've seen, use 3 wires.
http://en.wikipedia.org/wiki/Potentiometer
Unfortunately, just about everyone uses these terms interchangeably.

It's the sustaining pedal on a 7 year old CLP-970. The sensor looks for
all the world like a pot.

http://music.yamaha.com/products/main.html?productId=109
Release data is Jan 1, 2000 which makes it at the bitter edge of when
Yamaha started using optical pedal mechanisms. It doesn't specify the
pedal mechanism, but only says "Half pedal effect (Right)" which makes
me wonder why they bothered to use a pot instead of two switches.

Fortunately, the CLP-970 is heavy enough to require two people to lift,
so there's no problem of its moving around.

Well, yeah. The pedals are part of the cabinet.

I'm rather surprised that you opted to get the original part from
Yamaha. Potentiometers are fairly generic and available. If you have
the old part, try to determine the taper (linear, log, audio, weird,
etc), and the value of the pot with an ohms-guesser. Also the shaft
type. Then go shopping online for a suitable replacement. If you
need help, do some measuring and supply a photo.



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.

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.

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.

Sylvia.