[sdiy] Hacking the expression pedal input on a JD-800

[Cornutt, David K]

I've been contemplating that expression pedal input on my JD-800...
I'd like to figure out a way that I can drive a CV input into it.
The output is intended to take a Roland expression pedal.
It uses a TRS 1/4" jack.  From looking at the schematic,
the tip is the input, the ring is a +5V output, and the sleeve
is ground.

I've tried just connecting a CV source to the input, but that
doesn't work.  I'm not sure, but I'm guessing that this is
some kind of current-loop interface; it expects to see a
current flowing from the ring to the tip, so if you just hit the
tip with a CV it doesn't draw current from the ring and
therefore doesn't do anything.  I'm thinking that I can use
an optocoupler as, in effect, a voltage-controlled resistor
to vary the current.  I'm looking at the Fairchild H11F1.
This is an optocoupler with a linear bilateral detector.

According to the specs the maximum voltage across the
detector is 30V, which would seem to be well within my
needs.  However, the spec sheet indicates a linear region
with voltages across the detector in millivolts and output
currents in microamps.  It occurs to me that I really don't
know what the specs of the Roland expression pedal are
or what kind of current the input is looking for.  Now, I know
I could hack up a pot and do some experimenting.  But
I'm at work right now, and anyway, it's easier to ask here. :-)
So the question is: has anyone opened up a Roland 
expression pedal?  Does it consist of just a pot, or is
there active circuitry in it?