Vintage Synth Repair

The lightbulb shines onto a photocell which changes its resistance
according to how much light falls on it.  I think it is in the direction of
more light reducing the resistance.   It's a semiconductor device with the 
light taking the place of the base junction in a transistor.

This has two significant advantages for a volume pedal.  It can't get 
scratchy, like a dirty potentiometer, and it doesn't require any gearing 
or mechanism that would break down over time.  It's probably cheaper, too.

So, the pedal just moves a little shutter that shades the photocell.   On 
one unit I worked on, the shutter was actually a piece of plastic with a 
V-shaped slot in it.  The pedal moved this past the photocell, allowing 
most of the area to be hit by the light at the wide end of the V, and then 
reducing the exposed area very nicely and slowly.   

It's an elegant piece of design, and apparently very reliable.   Putting 
any kind of unsealed pot in a pedal is just asking for dirt to get in.

Obviously light-bulb life is the major limiting factor.

I think that just about any bulb of the right size and voltage would 
probably work, perhaps throwing off the calibration a bit if it were 
significantly brighter or dimmer.   I also expect that the bulb is being 
run on a lower than rated voltage, to get longer life.

If you can't read the legend on the bulb, I would try measuring the 
voltage that is fed to it.   I'd try a bulb that is rated a bit above 
that, maybe half again.   The voltage may read read, high, though, because 
the bulb would normally load it down.   You may have to experiment with a 
couple of bulbs.   They shouldn't be at all expensive.

Eirikur