Enabling the Vibrato in a Wurlitzer 726
We removed these switches from a Wurlitzer 726 electronic piano. This model was the student version of the 720 (itself the console version of the 140b). One switch was for toggling between "self" and "ensemble" modes, while the other switched between the built-in speaker and the hardwired headphones.
Since the 726 is a student model, it does not have vibrato enabled. However, all of the necessary components are already on the circuit board, including the heart of 140b vibrato: the optocoupler. The optocouplers of today are tiny sealed black boxes, but in the 140b it's actually a small neon lightbulb and a light-sensitive resistor in a golden housing. The bulb glows like a firefly; the resistor absorbs the light and mutes the reed bar signal accordingly. Not all vintage optocouplers are so elegant; in a 1960s Gibson Starfire, for instance, it's a bulb essentially plastic-wrapped against a resistor.
Optocoupler in a Gibson Starfire
Optocoupler in a Wurlitzer 140b (gold box)
Anyway, since the vibrato components are already on the circuit board in the 726, all we had to do to make it functional was add a knob. To avoid making extra holes in the piano's wooden body, we removed the self/ensemble and headphone/speaker switches. The self/ensemble switch was useless without the other pianos in the classroom network, and 1960s headphones aren't exactly great for close listening. We replaced the switches with a vibrato knob and an input jack.
The vibrato sounded pretty fabulous, as you can see below.
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