My guess is that the 10V zener is for protection when using a power supply. (I could be wrong of course.)
If you can trace the circuit or draw the tracks and components around the resistor, zener and power socket then I am sure someone will be able to tell you an approximate value once it is known what it's function is.
Has the pedal been used with an AC power supply, or one with greater than 9V DC or, one with reversed polarity on the connector?
Politics is the art of so plucking the goose as to obtain the most feathers with the least squawking. - R.G. 2011
Jeez, she's an ugly bastard, she makes my socks hurt. I hope it's no ones missus here. - Ice-9 2012