You can get an idea by looking for numbers on the diode itself and where it goes in the circuit, would imagine its a reference diode or shunt. Get a mag glass and look for the numbers and if no luck trace back the circuit itself. Im willing to bet its a 6.8v reference or 12v shunt. If its across the source/gate of the switching transistor Im almost sure it will be a 18.5v working as a snubber for scrubbing off the high leading edges of the square wave making its way to the switcher transistor.

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