Need help identifying a zener diode

[tobor]

So, I'm refurbishing my AR X-06 receiver, and on the main power supply board, there is a broken zener diode.  The service manual references is as a UZ-20 (M, H) Zener Regulator.  That's not enough information for my limited experience to figure out what the appropriate specs for a replacement would be.

I'll attach the schematic for the board as well as a photo.  It is component D903.  D904 is an identical component, so, one solution that comes to mind, would be if I could determine through testing what the specs are on D904.  However, if I do so, I need to be careful not to destroy it as well.

Any help, on either a direct ID from the description, from it's use in the circuit, or on a method of determining the specs of the good component with little or no chance of damaging it, would be greatly appreciated!