Hi Rabbit,
Well yes D103 is indeed a zenner diode and i have managed to identify it. The markings on the device itself are misleading and are probably a house number for Sony, i.e. only Sony and Authorised dealer/Repair centers know its true value but persistance pays off.
The device number that you need to track down is RD10ES produced by NEC Electron Devices.
Check this page for its characteristics,
http://www.ic.nec.co.jp/discrete/eng.../rd/rd_es.html
The positioning of the diode in the circuit would suggest that its value is critical to the units correct operation and safety.
Also check here for a schematic.
http://psxhelp.users4.50megs.com/9002afullsd.htm
Now you know what it is, its up to you to find it.
Try Farnell (Mail order from Leeds), or RS Components.
Have fun.