<!-- Begin schematic: In order to preserve an editable schematic, please
don't edit this section directly.
Click the "edit" link below the image in the preview instead. -->
<!-- End schematic -->
It could very easily just be this. A shunt 8V zener diode. I don't know your frequency or voltage parameters. Is there a power/voltage rating on this little box? Is it custom or is there a mfg. part number?