It's a fast SCR ( Silicon controlled rectifier ) E0122 (or E0102YA). It's part of the power supply circuit.
http://pdf.datasheetarchive.com/datasheetsmain/Datasheets-35/DSA-697395.pdf
http://pdf.datasheetarchive.com/datasheetsmain/Datasheets-35/DSA-697395.pdf