Error:  ORA 10500
Text:   turn on traces for SMON
-------------------------------------------------------------------------------
Cause:
Action:
        Level:  <=5  trace instance recovery
        > 5  trace posting of SMON

To set event 10500:

For the instance:
a.  Shutdown database
b.  Edit the initialisation parameter file and add:
    event="10500 trace name context forever, level <value>"
c.  restart the database

For the SMON session:
Post the SMON process using oradbx (Oracle 7) or oradebug (Oracle 8).
For oradebug from server manager issue:

oradebug setospid <OS PID>
oradebug event 10500 trace name context forever, level <value>

For further information about oradebug Note 29786.1
                              oradbx   Note 28863.1

<value> is 'Level' as per above