accessing Phaser 350 hidden service menu to increase voltage to head

[XeonTux]

I know that somewhere there is a hidden service menu on the Tektronix/Xerox Phaser 350 printers, where there is an option to increase the voltage to the print head. This is useful for unclogging jets that remain clogged after performing the normal cleaning cycle. I have a service manual (actually for a 340) but this manual does not give this information.

Anyone here by chance know this procedure?

Thanx in advance