The only info on Zenith I've ever seen or used, this from
http://www.cyberspace.org/~awh/


5.3 ZENITH
Read the _ SERVICE MODE WARNING _ before proceeding.

Using the television's front panel controls:

To access the service menus, press and hold the menu button, then
press and hold the volume down (or volume up) and channel down (or
right adjust) so that all three are depressed simultaneously. The
standard menu that was displayed will change to one of the service
menus. Now the standard controls are used to search through the menus
(the remote can be used to do this). Use "select" to choose a menu
item and "adjust" to alter the setting. Enter commits to a setting,
and menu itself closes the service menu. When menu is pressed again
the standard menu will return, unless the three key sequence is used.

Using the television's remote control:

Turn the TV on. Hold down the [MENU] key on the remote until the menu
clears or "Welcome to Zenith" appears then release the [MENU] key and
within two (2) seconds hit the following five keys:

[9], [8], [7], [6], [ENTER]