About 4/5ths of the way down on http://myweb.accessus.net/~090/how2adj.html
in the service mode section:
start of section >>>>>>>>>>>>>>>>
5.4 GE TELEVISIONS / RCA TELEVISIONS / PROSCAN TELEVISIONS
Read the _ SERVICE MODE WARNING _ before proceeding.
To enter service mode, push and hold the [Menu] button on the set. While holding, push and release the [Power] button, then the [Vol +] button.
You will now have some information on the screen: "P0 V0"
To actually be able to change these values, you must push [Vol +] on the remote so the information reads V76. Then pushing on the Channel +/- will change the "P" value.
P1 - 3 are error codes. If there is a problem with the set that can be self-diagnosed, the "V" will report the code. These codes are available in the RCA repair manual.
P4 - 7 control the horizontal size, shift, vertical size and shift.
You may also find a summary leaflet for the service tech clipped somewhere inside the chassis. The procedures for getting into the service mode, as well as the service items of interest will be there.
If you're attempting to adjust color or geometry errors, you'll need the Video Essentials DVD, Laser Disc or some other source of test material.
<<<<<<<<<<<<<<< end section
Of course, this otherwise very good resource does not tell you how to EXIT GE/RCA service menus!