In the course of the recent Comcast debacle, I kept my eyes peeled, and gleaned a bit of info about the diagnostic menus on the Motorola DCT3412 I ("Dual Tuner DVR / HDTV Capable").
To get into the main diagnostic status menu, grab the remote, and make sure the box is plugged in, but "off" (press Cable, then Power if it's on). Then press Select. This bring up a menu of the status pages which you can look up to see what your box is doing, but as far as I can tell, you can't change anything here. Scroll to Exit and Select to get out of this menu or just press Power.
To get into the User Settings menu, again make sure the box is off, then press Menu. This will let you change:
TV Type:
- 4:3 Letterbox
- 4:3 Pan Scan
- 16:9
YPbPr Output:
- 1080i
- 720p
- 480p
- 480i
4:3 Override:
- 480i
- 480p
- Stretch
- Off
The 4:3 override seems to only operate when you're component (YPbPr) output is set to one of the HD signals, and sets the 4:3 SD channels to output according to this setting. 480i and 480p are just as they say, "Stretch" takes the 4:3 signal, and stretches it to fill whatever 16:9 HD output you have selected, and "Off" uprezes the 4:3 image proportionally to fit in the HD frame, pillarboxed.
This menu will also let you change the Closed Captioning, with all sorts of language, font and color settings. More info about the settings can be found starting on page 9 of this manual [Unfortunately now offline —Ed. 2015-08-15]. Press Power or Menu to get out of this User Settings menu. That's it!