Zenith Z-100 PC series Service Manual page 295

Page
4.9
Troubleshooting
Video Output Adjustments
The following paragraphs detail the procedure for setting the video output
black level and contrast.
Connect the oscilloscope vertical input to the monochrome video output
connector, P305. Connect oscilloscope external sync input to pin 5 of
P303, HS. Obtain a stable waveform resembling Figure 4.1 .
Figure 4.1. Video Output Adjustment Waveform
Adjust black level control, R319, for a pulse height of .3 volt. Then adjust
the contrast control, R320, for a peak waveform height of 1 volt. The
adjustments may be slightly interactive, so repeat these two operations
as necessary to obtain the specified results.