Zenith Z-100 PC series Service Manual page 282

Page
3.35
Circuit Description
When horizontal and vertical retrace are inactive, the composite RGBI
video signal from the emitter of Q301, is applied to the base of Q302
and to the base of Q303. The output at the emitter of Q302 provides
the composite video signal to internal monitor connector P303, and
the output at the emitter of Q303 provides the composite video signal
to external monitor connector P305.
The RFI filters are configured as pi-type capacitive input filters. The
value of the inductor is 2.2 microhenries, and the value of each
capacitor is 56 picofarads.
Capacitors C362, C363, and C364, in conjunction with inductor L301,
provide filtering and decoupling of the 5 VDC supply before use by
the video output transistors.
The light pen input signal is interfaced to the gate array through con­
nector P304. Jumper J301 is used to establish light pen polarity and
pin 10 of U344 provides pin 4 of the gate array with the light pen
switch (LPSW) signal.
• Contrast and Black Level Adjustments - Potentiometer R319 allows
for adjustment of the dark-to-light screen area ratio (contrast) and R320
allows for adjustment of the black level for different monitors. R320
should be adjusted to obtain a reading of 1 volt peak-to-peak at the
base of Q301 (TP301).