Repair manual

2-47
THEORY OF OPERATION
One output of the horizontal timing circuitry is the synthesized
Horizontal Sync (/HSYNC) signal. This signal is used as the feed-back
input to the phase detector.
For a detailed description of a typical phase-locked frequency
synthesizer, refer the Motorola MECL Device Data Book.
7. Auto Luminance (Brightness) Control
The Auto Luminance Control (Figure 2-9, sheet 7) automatically monitors and
maintains the CRT beam at its adjusted luminance (brightness).
When the host computer starts a sequential exposure of the computer-generated
image, a digital VIDEO signal generated by the Video Signal Generation circuit
is applied through connector J9 to the input of the Cathode Driver circuit on the
Monitor P.C. Board causing the CRT beam to turn on. Refer to paragraph 5 in
Section E of this Service Manual for a functional description on how the Cath-
ode Driver circuit processes the applied VIDEO signal.
As the CRT beam turns on, the photodiode assembly which is mounted on the
Digital Palette housing senses its brightness. The photodiode assembly
converts the sensed brightness into a signal that is applied through connector J13
to the input of the Auto Luminance circuit. The Auto Luminance circuit
processes the applied signal and then inserts the processed auto luminance data
onto the A/D communications bus. The microprocessor inputs and decodes the
auto luminance data.
Once the microprocessor decodes the auto luminance data, it flags the internal
auto luminance program stored in the EPROM to determine if the CRT bright
ness needs to be adjusted. If no adjustment is required, no action is taken.
If it determined that the specified CRT brightness has changed (gone above or
below the specified brightness), the microprocessor decodes the auto luminance
instructions from the EPROM and then applies the necessary auto luminance
data onto the A/D communications bus. The Auto Luminance circuit processes
this applied auto luminance data and then generates a luminance signal (LUM
CTL). The LUM CTL signal is applied through connector J9 to the input of the
Luminance circuit on the Monitor PC Board. The Lum CTL signal causes the
Luminance circuit to re-adjust the CRT beam to the specified brightness by
changing the bias on grid 1 of the CRT. A more negative bias decreases elect-
ron flow turning down the brightness. Refer to paragraph 8 in Section E of this
Service Manual for a functional description on how the Luminance circuit
adjusts the CRT beam to the specified bridghness.