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4.6.2 CHARACTER REGISTRATION AND TIMING
As the carriage moves, the optical pick-up head and light source
on the video amplifier assembly generates the video signal for controlling
the print timing.
As the print head and optical head assembly moves across
the timing fence, the vertical slots on the timing fence interrupt light
to the optical pick-up head, generating a video signal. The VIDEO AMP
output then triggers the STROBE one-shot ME23 on the logic card, initiating
the print timing shown in Figure 4-6.
The STROBE one shot is adjusted for 475-525 usec.
If a 9 x 7
matrix is used, the leading edge of STROBE also triggers a delay one-shot
(ME28-4) adjusted for a 600 usec output pulse. The trailing edge of this
pulse triggers the Delayed Strobe (DLYSTB) one-shot which is adjusted
to the same pulse width as STROBE.
In normal character printing,STROBE
is used for full-step timing and DLYSTB for the half-step timing.
4.6.2.1 Video Amplifier
The circuit used to amplify the video signal generated by the
timing fence is located on the video amplifier assembly board, contained
on the print head carriage.
The video amplifier consists of a high gain amplifier with positive
feedback.
When the photo cell is dark, no current flows through it and the
base of Q2 is held at +5 volts through resistor Rl. When Ql is turred off,
Q2 is turned on through resistors R2 and R7.
Q2 being on also turns on Q3
through resistor R4.
Because Q3 is on, the collector is held at approximately ground,
thereby allowing the current to flow through R7 and holding Q2 on through
the positive feedback.
When the photo transistor detects light, current
is allowed to flow through it, thereby drawing current through transistor
Ql and resistor R2.
Ql then turns on and turns transistor Q2 off by
shunting the current away from the base of Q2. When Q2 turns off, Q3 also
turns off and the collector of Q3 is held to +5 volts through R6.
R7 serves
to drive Q2 further into the cut-off region. Capacitors Cl and C2 are used
for proper frequency response and noise suppression.
Resistor R3 is used
to prevent leakage by keeping Q2 from turning off.
4-17
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