Heathkit IO-4550 - Page 22

Page 20
Trigger slope selection is determined by the TRIG switch
and the slope selector gate, 1C202, as shown in Figure12.The
slope selector gate functions as a two input multiplexer and
couples the pin 11 or pin 10 output of 1C201 to the sweep
circuits, as determined by the TRIG switch. When the
control line is at a high level (+ slope), pin 4 of gate B is ata
logic high. This enables gate B so the pin 10 output of 1C201
is inverted and coupled through to gate C. The logic high at
Pins 12 and 13 of gate D is inverted and coupled to pin 2 of
gate A. The logic low at pin 2 disables gate A and forces its
output high. A high on pin 10 of gate C enables gate C, to
invert and couple the signal from gate B to the sweep
control circuit and auto-baseline circuit. When the control
line from the TRIG switch is at a low level, the slope
selector gate couples the signal from pin 11 of 1C201 to the
sweep control circuit and auto-baseline circuit.
TRIGGER
SWITCH
¥1
=
fio
SLOPE
SIGNAL
;
Diuip
SELECTOR
af
Net
3
GATE
10202
SLOPE SWITCH
IN + POSITION ‘
Jl = 1 + LOGIC HIGH
0 = LOGIC LOW
(71) = SLOPE SWITCH IN
- POSITION
Figure 12
SWEEP
The Negative edge of the trigger pulse activates 1C203B and
turns transistor Q203 off. This lets the timing capacitor
charge through the bootstrap current source, and generates a
linear voltage ramp. The ramp (sweep signal) is coupled to
the horizontal deflection circuit, and the remaining sweep
circuits. When the ramp reaches a predetermined level, the
CRT is blanked, 1C203B is reset, and Q203 is turned on to
provide a discharge path for the timing capacitor (ramp
returns to zero level).
Refer to Figure 13 for the following discussion.
When transistor Q203 turns off, the timing capacitor begins
to charge. After approximately 10 nS, the CRT unblanks
and the trace becomes visible. This short delay hides any
switching transients. At a preset ramp level, the CRT is again
blanked before the trace is stopped (to give the CRT time to
fully blank). After the short delay, transistor Q203 is turned
on and the timing capacitor is discharged.
=10nS = 100nS
SWEEP CRT CRT START sweep STOPS
sa ae TO a i? (RETRACE)
11 CM TRACE
TRIGGER
HOLD-OFF
TRIGGER
HOLD-OFF
10CM
VISIBLE
TRACE
Q203 Q203 Q203
ON OFF ON
Figure 13
3)