B Sweep Timing Selections - Tektronix 2246 1Y Service Manual

SPEED
1
tim in g
20 ns 2 mA
50 ns 800 |xA
100 ns 400 ixA
100 ms 200 jxA
100 ms 80 (xA
1 40 jxA
|XS
2 |xs 20 |xA
5 jxs 8 |xA
10 jxs 4 mA
20 |xs 2 mA
50 |is 800 jxA
100 )j.s 400 jxA
100 jxs 200 jxA
100 jxs 80 jxA
1 ms 40 jxA
2 ms 20 |xA
5 ms 8 jxA
RUNUP STATE. With Q304 and Q308 off, the
charging current from the timing circuit can begin
charging the timing capacitors, and the voltage at
the emitter of Q304 ramps up linearly. That ramp is
buffered by the A Sweep Buffer (U310A and B and
Q312) to drive the Horizontal Output Amplifier. As
the ramp is running up, it is being compared with a
fixed reference level by the Sweep End Com­
parators. When the ramp level reaches the com ­
parison level, the A SWP END signal goes high. That
signals the Trigger Logic 1C, U602, to end the A
GATE signal, and the sweep is switched to the
retrace state. The sweep ramp is also being fed
back to the base of Q303. At the point in the ramp
that the base voltage of Q303 exceeds that on the
base of 0302 (the -2 V baseline reference), Q303 is
biased off and Q302 conducts. This biasing con­
ditions disables the feedback loop that stabilizes the
baseline voltage level, and it remains off until the
Theory of Operation—2246 1Y and 2246 Mod A Service
Table 3-23

B Sweep Timing Selections

bsO
C tim in g
C329/C330 0
I I
0
I I 0
I I 0
I I 0
0
I I
0
I t
I I 0
C307C 1
I I 1
1
I I
I I
1
1
I I
I I 1
1
I I
I I 1
I I
1
bsl bs2
1 1
0
0
1 0
0 1
0
0
1
0
0 1
0
0
1 0
0 1
0
0
1 0
1
0
0
0
1 0
0 1
0 0
feedback voltage during the retrace period falls back
to near the -2 V baseline reference voltage on the
base of 0302.
RETRACE STATE. At the end of the sweep, the gate
signals reverse state. Transistor Q309 is biased off,
and Q308 is biased on. Retrace current supplied by
Q308 quickly returns the voltage across the timing
capacitor to a little below the baseline voltage level.
That retrace current is regulated by Q311 and Q326
to produce a rapid, yet rate-controlled retrace. At
the point of the fall in feedback voltage where Q303
starts to turn on, base current becomes available to
Q304 to turn it on, and the feedback loop that
stabilizes the voltage at the baseline level again
becomes active.
bs3 bs4
1
0
1
0
0 1
0 1
1
0
0
0
0 0
0 0
1 0
1 0
1 0
0 1
1
0
0 1
0 0
0 0
0 0
3-35