Tektronix 3B5, Instruction Manual

Page: 50 / 187

Circuit Description — Type 3B5
For all sweep rates, the VARIABLE control, R397, adds
additional resistance in series with the Timing Resistor when
not in the CAL detent. This provides continuously variable,
uncalibrated sweep rates between the 1-2-5 steps. Also note
that this control is ganged with SW397 to turn on the UNCAL
readout and turn off the s/DIV readout (see s/DIV and
UNCAL Readout Control).
s/DIV and UNCAL Readout Control
SW397 is ganged with the VARIABLE control, R397. When
the VARIABLE control is in the CAL detent, B999, s/DIV
readout, is connected into the circuit. However, whenever
the VARIABLE control is turned away from the CAL detent,
SW397 turns off B999 and turns on B987, UNCAL readout,
to indicate that the sweep rate is not calibrated. The current
through B987 or B999 from the —12.2 volts supply provides
current to the — 6 volt line for the heater of V1 63. This
current is provided in parallel with current supplied by the
mode-indicating bulbs in the Operating Mode Power circuit.
X10 and X100 Magnifier Gates
Q364 is the X10 Magnifier Gate and Q324 is the XI 00
Magnifier Gate. When both of these transistors are on, the
sweep magnification is X 1. X10 delayed sweep magnifi­
cation is provided when Q364 is off and Q324 remains on.
When both Q364 and Q324 are off, the delayed sweep
magnification is X1 00.
R381, R384 and R387 are the X1 timing resistors. Only
one of these resistors supplies timing current for X 1 sweep
magnification as selected by the multiplier logic. For
example, Fig. 3-14 shows the timing-current path for a sweep
rate of one millisecond/division with X1 sweep magnifica ­
tion. The 1 ms (d) decade logic actuates K310 and K313 to
select the 1 ms decade. D392 and D393 are forward biased
by the multiplier logic (approximately zero volts) applied to
the "2 ” and “ 5” logic lines and the "2 ” and ‘‘5 ” timing
currents are shunted. — 12 volts on the "1" logic line reverse
biases D391 and allows the "T" timing current to pass to
the Timing Capacitor through D317. The X10 Magnifier
Gate is held on by the XI magnifier logic applied to the
Delay Comparator (see Delay Comparator discussion). This
places the collector of Q364 near zero volts and the X10
timing current is shunted through D367, D366 and Q 364.
The X 1 00 Magnifier Gate is held on by the X 1 00 (bar
X
1 00 magnifier) logic. The X 1 00 timing current is shunted
through D327 and Q324. Therefore, only the timing current
through R381 charges the Timing Capacitor.
For X10 delayed sweep magnification, X10 timing cur ­
rent through R382, R385 or R388 is added to the X1 timing
current. The Delay Comparator switches Q364 off at the
sweep level selected by the DELAY control (note exception
for 0.1 μ s and 10 ns decades; see Delay Comparator discus ­
sion). When the comparator switches, the collector of Q356
goes positive. This decreases the current through tunnel
diode D365 and it switches from its high-voltage state to
its low-voltage state. D365 produces a fast voltage change
at the base of Q364 to turn it off quickly. When Q364
turns off, its collector drops to about — 12 volts and the
X10 shunt diodes D367-D368-D369 are reverse biased. X10
timing current is added to the XI timing current in the
timing group that is charging the Timing Capacitor (as
determined by the multiplier logic). The timing current in
the other two timing groups is shunted away from the timing
capacitor. For example, Fig. 3-15 shows the timing current
path for X10 delayed sweep magnification at a magnified
sweep rate of 0.1 millisecond/division. Note that the same
Timing Capacitor is used as in the previous example for
X 1 magnification. D391 is reverse biased by the multiplier
logic applied to the "1 "-logic line to allow timing current
to charge the Timing Capacitor through D317. The total
X10 timing current is supplied through both R381 and R382.
This total timing current is 10 times greater than the X1
timing current to increase the sweep rate 10 times. The
X100 timi ng cur rent is shunted by D327 since Q364 is held
on by the X 1 00 magnifier logic level.
For X 100 delayed sweep magnification the X10 Mag ­
nifier Gate still controls the starting point of the magnified
sweep as described for X10 operation. The X 1 00 timing
current is a combination of the X 1, X10 and X 100 timing
current. R383, R386 and R389 are the X 1 00 timing resistors.
The X 1 00 Magnifier Gate is controlled by several logic
levels. For the 1 μ s timing decade, the 1 μ s (g) logic line
is at —12 volts. Since its base is held negative, Q324 is
conducting and the X 100 timing current is locked out
through D327, D328 or D329. In the 0.1 μ s and 10 ns timing
decades, the 0.1 μ s (h) logic line is at —12 volts and the
X 1 00 timing current is similarly locked out. These logic
levels prevent sweep rates faster than 10 microsecond s/divi-
sion from being magnified 100 times. In addition, the X 1 00
magnifier logic holds the X 100 Magnifier Gate on for X 1
and X10 sweep magnification. However, for X100 sweep
magnification the X 1 00 magnifier logic is zero volts and
Q324 is reverse biased. The collector of Q324 goes to about
—12 volts and the X 1 00 shunt diodes, D327-D328-D329,
are reverse biased. Since the X10 Magnifier Gate still con­
trols the turn-on point for the delayed sweep magnifier, the
X 100 timing current is shunted along with the X10 timing
current. Then, as the X10 Magnifier Gate is turned on,
both the X10 and X100 timing current are added to the
X 1 timing current to charge the Timing Capacitor. This pro­
duces a sweep rate 100 times faster than the basic X1
sweep rate. Fig. 3-16 shows the timing current path for
delayed sweep magnification at a magnified sweep rate of
10 microseconds/division with X100 magnification. The
total X 1 00 timing current is supplied through R381, R382
and R383. This total current is 100 times greater than the
X 1 timing current to increase the sweep rate 100 times.
NOTE
Although the DLY’D SWP MAG switch can be set
to X 1 00 for sweep rates of 1, 2 and 5 μ s, only
X10 delayed sweep magnification is provided.
Sweep rates between 10 ns and 0.5 μ s cannot be
magnified with the delayed sweep magnifier.
Delayed Sweep Magnifier Intensifier
Q374 provides an intensifying pulse to the grid circuit of
the indicator oscilloscope CRT to compensate for the lower
display intensity when the sweep is magnified _ [due to
increased sweep rate.) For X 1 magnification, the X1 mag ­
nifier logic level at the anode of D371 is zero volts. This
clamps the base of Q374 s o it ca n not go more negative
than about zero volts. The X 1 00 magnifier logic level at
the anode of D376 is —12 volts. D376 is reverse biased and
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