Tektronix 485, Instruction Manual

Page: 18 / 197

Circuit Description—485/R485 Service
Q908 disables B Sweep in A HORIZ DISPLAY mode and
Q914 introduces an offset at 10 and 20 ns/div. Switch A9
opens when A Sweep speed is 1, 2, or 5 ns/div to start B
Sweep with minimum delay, since the B Sweep Generator
produces these speeds regardless of the HORIZ DISPLAY
mode.
B Sweep generator is similar to A except that the
maximum output rate is 5 times faster. The sequence of
timing components is different from A Sweep. Q1238
prevents large positive voltages that would occur at the
miller input after the end of the sweep but before the end
of the gate.
For A Sweep speed settings of 10 ns/div or slower, A
HORIZ DISPLAY mode displays A Sweep. B HORIZ
DISPLAY mode displays B, ALT, INTEN, and INTEN (A)
function. However, when A Sweep speed is set at 1, 2, or
5 ns/div the Horizontal system displays B Sweep which has
been gated on by A trigger. Switch A9 is open so that the
Delay Time Position control is effectively set to zero. A t A
= 5 ns/div, the displayed B Sweep is positioned horizontally
in the amplifier. A t A = 1 or 2 ns/div, the POSITION
control is disconnected from the Horizontal Am plifier via
switches B19 and B20. The POSITION control is connected
to a variable delay circuit Q1102 which time-positions B
Gate for about 30 ns after A Gate.
H O R IZ O N T A L A M P L IF IE R
The A sweep or B sweep may be selected by saturating
Q1312 or Q1318 respectively. This is done by the
horizontal control logic level at N4. If N4 is below —5 V,
Q1336 is o ff and Q1338 saturates. This allows the node
R1313, R1314 to go up and turns o ff Q1318 and saturates
Q1312. The output of A sweep generator has been selected.
Q1338 being saturated also shunts to ground any signal
from B sweep generator that might be present in ALTernate
mode. If N4 is —3 V, B sweep is selected as Q1336
saturates, turning o ff Q1338 and pulling down on R1314,
saturating Q1318 and turning Q1312 o ff. CR1311 will
shunt any signal from A sweep.
A +1 V at N4 w ill turn Q1342 on which turns off
Q1358, disconnecting A and B sweeps and saturating
Q1356, selecting the X signal for X-Y.
In A = 1, 2, or 5 ns, B18 closes and Q1331 turns on,
saturating Q1322, a 2X magnifier. To provide offset to
enable viewing at sweep start, in 5 ns B22 closes and offset
through R1327 is provided. In 1 and 2 ns, the base of
Q1328 is moved and a current from Q1328 through Q1331
and the base collector of Q1332 is inserted to provide
offset.
Q1362 and Q1364 compose a HI gain amplifier. The
feedback R is R1364 and R1366. The input resistance in B
sweep is R1321 and R1322. If A sweep is selected the input
resistance is R1307 and R1308.
By selecting B sweep and adjusting R1366, the hori­
zontal gain is set. Then by selecting A sweep and adjusting
R1308, A sweep may be calibrated. Both adjustments are
done in 0.1 ms/div. To get A and B sweep to start at the
same point on screen, R1325 (B REGISTER) provides
some offset current for positioning B sweep relative to A
sweep. This is done in A LT horizontal mode. Q1378 and
R1368 provide a constant current for the operational
amplifier Q1362, Q1364. Q1366 is saturated in Beam
Finder by S600B. This cuts the feedback resistor value by
2/3 and Rf/RI is reduced. C l364 provides linearity control
for 1, 2, and 5 ns/div.
Q1372 and Q1374 are a paraphase amplifier to provide a
differential signal to the horizontal amplifier. C l372,
C1374, R1372, and R1374 provides thermal compensation
for Q1372 and Q1374. Since the horizontal is symmetrical,
we will deal with one half only. Q1172 provides isolation
between the CRT capacitance and the input amplifier
Q1154. Q1172 and Q1174 provide the current path to
drive the deflection plate. Q1192 is an emitter follower to
provide feedback to Q1154. By using active feedback the
actual feedback resistance may be lowered without sacrific­
ing gain. This is accomplished by utilizing the impedance of
Q1192, emitter to base, and the divider circuit of R1177
and R1178. CR1156 provides a - 5 V reference for Q1154
and CR1152. CR1153 limits the voltage swing to keep
either half of the horizontal from saturating. This allows
the horizontal to be very linear.
H O R IZ O N T A L CO N TR O L
This circuit is activated by the HORIZ DISPLAY
controls. Several output signals are derived; B disable,
horizontal control, A-B control autofocus, trace separation
and vertical control drive.
The basic circuit consists of U1535A which is controlled
by the HORIZ DISPLAY. The HORIZ DISPLAY switch is
overridden when B sweep is used in A = 1, 2, or 5 ns or is
X-Y. A truth table is shown on the diagram 12.
The main function of U1535 is to provide a HORIZ
CONTROL signal to the horizontal B preamplifier. With the
X-Y control of Q1520 (which saturates in X-Y), this signal
on pin N4 has 3 levels. In the ALT mode, U1535 switches
from one state to another each time the A GATE goes
negative. U1530C inverts the A GATE to get the positive
edge required by U1535A. (It is important to maintain
correct timing with the sweep control since switching must
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