Horizontal - TEK 2225 Service Manual

Theory of Operation-2225 Service
Auto Baseline Generator and also puts a LO on the
TRIGGERED line. At the end of a sweep, the holdoff
pulse is latched by U530A via U520B and U550C,
and the D input of U530B is driven HI. Thus the
sweep will not start on receipt of a trigger. This
condition is cleared by a pulse from single-shot
monostable U500A, that clocks the LO on the D
input of U530A to the Q output, allowing the next
trigger to initiate a sweep. U500A is used as a switch
debounce circuit. Timing components R506 and
C506 are chosen to give a pulse width of about
30 ns, a pulse that is shorter than the fastest sweep
speed. U500A also sets U510B, turning the TRIG'D
light on via U550A. When the holdoff period is
initiated (and U500A has timed out), U500B will clock
a LO back onto the Q output of U510B, allowing the
TRIG'D light to be turned off.
Alternate Magnification
The ALT Magnification mode is controlled by S601.
In the X I mode, is LO to set flip-flop U510A. The
Q output of U510A ( m ) is therefore HI. This HI is
inverted and level shifted by Q514 to drive the MAG
line LO to the Horizontal Amplifier. In MAG mode,
the
K G
line from S601 is LO, and flip-flop U510A is
reset.
SEP
is therefore LO, driving the MAG line HI to
the Horizontal Amplifier. The
SEP
signal line controls
the trace separation circuitry in the Vertical Ampli-
fier. In the ALT mode, U510A divides the ALT SYN
signal by two so that on every other sweep the
SEP
and MAG lines are TRLIE.
Alternate Channel Switching
The ALT SYNC signal is provided for the channel
switching circuit so that when ALT Vertical MODE is
selected, channel switching will be synchronized
with the timebase. When ALT MAG is not selected,
the alternate switching pulse (ALT SYNC from
U515A, pin 3) is supplied at the end of each sweep
to the channel switching logic circuit. When ALT
MAG is selected, flip-flop U510A divides ALT SYN by
two so that the ALT SYNC channel switching pulse is
supplied after each second sweep. This produces
the following sequence of displays:
(2511. Diode CR511 clamps the cathode of CR510 to
about -0.6V, so about 0 V is applied to the SET pin
of U510A to set that flip-flop. The
Q
output of U510A
is therefore HI, disabling the sweep separation and
MAG circuits.
HORIZONTAL
The Horizontal Amplifier circuit, shown on diagram
5, provides the signals that drive the horizontal
deflection plates of the crt. Signals applied to the
Horizontal Preamplifier may come from either the
Miller Sweep Generator (for sweep deflection) or
from the X-Y Amplifier (when X-Y display mode is
selected). See Figure 3-4 for the block diagram of
the Horizontal Amplifier.
The Horizontal POSITION control, X5, X10, X50
Magnifier circuitry, and the horizontal portion of the
Beam Find circuitry are also part of the Horizontal
Amplifier circuitry. The Horizontal Preamplifiers
amplify input signals for application to the Horizontal
Output Amplifier.
X11X5
Horizontal Preamplifier.
The X I 1x5 amplifier is a differential stage consisting
of (2747, (2748, and associated components. When
the X5 MAG line is LO, the X1 gain is set by resistor
network R775 and R753, with current supplied
through (2750. When X5 MAG is selected (HI), Q750
is switched off, and current is supplied through
R730. Potentiometer R730 is adjusted to balance the
current through Q747 and (2748. The X5 gain is set
by R753, R755, R731, and R749. When in XI mode,
CR747 and CR748 are reverse biased so that the X5
stage has no effect.
X I 1x1 0
Horizontal Preamplifier
The X I 1x1 0 amplifier is a cascode differential ampli-
fier consisting of U745, U755, and associated
components. Signals from the X I 1x5 Preamplifier
are buffered by emitter followers Q759 and Q760
before being applied to the bases of U745C and
U745D. When the X1 0 MAG line is LO (XI selected),
U755B and U755E are biased off, and U755A and
U745E are biased on. Diodes CR773 and CR774 are
reverse biased. ' T he gain will then be set by R763.
CHI MAG
When XlO MAG is HI, U7558, U755E, ~ ~ 7 7 3 , and
CHI X I
CR774 are biased on, and U755A and U745E are
CH2 MAG
biased off. The gain of the X I 0 stage is set by R763,
CH2 X I
R767, and R777. Potentiometer R782 balances the
currents in the preamplifier so that there is no hori-
When BEAM FIND switch S390 (diagram 6) is zontal trace shift when switching between X I and
pressed, the emitter of Q776 (diagram 5) goes LO XI0 modes. Capacitors C773 and C755 damp the
to about -8V. That voltage is applied to R510 and high-frequency gain of the preamplifier.