Tektronix 7623, Instruction Manual

Page: 60 / 266

Circuit Description—7623/R7623 Service
Display
Right
Command
Plug-In
Alternate
Command
Fig. 3-12. Logic diagram of Plug-In Binary and Plug-In Alternate Buffer stages.
Output Buffers
The output switching commands from the Logic circuit
are provided through buffer stages Q142Q137,
Q142-O147, Q162-Q167, and Q182-Q187. Each of these
stages includes a common-base input transistor to provide a
low-impedance load for the associated driving stages. The
output transistor is connected as an emitter-follower to
provide isolation between the Logic circuit and other
circuits within this instrument or the plug-in units.
TRIGGER SELECTOR
The Trigger source switch determines which vertical
signal is connected to the time-base unit, and which vertical
signal, that is provided at VERT SIG/OUT connector on
the rear panel. Fig. 3-13 shows a detailed block diagram of
the Trigger Selector circuit, along with a simplified diagram
of all the circuitry involved in selection of the trigger
source. A schematic of the Trigger Selector circuit is shown
on diagram 3 at the rear of this manual. Also, see diagrams
6 and 7 for the signal selection circuitry not shown on
diagram 3.
Trigger Mode and Add Signals
General.
The circuitry shown on the left side of the
simplified diagram in Fig. 3-13 determines the operation of
the Trigger Channel Switch stage. TRIG SOURCE switch
S1011 controls Trigger Channel Switch U324. When the
TRIG SOURCE switch is set to the VERT MODE position,
the setting of the VERT MODE switch determines the
trigger selection. In the LEFT or RIGHT positions, the
trigger signal is obtained from the indicated vertical unit.
The following discussions give detailed operation in each
position of the TRIG SOURCE switch.
the base of Q314 is connected to ground through the ALT
and RIGHT sections of S1021, CR1021 and CR1026, and
S1011. This holds Q314 reverse biased to provide a LO
level to pin 4 of U324 (see Fig. 3-14).
When the VERT MODE switch is set to ALT, +5 volts is
applied to the base of Q314 through CR1021 and S1011.
Q314 is forward biased and its emitter level is determined
by the Mainframe Vertical Mode Command signal from the
Logic circuit applied to its collector. This signal switches
between the HI level (Right Vertical unit to be displayed)
and the LO level (Left Vertical unit to be displayed) at the
end of each sweep. When the Mainframe Vertical Mode
Command is HI, it provides a positive collector voltage to
0314. Q314 is saturated due to CR1021, and its emitter
level is very near the collector level. This provides a HI
output level to the Trigger Channel Switch stage. As the
Mainframe Vertical Mode Command goes LO, the collector
supply for Q314 also goes negative. Q314 remains saturated
and the output again follows the collector level to supply a
LO output level to U324.
For ADD and CHOP vertical mode operation, +5 volts is
connected to pin 14 of U324 through CR1023 or CR1024
and S101 1. At the same time, the base of Q314 is held LO
by the ground connection through the ALT and RIGHT
section of S1021 so the level at pin 4 of U324 is LO also
(produces an ADD mode in Trigger Channel Switch; see
description of this circuit which follows). In the RIGHT
position of the VERT MODE switch, +5 volts is connected
to the base of Q314 through CR1026 and S1011 to
forward-bias the transistor. The Mainframe Vertical Mode
Command signal connected to the collector of Q314 is also
HI in this mode, and a HI output level is produced at the
emitter of Q314.
Vert Mode.
In the VERT MODE position of the TRIG
SOURCE switch, the setting of the VERT MODE switch
determines the operation of the Trigger Channel Switch
stage. In the LEFT position of the VERT MODE switch.
Left. When the LEFT trigger source is selected, the
VERT MODE switch is disconnected from the trigger
selector circuitry. Now the ground connection through the
3-16