Tektronix 7623, Instruction Manual

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Circuit Description —7623/R7623 Service
+ 130-Volt Supply
The + 130-V Rectifier CR806 provides the rectified
voltage for the + 130-Volt Supply. However, this secondary
winding of T801 does not supply the full potential
necessary to obtain the +130-volt output level. To provide
the required output level, the +50-Volt Supply is connected
in series with this supply through Q850. Basic regulation of
the output voltage is provided by +130 V Feedback
Amplifier Q852, and +130 V Series Regulator Q850.
The output voltage of this supply is connected across
divider R855 R856. This divider provides a quiescent level
of about +50 volts at the base of Q852. The reference level
for this supply is provided by the +50-Volt Supply
connected to the emitter of Q852. If the output of this
supply changes, this change is sensed by Q852 and an
amplified error signal is connected to the base of Q850.
This error signal changes the conduction of the +130 V
Series Regulator Q850 to correct the output error. Fuse
F855 protects this supply if the output is shorted.
However, since the response time of F855 is slow to a
shorted condition, VR851 provides additional current to
the base of Q850 to protect it from damage due to over
voltage. Diode CR852 limits the reverse bias on Q852 to
about 0.6 volt when F855 is blown.
signal is taken off the collector of Q618 and connected to
an output buffer Q620. CR621 and CR622 provide
protection against a high voltage inadvertently applied to
the output connector.
+ GATE OUT
The gate signal is connected to a comparator circuit
Q662 and Q666 through resistor R660. From the com­
parator the gate signal is connected to the emitter of an
output buffer Q672. Gate Selector switch connects one of
the gate signals to R660, the input of the Gate Amplifier.
Possible gate signals are MAIN gate and, with a dual-sweep
time-base unit,a DELAY or an AUXILIARY gate signal can
be selected. CR674 and CR676 provide protection against a
high voltage inadvertently applied to the output connector.
+ SAWTOOTH OUT
The sawtooth signal is connected to the Sawtooth
Amplifier through R36. Q631, Q634, and Q640 comprise a
negative feedback amplifier with a gain of two, determined
by the ratio of feedback resistor R645 to the combined
input resistance of R630 and R63. CR635 and CR676
provide protection against a high voltage inadvertently
applied to the output connector.
Graticule Light Supply
Power for the graticule lights is supplied by the Graticule
Light Supply. Rectified voltage for this supply is provided
by 5 V Rectifier CR820-CR821. Q835 operates as a series
regulator transistor. Emitter follower Q829 determines the
conduction of this series regulator as controlled by front
panel GRATICULE ILLUM Control R1095. Current­
limiting to protect this supply is provided by Q827. Under
normal operation, divider R830-R831-R833 sets the base of
0827 below its conduction level. However, if excess current
is demanded from this supply, the voltage drop across
R837-R838 increases until Q827 comes into conduction.
The collector of Q827 then limits the conduction of this
supply to limit its output current.
Divider R822 R823 provides a sample of the line voltage
in the secondary of T801 to the plug-in unit. This provides
a line-frequency reference to the plug-in units for internal
triggering at line frequency or for other applications.
SIGNAL OUT BOARD
VERT SIG OUT
The vertical signal is selected by the TRIG SOURCE
switch. The vertical signal selected is applied to the bases of
a differential amplifier Q606 and Q618. A single-ended
READOUT SYSTEM
The Readout System in this instrument provides alpha­
numeric display of information encoded by the plug-in
units. This display is presented on the CRT and is written
by the CRT beam on a time-shared basis. Schematics for
the total Readout System are shown on diagrams at the rear
of this manual.
The definitions of several terms must be clearly under­
stood to follow this description of the Readout System.
These are:
Character— A character is a single number, letter, or symbol
which is displayed on the CRT, either alone or in
combination with other characters.
Word-A word is made up of a related group of characters.
In this Readout System, a word can consist of up to ten
characters.
Frame-A frame is a display of words for a given operating
mode and plug-in combination. Up to six words can be
displayed in one frame. Fig. 3-24 shows one complete
frame (simulated readout) and the position at which
each of the six words is displayed.
3-31