Tektronix 7834

Theory of
Operation--834
+5 Volt Supply
The operation of the +5 Volt Supply is basically
the same
as described for the previous supplies . Error voltage to
the
base
of
Q1 560B is provided by divider R1564
and R1565,
referenced to the -50 V Sense line, from
the +5 V Sense
line . The divider ratio of R1564 and
R1565 is 10 :1, so the
base of 01560B is at zero volt when the supply
is operating
normally . The level on the -50 V Sense line is
held stable
by the -50 Volt Supply . Therefore, any change at
the out-
put of the +5 Volt supply appears at
the
base of
01560B as
an error signal . The output voltage is regulated
in the man-
ner described previously for the --50
Volt Supply . Diode
CR1589 limits the output
of this supply to about ---0 .6 volt
if it is shorted to
one of the negative supplies .
The +5 Volt Current Limiting stage (Q1 576A
and B) pro-
tects this supply from excessive output
current . Transistors
01576A and 01576B are connected as
a comparator to
detect excessive current through R 1587 . With
normal sup-
ply current through R1587, the voltage drop across R1587
is such that the base of 01576B is more positive
than the
base of Q1576A ; therefore, Q1 576A is cut
off . If the cur-
rent through R1587 increases above a safe
level, the base of
01576B becomes more negative than the base of Q1 576A .
Now, 01576B is cut off and 01576A conducts .
The
col-
lector current of Q1576A forward biases
CR1576 and
decreases the voltage on the base of 01582
. This limits the
conduction of 01588 to a safe current
level .
+15 Volt Supply
The +15 Volt Supply regulates in the same
manner as the
--50 Volt Supply ; current limiting operates
in the manner
described for the +5 Volt Supply
.
Error feedback
voltage to
the base of Q1436B is provided through divider
R1440 and
R1459, referenced to the -50 V Sense line,
from the +15 V
Sense line . The divider ratio of R1440 and
R1459 sets the
base of 01436B at zero volts . Any
change in the output
level of the +15 Volt Supply appears
at the base of Q1436B
as an error signal . This results in an
opposite change at the
collector of Q1436B . This
change is conveyed to the +15 V
Series Regulator stage
through 01455 to correct the error
in
the
output
voltage of the supply .
Diode CR1439 protects Q1436B against negative voltages
if
the +15 Volt Supply is shorted to
ground . Diode CR1459
limits the output of this
supply to about -0 .6 volt if it is
shorted to
one
of
the negative supplies .
+50 Volt Supply
Operation of the +50 Volt Supply is basically the same as
described for the ---50 Volt Supply ; current limiting operates
in a
similar manner as described for the +5 Volt Supply .
Error voltage to
the base of 01490B is provided by divider
R1412
and
R1429,
referenced to the --50 V Sense line,
from the 4-50
V Sense line . The divider ratio of R1412 and
3-5 8
R
1429 sets the base level of Q1 409B at
zero volts when the
output of
this supply is correct . The protection
diodes in
this
circuit operate similarly to those in the other
supplies .
Fan Circuit
The fan motor
used in this instrument is a
brushless do
motor,
using Hall Effect devices . The 2
Hall Effect devices
sequentially drive the 4 transistors (U1615
A, B, C, and D)
which, in turn, control the current flow
through the 4 field
windings . The fan motor speed is
regulated by limiting the
current flow through 01603 . Diodes
CR1611, CR1612,
CR1613, and CR1614
rectify the back emf produced
by
the 4 field
windings . This voltage is applied to the base
of
01603 through
resistive divider network R1614and
R1613 ;
the voltage
developed by this circuit is proportional
to the
motor
speed . If the morot speed starts to
increase, the cur-
rent drive to the base of 01603
will decrease, reducing the
current to the motor, thus maintaining
a constant motor
speed .
Graticule-Light Supply
The
Graticule-Light Supply provides
power to illuminate
the
graticule lights . The front-panel
GRAT ILLUM control
controls the output of this supply to set
the brightness of
the
graticule lights . - Transistors 01546, 01555,
and CR1549
form a pseudo
differential amplifier .
The output voltage at
the
collector of 01555 follows the voltage
set at the base of
01546
by the divider made up of
R1544, R1545, R1543,
and the GRAT ILLUM control
R917 (see Diagram 13) .
Resistor R1551 limits the output current
from this supply
to protect
01555 from damage due to a short
circuit .
MAIN INTERCONNECT
The Main Interconnect diagram shows the
signal and power
interconnections between the
Acquisition and Display Unit,
diagram 13, in the Diagrams section of
this manual .
VERTICAL
AMPLIFIER
The Vertical
Amplifier circuit provides final
amplification
for the vertical signal before it is applied to
the vertical de-
flection plates of the crt . The Vertical
Amplifier also accepts
inputs from the Readout System to block
the vertical signal
while readout information is displayed
and to produce the
vertical portion of the readout display .
An input is provided
for the BEAMFINDER
switch to compress an over-scanned
display within the viewing area
of the crt . In addition, the
Vertical Amplifier accepts the
Auxiliary Y-Axis input from
the Main Interface circuit . Figure 3-43
shows a block dia-
gram of the Vertical Amplifier
circuit and a schematic is
provided on diagram 14 in
the Diagrams section of this
manual .