Tektronix 7834

Theory of Operation-7834
Power
On Logic
The Power On Logic provides a signal to the Storage cir-
cuitry . When the instrument is first turned on the Power On
circuit produces a low state output for about 2 seconds
after which time a high state is produced indicating that
power is on . The low state is produced by the 7 .5 volt
supply rising and turning on U1278A through CR1266,
81265, C1263, CR1265 and 81265 . The high state is
produced after 2 seconds, when C1263 changes through
81260, turning on CR1261 and thus removing base drive to
U1278A .
When the instrument is turned off, this circuit produces a
low state output before the regulated power supply voltages
begin to drop . The inverter stop signal, pin 8 of U1275,
goes high allowing U1278B to
turn on, thus turning on
U 1278D through U1278E .
LV (LOW-VOLTAGE)
REGULATORS
The Low-Voltage Regulators convert semi-regulated voltages
from the
Converter/Rectifiers circuit to stabilized, low-
ripple
output
voltages . The regulators are series type, using
the --50 volt supply as a reference . Figure 3-42 shows a
detailed block diagram of the Low-Voltage
Regulator cir-
cuits . A schematic is provided on diagram
12 in the
Dia-
grams section of this manual
.
--50
Volt Supply
The
Semi-Regulated -54 volts from the Converter/Recti-
fiers circuit provides the unregulated voltage source for this
supply . Transistors 01508, 01522, 01534 and 01538
operate as
a
feedback-stabilized amplifier to maintain a
constant ---50 volt output level . Transistor 01508 is con-
nected
as a differential amplifier to compare the feedback
voltage at
the
base of
01508B against the reference voltage
at
the base
of
Q1508A . The error output at the collector of
Q1508B reflects the difference between these two inputs .
The change in error output level at the collector of Q1 508B
is always in the opposite direction to the change in the feed-
back input
at
the base of Q1508B . Zener diode VR1505
sets a reference
level
of about -9
volts at the base of
Q1508A . A sample of the output voltage from the ---50
Volt Supply is connected to the base of Q1508B through
divider network R1512, R1513, and R1514 . Resistor
R 1513 in this divider is adjustable to set the output level of
this supply . Notice that the feedback voltage to this divider
is obtained from a line labeled -50 V Sense . If the feedback
voltage were obtained at the supply, the voltage at the load
would not stay constant, due to the inherent
resistance
of
the interconnecting cable
between the supply and its load .
The sense configuration overcomes this problem by sensing
the voltage at the load . Since the current in the sense line is
small and constant, the load voltage is held constant regard-
less of the load current .
3- 56
Regulation of voltage
occurs as follows : If the output level
of this supply
decreases (becomes less negative) due to an
increase in load
or a decrease in input voltage (as a result of
line-voltage
changes or ripple), the voltages across divider
81512, 81513, and 81514 decreases also . This results
in
a
more positive level at the base of Q1 508B than that
estab-
lished
by
the --50 Volt Reference stage at the base of
Q1 508A .
Since the transistor with the more positive base
controls the conduction of the differential amplifier, the
output
current
at
the collector of Q1 508B increases . This
increase
in current through 01508B also causes successive
increases in current
through 01522 and 01534 resulting in
increased conduction of 01538, the -50 V Series Regula-
tor . The load current increases and therefore the voltage
across the load also increases (becomes more negative)
sufficiently to balance the input into differential amplifier
Q1
508A and Q1508B . The -50 Volts adjustment,
81513,
sets the output level of this supply .
The -50 V Current Limiting stage (01526) protects
the
-50 Volt Supply if excessive current is demanded from
this
supply . Since the load is connected to this
supply through
R 1537, all current from the -50 Volt
Supply must flow
through this resistor . Transistor
01526 senses the voltage
drop across 81537 . Under
normal operation there is insuf-
ficient voltage drop across 81537 to
forward bias 01526 .
However, when
excessive current is demanded from the
-50 V Series Regulator due to a short circuit or similar
malfunction at the output of this supply, the voltage
drop
across R 1537 increases until it is sufficient to
forward bias
01526 . The collector current of 01526 results
in a reduc-
tion of current through 01522 and
finally
to
limit the
conduction of 01538 . This
current limiting protects 01538
from damage due to excess power
dissipation .
Several protection
diodes are also included in this circuit .
Diode CR1539 prevents
the output of this supply from
going more positive than about +0 .6 volt
if it is shorted to a
positive supply . Zener diode VR1501
and diode CR1502
supply a turn-on voltage
for 01508 to start the ---50 Volt
Supply
when the instrument is first turned on . As soon as
the
-50 Volt
Supply turns on, CR1502 stops conducting .
--15
Volt
Supply
Basic operation of
all stages in the -15 Volt Supply is the
same as for the -50 Volt Supply
. Reference level for this
supply is established
by divider 81463 and 81464 between
ground
the the --50
V Sense voltage . The divider ratio of
R 1463
and R
1464 sets a level of --15 volts at the base of
Q1406A
. The level
on
the -50 V Sense line is held stable
by the --50 Volt
Supply . Any change at the output of the
--15
Volt
Supply appears at the base of Q1406B as an error
signal . The output voltage is regulated in the same manner
as described for the ---50 Volt Supply . Diode CR 1499
limits
the output of this supply from going more positive than
about +0 .6 volt if it is shorted to one of the
more positive
supplies . Diodes CR1468 and CR1469
provide reverse volt-
age protection for transistors
Q1406B and 01406A,
respectively .