Power Supply Assembly; Regulator Operation; Neon Lamp Drive Oscillator - HP 3400A Operating And Service Manual

Model 3400A
Section IV
4-34.
The emitter follower is
needed
to
match
the
high output
impedance
of
the
demodulator
to
the
low
input
impedance
of
the
meter
and thermocouple
cir-
cuits.
The
voltage
drop across
CR604
in
the collec-
tor circuit of
Q605
is
the
operating bias for Q604.
This
fixed bias
prevents
Q605 failure
when
the
base
voltage
is
zero
with respect to
ground.
4-35.
The dc level output,
taken
from
the
emitter
of
Q606,
is
applied
to
meter
Ml
and
to
the
heating
element
of
thermocouple TC402.
The
dc voltage
de-
veloped
in
the resistive
portion of
TC402
is
effectively
subtracted
from the voltage
developed by
TC401.
The
input signal to the
modulator then
becomes
the
differ-
ence
in
the
dc outputs
of
the
two thermocouples.
When
the difference
between
the
two thermocouples
becomes
zero the dc
from
the
emitter followers
(driving the
meter) will
be equal
to the ac
from
the
video amplifier.
4-36.
Noise on
the
modulated square wave
is
sup-
pressed by feedback
from
emitter of
Q606 through
C607 and C608
to the resistive
element
of
TC402.
4-37.
POWER SUPPLY ASSEMBLY
A7.
4-38.
The power supply
assembly
provides dc oper-
ating
voltages for the
tube
and transistors
used
in the
Model 3400A. See
power
supply
assembly schematic
diagram
illustrated
on Figure 6-4.
4-39. Either
115 or 230 volts
ac
is
connected to the
primary
of
power transformer Tl through fuse
Fl and
the
POWER
switch SI.
Switch S2 (slide
switch on
rear
panel)
connects
Tl primary windings
in
series
for
230-volt operation or
in
parallel for 115-volt
oper-
ation.
Neon lamp DS1
lights to
indicate
LINE power
ON
when
ac
power
is
applied
and
SI is
closed.
4-40.
REGULATOR OPERATION.
4-41.
The
series regulator acts as a
dynamic vari-
able resistor
in
series with the
power supply output.
A
control amplifier
senses changes
in
the output volt-
age by
comparing
the output with
a
fixed
reference
voltage.
The
control amplifier
then supplies any out-
put voltage
changes
to the
driver transistor,
which
in
turn changes the resistance
of
the
series regulator
to
oppose
the
change
in
output voltage.
Diodes CR704,
CR713
and
CR706
across
the
base emitter junction of
the series regulator
provide overload current
protection.
4-42.
+75
VOLT
SUPPLY.
4-43.
The +75
volt
supply consists
of
a full-wave
rectifier
(CR701 and CR702) whose
output
is
filtered
by
CIA
and
C1B
and
regulated by series regulator Ql.
The +75
volt
supply provides regulated +75
volts
which
is
used
as the plate
supply voltage for
V201.
Voltage
variation
from
the output is felt at
Q702 base
circuit
through C704, R715, and R716.
The C703
and R709
network provides phase
correction for
power
supply
stability.
The
regulation circuitry
is in
the negative
leg
of
the
+75
volt
supply,
and uses the -17. 5 volt
supply as
a reference.
4-44.
-17. 5
VOLT SUPPLY.
4-45.
The regulated -17. 5 volt
supply consists of
a
full-wave
rectifier
(CR711, and
CR712) whose
output
is
filtered
by
C706 and C707
and
regulated by Q2.
Breakdown diod
^R715
provides reference voltage
at
the
base
of Q'.
1.
Regulation operation
is
the
same
described in
Para^
°.ph
4-41.
4-46. -6.
3
VOLT
SUPPLY.
4-47.
The regulated -6. 3
volt
supply consists
of
a
full
-wave
rectifier
(CR716 and CR717) whose
output
is
filtered
by
C2
and regulated by Q3.
Emitter follow-
er
Q705
is
connected
to the -17.
5
volt
supply
which
provides a reference for the
-6.
3
volt
supply.
Series
regulator
Q3
acts as a
dynamic
variable resistor
in
series with the output to
oppose changes
in
output
voltage.
4-48.
NEON
LAMP DRIVE OSCILLATOR.
4-49.
The neon
lamp
drive oscillator consists
of
transistor
Q706, diode
CR718,
resistors R701, R702,
R712, R713, and capacitor C711.
Transistor
Q706
is
held on (conducting)
by the
base bias
developed
at
the junction of
R712
and CR718.
The collector
current
of
Q706 charges up capacitor
C711
through
R701 or
R702
depending upon
the illuminated
neon
lamp
on the
Chopper
Amplifier
Assembly
A6.
When
the
capacitor
reaches a
sufficient
charge to fire the
dark neon lamp,
the illumination
of
the
neon
lamps alternate
and
the
capacitor
discharges through
the previously
dark neon
lamp.
With
the
previously dark neon
lamp
illuminated,
the capacitor
charges up
in
the
opposite direction
until
firing the
previously illuminated
neon lamp.
The
cycle
described above repeats
at
a
frequency
of
90 to
100
Hz
as determined by
the
RC
time constants of
R701and C711, and R702
and C711.
4-3