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

Model 3400A
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 de level output, taken from the emitter of
Q606, is applied to meter Ml and to the heating
element of thermocouple TC402. The de voltage de­
in
veloped 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 de 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.
4-3S. Noise on the modulated square wave is sup­
pressed by feedback from emitter of QSOS through
CS07 and CSOS 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 81. Switch 82 (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 DSI lights to indicate LINE power
ac
ON when 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
turn changes the resistance of the series regulator to
oppose the change in output voltage.
CR713 and CR70S 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 CIB and regulated by series regulator Q1.
The +75 volt supply provides regulated +75 volts which
is used as the plate supply voltage for V201.
variation from the output is felt at Q702 base circuit
through C704. R715. and R716.
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 ':: 7 06 and C707 and regulated by Q2.
Breakdown
at the base of Q'.
described in Parai;, "t!>h 4-41.
4-46. -6.3 VOLT SUPPLY.
4-47. The regulated -S.3 volt supply consists of a
full-wave rectifier (CR71S and CR717) whose output
is filtered by C2 and regulated by Q3.
er Q705 is connected to the -17.5 volt supply which
provides a reference for the -6.3 volt supply.
regulator Q3 acts as a dynamic variable resistor in
series with the output to oppose changes in output
voltage.
4-48.
4-49. The neon lamp drive oscillator consists of
transistor Q706. diode CR71S. resistors R701, R702.
R712. R713. and capacitor C711. Transistor Q70S
is held on (conducting) by the base bias developed at
the junction of R712 and CR71S. The collector current
of Q706 charges up capacitor C711 through R701 or
R702 depending upon the illuminated neon lamp on the
Chopper Amplifier Assembly AS. 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.
in
the capacitor charges up in the opposite direction
until firing the previously illuminated neon lamp. The
Diodes CR704. cycle described above repeats at a frequency of 90 to
100 Hz as determined by the RC time constants of
R701 and C711. and R702 and C71!.
(liod· :R715 provides reference voltage
1.
Regulation operation is the same

NEON LAMP DRIVE OSCILLATOR.

Section IV
Voltage
The C703 and R709
Emitter follow­
Series
4-3