HP 400D, Инструкция по эксплуатации и обслуживанию

Страница: 24 / 50

Sect.
IV
P
a
ge
6
The following procedure s are listed .in a recom­
mended sequence for a complete test and calibration
operation. In general, tubes are the main cause
of
trouble and new ones should always be tried before
maldng adjusbnents or other component replacements.
See paragraph 4-4 before changing tubes.
The specifications for your �Model 4000, 400H, or
400L Vacuum Tube Voltmeter are given in the front
of this manual. The following test procedure con­
tains extra checks to help you analyze a particular
instrument. These extra checks and the data they
contain can not be considered as specifications.
A
ten to fifteen minute warm-up and pOwer supply
output voltage measurements are always recom­
mended before making any other test or adjustment.
NOTE
The test frequencies and the voltages specified in
many of the steps in this procedure are based upon
the use of the test equipment listed in paragraph 4-5.
Other frequencies and voltages can be used. Any
mm can be adjusted for optimum performance on
the range you most commonly use.
If
a Model 400D or 400H vtvm is involved, fol­
low the zero adjustment procedure
in
paragraph 4-2.
The mechanical zero for the special meter in the
Model 400L vtvm is preset during manufacture of
the mete r movement.
B.
CHECK
REGULATED POWER SUPPLY
1)
T
he B+ voltage at the output of the regulated
power supply should be 250 volts, :t5 volts. Measure
this voltage between the chassiS and the cathode of
the series regulator tube with the line voltage set
to 115 volts.
2) Vary the line voltage between 103 and
127
volts.
The regulated B+ will usually change no more than
2 volts from one extreme to the other. Ripple volt­
age in the regulated B+ is usually 3 millivolts or
less under these same test conditions.
The dc voltage at the plate of the series regulator
tube or at the cathode of the rectifier tube should
be between
400 and 420 volts wllh a 115 volt line .
Low voltage at this point may be from a defective
rectifier
tube,
filter capacitor, or power transformer.
To check the rectifier tube, connect a dc voltmeter
between chassis and the rectifier cathode. Reduce
line voltage from 115 to 103 volts. The voltmeter
reading should drop as line voltage is reduced and
then remain steady.
If
the dc voltage continues to
drop at a slow rate , the rectifier tube is probably
weak and should be replaced.
If
the regulated
Sf.
voltage Is not correct, replace­
ment
of
any or
all
four tubes in the power supply may
be necessary.
If
tube replacement does not correct
the dc voltage, precision resistors R62 and/or R64
may have changed value and should be replaced.
Abnormal current drain caused by component failure
elsewhere in
the
vtvm can result in poor power sup­
ply regulation.
Excessive ac ripple may
be
from a defective tube
in the power supply other than the rectifie r tube .
Capacitor C 3 6 may be open.
C.
SET DC HEATER VOLTAGE
1) Adjust control R66 to set the dc heater voltage
for tubes VI and V 2
to
6. 3 volts
:to.
2 volt. The dc
voltages across the heaters of tubes
V3
and V4 must
also be 6. 3 volts
:to.
2
volt.
DO NOT USE A VOLT­
METER THAT IS CONNECTED TO A COMMON
GROUND WITH THE VTVM UNDER TEST WHEN
MEASURING
THESE HEATER VOLTAGES
- A SHORl'
cmCUIT
WILL RESULT.
Slight readjustment of
control R66 will often bring these two voltages within
tolerance.
2)
If
both vOltages cannot
be
set to 6 . 3 vOlts,
:to.
2
volt, adjust R66 to bring the lowest of the two volt­
ages within tolerance. Connect a resistor in parallel
with the two heaters with the highest voltage. The
correct resistor value will enable you to bring
both
heater voltages within tolerance by readjusting
control R66.
Use
only one resistor in parallel with
one pair of tubes at a time.
A
resistor value below
220 ohms, 1 watt, should not be used.
3)
If
you are unable to obtain the correct heater
voltage after completing steps
I
and 2, check electro­
lytic capacitors
in the
heater circuit.
If Ix>th
heater
voltage s are low, check selenium rectifier SRI.
D. INPUT CIIlCUIT
RESPONSE ADJUSTMENT
1) Connect a I kc, 2 volt peak-to-peak, square wave
signal to the
INPUT
of the vtvm under test. Connect
the OUT P UT terminals of the same vtvm to the
vertical input of a high frequency oscilloscope such
as
the *
Model l50A.
.