HP 400D Operating And Servicing Manual page 24

IV ge
Sect. a 6
P
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
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.
ten to fifteen minute warm-up and pOwer supply
A
output voltage measurements are always recom­
mended before making any other test or adjustment.
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.
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
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.
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
The regulated B+ will usually change no more than
2 volts from one extreme to the other.
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
tube,
rectifier filter capacitor, or power transformer.
tried before
be
NOTE
Any
in
paragraph 4-2.
Measure
127 volts.
Ripple volt­
To check the rectifier tube, connect a dc voltmeter
between chassis and the rectifier cathode.
line voltage from 115 to 103 volts.
reading should drop as line voltage is reduced and
If
then remain steady.
drop at a slow rate , the rectifier tube is probably
weak and should be replaced.
If Sf.
the regulated voltage Is not correct, replace­
all
ment of any or four tubes in the power supply may
If
be necessary. tube replacement does not correct
the d c voltage, precision resistors R62 and/or R64
may have changed value and should be replaced.
Abnormal current drain caused by component failure
the
elsewhere in vtvm can result in poor power sup­
ply regulation.
Excessive ac ripple may
in the power supply other than the rectifie r tube .
Capacitor C 3 6 may
be
C .
SET DC HEATER VOLTAGE
1) Adjust control R66 to set the dc heater voltage
to
for tubes and 2 6. 3 volts
VI V
voltages across the heaters of tubes
:to.
also be 6. 3 volts volt.
2
METER THAT IS CONNECTED TO A COMMON
GROUND WITH THE VTVM UNDER TEST WHEN
THESE HEATER VOLTAGES
MEASURING
cmCUIT
WILL RESULT.
control R66 will often bring these two voltages within
tolerance.
If
2)
both vOltages cannot
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.
correct resistor value will enable you to bring
heater voltages within tolerance by readjusting
control R66. Use only one resistor in parallel with
one pair of tubes at a time.
220 ohms, 1 watt, should not be used.
If
3) you are unable to obtain the correct heater
voltage after completing steps
in the
lytic capacitors heater circuit.
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
to
signal the INPUT of the vtvm under test.
OUT P UT
the terminals of the same vtvm to the
vertical input of a high frequency oscilloscope such
the *
as Model l50A.
Reduce
The voltmeter
the dc voltage continues to
be
from a defective tube
open.
:to.
2 volt.
The dc
V3 and V4 must
DO NOT USE A VOLT­
- A SHORl'
Slight readjustment of
be :to.
set to 6 . 3 vOlts,
The
both
A resistor value below
I and 2, check electro­
If Ix>th
heater
Connect
.
2