4-1
INTRODUCTORY
This section contains instructions for maintaining
and trouble shooting the instrument. A tube re
placement chart lists the checks and adjustments
required after tube replacement, and a trouble
shooting chart assists in localizing most troubles
which might occur. Photographs soowing the physical
location of compOnents, a voltage and resistance
diagram, and a schematic diagram are also pro
vided fOr your convenience.
The more intricate adjustment procedures described
in this section are provided for those who have the
necessary test equipment. When qualified per
sonnel and test facilities are not available, it is
suggested that adjustments not
be
made in the field.
Instructions are given on the Warranty page in this
manual
for the procedure
to
be
followed should
repair service be required.
The following information appears in this section:
4-2
Meter Zero Adjustment
4-3
Case Removal
4-4
Tube and Crystal Diode Replacement
4-5
Required Test Equipment
4-6
Rapid Performance Check
4-7 Complete Adjustment and Test Procedure
4-8 Trouble Shooting
4-9
Power Supply Trouble Localization
4-10 Amplifier Trouble Localization
4-2
MnER ZERO ADJUSTMENT
The most accurate pOSitioning of the meter pointer
will result
if
the vtvm is allowed to warm-up for at
least 15 minutes with the cabinet in place.
A. MODELS 400D AND 400H
Turn warmed-up instrument off and allow about
two minutes for internal capacitors to discharge.
Rotate the meter mechanical zero adjusting screw
clockwise until the meter pointer
is
traveling down
scale toward zero and stop at zero.
H
you over-
Sect.
IV
Page
1
SECTION IV
MAINTENANCE
sOOot, continue rotating
the
adjustment screw clock
wise and again approach zero from the high side
of the scale. The adjustment screw should not
be turned counterclockwise during any part of this
procedure.
B.
MODEL tOOL
The special meter in the 400L is adjusted with
the vtvm turned ON since the meter pointer rests
against the stop at the left end of the scale when
the instrument is turned OFF. The mechanical
zero is preset during manufacture of the meter
movement.
4-3
CASE REMOVAL
The 1nstrument case (or dust cover) is fastened
to
the rear of the chassis with two screws. To remove
the case, remove the screws� and slide the case
to the rear and off the instrument.
4-4 TUIE & CRYSTAL DIODE REPLACEMENT
Tubes with standard EIA characteristics can be
used for replacement. In a great number of cases,
instrument trouble can be traced to a defective or
weak tube. Replace only those tubes proven to be
defective or weak. Check tubes by substitution.
Results obtained through the
use
of a "tube checker"
can be erroneous and misleading. Mark original
tubes to insure that they are returned
to
the same
socket
if
not replaced.
The power supply should be checked after replaCing
V6, V7� VB, or V9 in the power supply.
Heater voltage must be checked and adjusted after
replacing Vl, V2, V3, or V4. Factory selected
tubes are aVf\tlahle
for
VI and V2.
These
tubes are
tested for low noise and micro phonics. Contact
your
�
Sales Engineering Representative for the
�
Stock Numbers and prices on these tubes.
