Page 2
v-
400D-2C
D.
MODEL 400D SERIAL NO. 5130 AND BELOW
Transpose the connections to electrolytic capa-
citor C30
so
that C30B
is
20 pf and C3OC
is
40 pf.
This transposition
will
reduce power supply ripple.
E. MODEL 400D SERIAL NO. 14770 AND BELOW
MODEL 400H
S
ERIAL NO. 1137 AND BELOW
Replacement of control R29 or three-section resistor
R28 in the amplifier feedback
circuit
will
necessitate
revising and rewiring the feedback circuit.
The parts for this
circuit
revision
are
supplied as a
kit with complete instructions under
@
Stock No.
400D-95A.
The revised feedback
circuit
is
recommended for all
400D and 400H instruments whenever repair of the
feedback circuit
is
necessary. The revised circuit
is
superior to earlier circuitry but does not offer suf-
ficient
advantages to warrant modification merely to
include the new circuit in an instrument. The problem
of adjusting frequency response following any changes
in the feedback circuit would also make
it
advisable
to avoid making unnecessary changes,
F. MODEL 400D SERIAL NO. 16371 AND ABOVE
The regulated power supply was changed. Series
regulator tube V7 was changed from
a
6AU5 toa
12B4A. Control tube V8 was changed from a 6CB6
to a 6U8 and reference tube V9 was changed from an
OB2 to a type 5651. The circuit differences can be
determined by referring to the schematic diagrams
included in these Service Notes.
Field modification to incorporate this change
is
im-
practical and
is
not recommended.
SECTION
II
-
TUBE
&
Tubes with standard JEDEC 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 according to the
instructions in these Service Notes
after
replacing
V6, V7, V8, or V9 in
the
power supply.
Heater voltage must be checked and adjustedafter
re-
placing
V1,
V2,
V3,
or V4. Factory selected tubes
are
available for
V1
and V2. These tubes are tested
for low noise and microphonics. Contact your@ Sales
Office for the
@
Stock Numbers and prices on these
tubes.
G.
MODEL 400H SERIAL NO. 1338 THROUGH 1537
Resistor R83 should be changed in value and
location. The
new
value
is
470,000 ohms
*lo%,
1/2
,‘-
watt instead of 100,000 ohms.
A
second
resistor
with
a value of 470,000 ohms
&lo%,
1/2 watt and a sche-
matic
reference
of R85
is
connected in
series
with
R83.
The
series
combination of R83 and R85
is
then
connected between the regulated B+ output at the cath-
ode of the regulator tube and chassis ground. The
transformer winding supplying heater voltage for V6,
V7, and V8
is
connected
to
the junction of R83 and R85.
These connections can be made by mounting both
re-
sistors between terminals on the socket for V7.
4
H. MODEL 400D SERIAL NO. 17970 AND BELgW
MODEL 400H SERIAL NO. 2237 AND,BELaW
Residual noise as indicated by the meters in these
instruments can be reduced by a modification of the
input attenuator.
A
higher degree of residual noise
reduction
is
obtained on the upper voltmeter ranges
by this modification.
The input attenuator
is
changed from a capacity divider
to a
resistive
divider system by the addition of a kit
of parts available under
@
Stock No. 400D-95B. The
instructions supplied with the kit give all necessary de-
tails
for changing the Input Attenuator Resistor Board.
Basic calibration and frequency response adjustments
will
be required after this modification has been
completed.
IC
L/
I.
MODEL 400D SERIAL NO. 25970 AND BELOW
MODEL 400H/L SERI AL NO. 5225 AND BELOW
The value of resistor R45 can be varied to control low
frequency response
if
cathode resistors R38, R47, and
R53 are changed from 120 ohms to 100 ohms
&lo%,
1
watt.See Section IV stepM1-d. The vtvm performance
will
be the same after changing the resistors but low
frequency response adjustments are simplified.
CRYSTAL
DIODE
REPLACEMENT
Special high-performance junction type silicon diodes
are available for replacement of diodes CR1 andCR2.
These special diodes are manufactured by
@
and are
available under
@
Stock No. G-29B-5. Order directly
from the factory or from your
@
Sales Office.
The
@
Stock No, 212-1N82S diodes once specified for
CR1 and CR2 have been superseded by the superior
G-29B-5 diodes. The cathode end of the diode has
a
gold plated
wire.
The anode end of this diode has a
silver
plated
wire.
The G-29B-5 diodes have a junction that
is
less
than
1/2 mil in diameter. If the diodes are dropped, a
mechanical failure may occur at the junction.
After
installation the diodes
will
withstand any shock that
the entire instrument can withstand.
Voltage calibration and frequency response should be
checked and adjusted after replacing
V1,
V2, V3,
V4,
V5, CR1, or CR2.
4
00665-3
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