Model 4S6A
equivalent, and two Mallory Type TR 233R batteries
in series. BE SURE TO INSTALL THE BA TTERIES
WITH THE SAME POLARITIES AS MARKED ON
THE BATTERY HOLDER. Note that the small termi
nal on the battery is
not positive as with
dry cells .
BE SURE THE BATTERIES ARE MAKING GOOD
CONT AC T WITH THE BATTERY HOLDERS.
If
necessary, bend the battery holders so that they do
make good contac t. In the holder where two batteries
are used in s eries cut off part of the ca rdboa rd tubing
a round the batteries at the ends which meet in order
to get good contact between them.
4 -6.
C O N VE R TING TO AC-O PERATED M O DE L .
T h e battery-operated model c a n b e quickly a n d easily
converted to an ac-powered model .
Remove the
instrument from the cabinet as ins tructed in pa ra
graph 4- 2. Unsolder the wires going to the battery
holder at the battery holder end . Remove the twO
sc rews
the battery
chas s is. Remove
the battery holder chassis and batteries, if any.
Substitute the ac power supply (� stock
#456A-95B)
in the space formerly occupied by the ba ttery holder
chassis .
Fasten the supply with the same screws used to mount
the battery holder chassis. Solder the loose wires
to the terminals (marked with the corresponding
colors ) on the boa rd. Replace the cabinet.
4 -7.
TEST EQUIPMENT REQUIRED.
The following tes t equipment is required to test the
Mode14S6A :
1) Vacuum Tube voltmeter accurate to within
±
1%.
such as � Model 403A or 400H/L.
2) Signal Generator with output to 4 me, such as
�
Model 6S0A .
3) Shunt Resis tor of 1 0 ohms accurate to at least
±
5%
such as � Model 470C .
4)
DC
Vacuum Tube Voltmeter, such as
�
Models
4l0B or 4l2A .
5)
Variable Transformer continuously adjustable
from 1 00 to 130 volts , equipped with a monitor volt
meter accurate within ±
1
volt.
6) Clip-On Milliammeter, such as
�
Model 428A.
4-8.
FRONT PANEL P E RFO R MAN C E C H E CK.
Perform this check upon receiving this instrument
or whenever trouble is suspected.
A. NOISE.
1 ) To check ac-powered 456A turn instrument off
and read the output nois e on an oscilloscope or meter.
If there is appreciable hum see paragraph 2-50, Ground
00395- 1
Section IV
Pa ragraphs 4-6 to 4-9
Loops . Follow the procedure in that paragraph until
any appreciable hum is eliminated.
2)
Turn on the 456A with no input to the probe. The
noise as read on the meter or oscilloscope should be
less than 7
5
Jl
volts (100
Jl
volts with ac -powered
model).
If not check C94, power supply capacitor
(if ac powered), or try replacing CRI , Q1 , or
Q2.
Check with 456A POWER s witch off to see if ground
loops exist . If they do, see pa ragraph 2-50.
B.
CALIBRATION A ND FREQUENCY RESPONSE.
Perform the procedure illus trated in figure 4-2.
Note that this test is used only for rough testing with
readily available equipmen t . In almost a l l cases the
test accuracy will be much poorer than the probe
deviation from specification.
In this case assume
that the probe is within specifications.
4-9.
TR O UB L E LO CALIZAT IO N .
Adopting a sys tematic approach to troubleshooting will
enable you to find the trouble in the shortest possible
time and eliminate the possibility of damaging the
transis tors or other parts of the instrument. Whenever
trouble is suspec ted, perform the following s teps in
the order gi ven until the trouble is located. A fter the
trouble has been loca ted proceed to the next paragraph .
1) Perform the front-panel performance check as
given in paragraph
4-8.
If ins trument does not meet
specifica tions proceed to next step.
2) Clean probe jaws as described in paragraph 2-5A3
and figure 2-3.
3) If instrument does not operate at all check con
tinuity of both cables with an ohmmeter . Check probe
by substitution if another probe assembly is availabl e .
4) Check the voltage o f the ba tteries as described
in paragraph 4-4, Checking the Batteries . At the
BA TIER Y TEST jacks on the front panel of an ac
operated instrument, the VOlta ge should be approxi
mately 8 volts
±
10%
with a n input of 1 15 volts ac . If
the voltage is much below 8 VOlts, either the s upply
is faulty or the inst rument is drawing too much current.
5) Meas ure the current in the red ( B+) lead running
around the front of the etched-circuit boa rd with a
milliammeter, such as the
�
Model 428A Clip- On
Milliammeter. The cu rrent should be S . 5ma±0.5 rna.
It too high , check voltages across CRl ,
Ql,
and Q2.
6) Measure current in violet-white lead (ground)
going to switch . There should be less than 0.02 rna
in this lead. If there is more cu rrent, s omething i s
wrong in the amplifier (provided the supply voltages
a re correct).
7)
Meas ure the rippl e on the ac s upply by connecting
an ac voltmeter, such as the
�
Model 403A (or Model
400D/H/L). from any pink (B-) lead to any red
(B+)
lead. This is a very difficult measu rement to make
because eliminating all ground loops is necess a ry .
Refer t o paragraph 2-50 for instructions concerning
4-3
