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Model 4S6A
equivalent, and two Mallory Type TR 233R batteries
in series.
BE SURE TO INSTALL THE BA TTERIES
WITH THE SAME P OLA RITIES AS MARKED ON
THE BATTERY HOLDER. Note that the s mall termi
nal on the battery is negative not positive a s with
dry c ells .
BE SURE THE BATTERIES ARE MAKING GOOD
CONT AC T WITH THE BATTERY
necessary, bend the battery holders so that they do
make good contac t. In the holder where two batteries
a re u s ed in s eries cut off part of the ca rdboa rd tubing
a round the batteries a t the ends which meet in order
to get good contact between them.
C O N VE R TING TO AC-O PERATED M O DE L .
4 -6.
T h e battery-operated model c a n b e quickly a n d easily
converted to an ac - powered model .
instrument from the cabin et as ins tructed in pa ra
graph 4- 2.
Unsolder the w i res going to the batter y
holder at the ba ttery holder end .
sc rews h olding the battery h o l der chas s is. Remove
the battery holder chassis and batteries, if a n y .
Substitute the ac power supply ( � stock #456A-95B)
in the s pace formerly occupied by the ba tte ry holder
chassis .
Fasten the supply with the same sc rews used to mount
the battery holder chassis.
to the terminals (marked w ith 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 acc urate to within
s uch as � Model 403A or 400H/L.
2) Signal Generator with output to 4 me, s uch as
�
Model 6S0A .
3) Shunt Resis tor of 1 0 ohms accurate to at least
s uch as � Model 470C .
DC
4)
Vacuum Tube Voltmeter, s uch as
4l0B or 4l2A .
5) Variable
Tra nsformer
from 1 00 to 130 volts , equipped with a monitor volt
1
meter accu rate within ±
6) Clip-On Milliammeter, s u ch as
4-8.
FRONT PANEL P E RFO R MAN C E C H E CK.
Perform this chec k upon receiving this instrum ent
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 mete r .
I f there i s appreciable hum s e e paragraph 2-50, Ground
00395- 1
HOLDERS.
If
Remove the
Remove the twO
Solder the loose wires
1%.
±
±
5%
�
Models
continuously adjusta b le
volt.
�
Model 428A.
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
Jl
less than 75
volts (100
model).
If not check C94, power supply capacitor
(if ac powered), or try replacing C RI , Q 1 , or Q2.
Chec k with 456A POWER s witch off to see if ground
loops exist . If th ey do, see pa ragraph 2-50.
B.
CALIBRATION A ND FREQUENCY RESP ONSE.
Perform the proc edure illus trated in figure 4-2.
Note tha t this test is used only for rough testing with
readily available equipmen t .
test acc u racy will be much poorer than the probe
deviation from specification.
that the probe is wi thin s pecification s .
4-9.
TR O UB L E LO CALIZAT IO N .
Adopting a sys tematic a pproach to troubleshooting will
enable you to find the trouble in the shortest poss ible
time and elim inate the poss ibility of dam agin g the
transis tors or othe r parts of the instrument. Wheneve r
trouble is suspec ted, perform the following s teps in
the order gi ven u ntil the trouble is located. A fte r the
trouble has been loca ted proceed to the next paragraph .
1) Perform the front-panel performa nce check as
giv en in paragraph 4-8. If ins trumen t does not m eet
specifica tions proceed to next step.
2) Clean probe jaws as desc ribed in paragraph 2-5A3
and figure 2-3.
3) If instrument does not operate at all check con
tinuity of both cables w i th an ohmmeter . Check probe
by substitution if another probe assembly is availa bl e .
4) Check the voltage o f the ba tter ies as described
in paragraph 4-4, Checking the Batteries .
BA TIER Y TEST jacks on the fron t panel of an ac
operated instrument, the VOlta ge should be approxi
10%
±
mately 8 volts
with a n input of 1 1 5 volts ac . If
th e 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 fron t of the etched-circuit boa rd with a
milliammeter, s uch as the
Milliammeter. The cu rrent should be S . 5ma±0.5 rna.
It too high , check voltages across CR l , Ql, and Q2.
6) Measure c urrent 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
w rong 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, s u ch as the
400D/H/L). from any pink (B-) lead to any red (B+)
lea d .
This is a v ery diffic ult measu rement to make
b ecause eliminating all ground loops is necess a ry .
Refe r t o pa ragraph 2-50 for i n struc tion s conce rning
Section IV
Pa ragraphs 4-6 to 4-9
Jl
volts with ac -powered
In almost a l l cases the
In this case a s s ume
At the
�
Model 428A Clip- On
�
Model 403A (or Model
4-3
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