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Section IV
4 - 1 0
Paragraph
to 4- 1 3
the elimination of ground loops . If, after eliminating
the ground loops , the ripple is still too great check
CR92 and Q90.
Diode CR92 should have 14 volts
*
2 vol ts ac ross it.
8) if the instrument section is faulty , locate the
trouble by measuring the vol tages as shown on the
schematic diagra m .
CAUTION
TAKE CARE N OT TO SHORT
WHEN TROU B LESHOOTING.
MENT A RY S HORT MAY BURN OUT A TRAN
OR mODE.
S ISTOR
The transis tors can be checked in a similar manner.
Since the transistors used in this instrument a re of
the pnp type the voltage at the emitter should be
slightly positive (w ithin 0.4 volt) of the voltage on the
base.
The vol tage on the collector should be nega
tive with respec t to the base and of the value shown
on the schematic diagram.
Unfortunately , due to the large amount of feedback
any trouble ma y cause all of the voltages to be off.
First, try to find the trouble by resistance measure
ments .
If this fails you m ay have to remove the
transistors and diode, and test them on a simple dc
tester .
4 -1 0 .
S EMI - C O N DU CTOR RE P LACEME N T .
Do
NOT replace the trans istors or diodes unless
there is a definite indication that these componen ts
are faulty , as you may damage them .
before doing any soldering as a c
strument OF F
lea kage from the soldering i ron m a y overload and
damage the transistors .
iron on any semi-conductor lead, clip a pai r of long
nosed pliers between the iron a nd semi-conductor .
The pair of pliers will ac t as a heat sink and absorb
hear helping to prevent damage to
Table 4-1 lists the tes ts which should be made after
replacing the compon ents .
4 -1 1 .
CAL I BR AT I O N .
Perform the procedure in figure
R9, Gain Set Potentiometer, so that the voltage out
0. 1
is exactly
volt (millivolts output equals m illi
amperes input).
If
you can't reach calibration , check
R 7 , 9, 1 0 and 1 2, and the probe .
4 - 4
TH E
WIRING
EVEN A MO
Turn the in
In addition before using an
the
s emi- conductor .
4 -2
at 1 kc . Adjust
4 -12.
F R EQU E N C Y
R ESP O N S E.
Perform the procedure giv en in figure 4- 2.
4 -1 3 .
R E P LAC I N G T H E PRO B E C A B L E .
A .
GENERAL.
Th e cable as sembly used i n the
45 6A-2 1 A P robe As semb ly includes in the probe
cable assembly a s peCial cable terminated at one end
with a Mic rodot miniature connector and at the other
end with a c rimp- on ground lead .
nectors may b e replaced b u t it is recommended that
a new probe cable as sembly,
be installed instead becaus e of difficulties encountered
in replacement.
B .
MIDRODOT M INIATURE CON N ECTOR .
mic rodot connec tor terminates the end of the
cable under the handle (see figure 4- 3). To replace
this connec tor four new parts and a special hand tool
for ass em bl y a re required.
c!j!l
for the
425A- 2 1 B Microvolt Am m eter P robe for
ins tructi ons .
CRIM P -ON GROUN D CON NEC TOR.
C.
nector is crimped t o a void soldering a n d conse
quent melting of the cable ins ulation . Crimp the ground
lead on again. if possible. If the ground lead must be
soldered , clamp a pai r of pliers between the iron a nd
insulation to act as a h eat sink.
Table 4- 1 .
Componen
None
CRI
Check power supply (pa r . 4- 9)
CR90
CR91
Check power supply (pa r. 4-9)
CR92
Chec k pow e r s upply ( pa r. 4 - 9)
None
Q l
Check calibration (par . 4- 1 1 )
Q2
0
Q9
Check power s upply (pa r . 4-9)
P robe
Check calibration (par. 4- 1 1);
c heck frequency response (par . 4 - 1 2:
Model 456A
Both of these con
�
s tock # 456A - 2 1 A - 8,
The
See the ope rating note
This con -
Replac ement
Test
00395- 1
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