HP 456A, Инструкция по эксплуатации и обслуживанию, Страница: 17 / 24

Section IV
Paragraph
4 - 1 0 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
volts ac ross it.
8)
if the instrument section is faulty, locate the
trouble by measuring the vol tages as shown on the
schematic diagram.
CAUTION
TAKE CARE NOT TO SHORT TH E WIRING
WHEN TROUBLESHOOTING. EVEN A MO­
MENTARY S HORT MAY BURN OUT A TRAN­
S ISTOR OR mODE.
The transis tors can be checked in a similar manner.
Since the transistors used in this instrument are 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 respect to the base and of the value shown
on the schematic diagram.
Unfortunately , due to the large amount of feedback
any trouble may cause all of the voltages to be off.
First, try to find the trouble by resistance measure­
ments . If this fails you may 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 transistors or diodes unless
there is a definite indication that these components
are faulty, as you may damage them. Turn the in­
strument OF F before doing any soldering as ac
leakage from the soldering iron may overload and
damage the transistors . In addition before using an
iron on any semi-conductor lead, clip a pai r of long­
nosed pliers between the iron and semi-conductor .
The pair of pliers will act as a heat sink and absorb
hear helping to prevent damage to the semi- conductor .
Table 4-1 lists the tes ts which should be made after
replacing the components .
4 -11 .
CAL I BR AT I O N .
Perform the procedure in figure 4-2 at 1 kc . Adjust
R9, Gain Set Potentiometer, so that the voltage out
is exactly
0. 1 volt (millivolts output equals milli­
amperes input). If you can't reach calibration, check
R7 , 9, 1 0 and 1 2, and the probe .
4 - 4
Model 456A
4 -12. F R EQUEN C Y R ESP O N S E.
Perform the procedure given 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. The cable assembly used i n the
456A-2 1 A P robe Assembly includes in the probe
cable assembly a speCial cable terminated at one end
with a Mic rodot miniature connector and at the other
end with a crimp- on ground lead . Both of these con­
nectors may be replaced but it is recommended that
a new probe cable as sembly,
�
s tock # 456A - 2 1 A - 8,
be installed instead because of difficulties encountered
in replacement.
B . MIDRODOT M INIATURE CON N ECTOR. The
microdot connector terminates the end of the
cable under the handle (see figure 4- 3). To replace
this connector four new parts and a special hand tool
for ass em bl y a re required. See the operating note
for the
c!j!l
425A- 2 1 B Microvolt Ammeter P robe for
ins tructi ons .
C. CRIM P -ON GROUND CON NEC TOR. This con-
nector is crimped to avoid soldering and 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 pair of pliers between the iron and
insulation to act as a heat sink.
Table
4- 1 .
Replacement
Componen Test
CRI None
CR90 Check power supply (pa r . 4-
9)
CR91 Check power supply (pa r. 4-9)
CR92 Check power s upply (pa r. 4 - 9)
Q l
None
Q2
Check calibration (par .
4- 1 1 )
Q9
0 Check power s upply (par. 4-9)
P robe Check calibration (par. 4- 1 1);
check frequency response (par .
4 - 1 2:
00395- 1