5-20.
Output Impedance Test.
a. Connect 50 ohm output of 652A directly to ac
voltmeter.
b. Set 652A controls as follows:
FREQUENCY RANGE . . . . . . . . .
.
.
.
. .
X l lC
FREQUENCY dial
. • . . . . . . . . . . . . . • . . . .
I
OUTPUT ATTENUATOR . . . . . . . . . . 3.0 V
OUTPUT MONITOR . . . . . . . . . . . .
NORMAL
c. Adjust 652A OUTPUT AMPLITUDE and VERNIER
controls for a 6.0 V rrns indication on ac voltmeter.
d. Insert 50 ohm feedthru load between 652A and ac
voltmeter. AC voltmeter indication should drop to 3.0 V
± 0. 1 5 V rms, verifying a 50 ohm output impedance.
e. Remove 5Q ohm feedthru load and disconnect cable
to 50 ohm output of 652A.
f. Connect 600 ohm output of 652A directly to ac
voltmeter.
g. Adjust 652A OUTPUT AMPLITUDE and VERNIER
controls for a 6.0 V indication on ac voltmeter.
h. Insert a 600 ohm load between 652A and ac
voltmeter. AC voltmeter indication should drop to 3.0 V
± 0.1 5 V rms, verifying a 600 ohm output impedance.
5-2i.
ADJUSTMENT AND CALI BRATiDN PROCEDURE.
5·22. The following is a complete adjustment and calibra·
tion procedure for the Model 652A. The adjustments
should be performed only if it has been determined by the
Performance Tests that the 652A is not within specifica·
tions. Figure 5·7 shows the location of all internal
adjustments.
5·8
The 652A contains high impedance, high
frequency circuits. Contamination of the
switches, circuit boards or tuning capacitor
will cause high impedance leakage paths and
subsequent deterioration of the performance
of the instrument. A void touching any of these
circuits with the bare fingers, as skin oils are
extremely contaminating. If handling is neces
sary, wear clean cotton or rubber gloves. Do
not use a pencil to trace circuits in the
instrument, Graphite penci/ lead is an extreme
ly good conductor and an accidentally intra·
duced path of this type is sometimes difficult
to locate. To avoid surface contamination of a
printed circuit or switch. clean with a weak
solution of warm water and mild detergent
after repair. Rinse thoroughly with clean water
Section V
and allow it to dry completely before opera·
ting. Do not use alcohol or any other cleaning
solution except mild detergent and water. Do
not apply any commercial moisture sealing
spray
10
the boards; application of these agents
may cause leakage paths.
5·23.
Cover Removal.
5-24. To remove top or bottom cover, remove two
retaining screws from sides of cover; slide cover about 1 /2
inch to rear; lift it off. To replace cover, reverse procedure.
5-25. To remove side cover, remove two retaining screws in
cover and lift it off.
5·26.
Power Supply Voltage Adjustments.
a. Connect dc voltmeter to power supply positive out
put, connector point I . (Refer to Figure 6·4.)
b. Adjust A I R40
(+
30 V ADJUST) for a de voltmeter
indication of
+
30 V
±
0.75 V.
c. Connect dc voltmeter to power supply negative out
put, connector point 2. Adjust A l R41 (
.
25 V ADJUST)
for a dc voltmeter indication of· 25 V
±
0.75 V.
5·27.
FREQUEN CY CALIBRATION PROCEDURES.
a. During this procedure each range is calibrated individ
ually; the capacitors for each range are adjusted at the high
end (FREQUENCY dial at 1 0) and the resistors are
adjusted at the low end (dial at I ). The tuning capacitor
(Cl A, B, C) mechanical adjustments are made on the
XI
K
RANGE, also the two test point voltages (A2TP I , and
A2TP2), which are interactive, are determined on this
range. 111e voltage at A2TPI effects distortion so must not
be set outside the limit of 1 10 ± 10 mV rms.
b. The negative dc voltage at A2TP2 effects the
instrument output flatness; -it is important to ensure that
this voltage is within limits at both the high and low end of
each range ; t.he limits are given in the procedure for each
range. Towards the center of each range the voltage can
vary; the required limits are given in the procedure for each
range but no adjustments can be made. On some instru
ments the voltage may drop below the required limit;
provided that it does not drop below - 200 m V and the
instrument still meets output flatness and distnrtion specifi�
cations then this is acceptable. If it does drop below
�
200 mV or if flatness or distortion is not met then it will
be necessary to change components, by trial and error, on
the A2 Board and repeat the calibration procedure for all
ranges (the most likely components to change would be
A2CR6, A2CR 7, A2Q2 and A2Q3).
Model 652A