HP 654A Operating And Service Manual page 39

Model 654A Section V
null. If it is not change the value of A4C1* to
bring the 654A level within tolerance (decrease
A4C1* to increase the output level and vice-versa).
c. Sweep the 654A frequency slowly down to 1MHz
and observe the dc null voltmeter indication which
should remain within +/-AE of null. If it does not
repeat step b of this paragraph but select a value
for A4C1* which allows the flatness to be met
across the 1MHz to 10MHz frequency range.
d. Check the 654A flatness over the 10Hz to 1MHz
ranges as before.
5-68. 135, 150 and 600 BAL FLATNESS.
5-69. Check the flatness for the balanced impedances by
performing Paragraphs 5-18, 5-19 and 5-20. There are no
adjustments to be made for these ranges.
5-70. TROUBLESHOOTING THE 654A
5-71. This section contains information and procedures
designed to aid in the process of isolating malfunctions.
Troubleshooting should be undertaken only after it has
been determined that the malfunction cannot be corrected
by performing the adjustment and calibration procedures.
5-72. When a malfunction occurs first ensure that the
trouble is not caused by conditions external to the
instrument; then make the front panel checks described in
Paragraph 5-74 before proceeding to the Troubleshooting
Tree.
5-73. The Troubleshooting Tree (Figure 5-18) illustrates a
systematic method of locating a faulty circuit. Additional
checks (including visual) and measurements will be required
to isolate the faulty component.
5 74. FRONT PANEL CHECKS.
a. Check that the LINE ON lamp is lit; if it is not,
check the setting of the 115/230V slide switch,
check the fuse (F1) and if necessary check the
primary circuit of the power transformer (Tl on
schematic No. 4, Figure 7-5).
b. In this procedure the 654A will be swept across its
frequency range while the following points are
monitored:
A. the COUNTER OUTPUT
B. the 654A meter indication
C. the front panel output connectors.
By applying the observation made to Table 5-5 it
should be possible to localize any problems to a
particular area in the instrument (refer also to the
Block Diagram, Figure 7-1).
1) Set the 654A controls as follows:
AMPLITUDE fully clockwise
OUTPUT LEVEL dBm + 10,0
IMPEDANCE 50 UNBAL
2) Connect an oscilloscope or ac voltmeter,
through a 50 ohm feedthrough termination, to
the 654A rear panel COUNTER OUTPUT.
3) Connect an oscilloscope or ac voltmeter,
through a 50 ohm feedthrough termination, to
the front panel UNBAL output connection.
4) Sweep the 654A FREQUENCY dial slowly
from 1 to 10 ( for all positions of the
FREQUENCY RANGE switch) while observing
the three monitoring points A, B and C.
5) Select from the left hand column of Table 5-5
the ABC combination which corresponds with
the observations made in step 4). The center
column of Table 5-5 gives the most likely
trouble atea for each combination and the right
hand column indicates the next step to make in
troubleshooting.
5 75. TROUBLESHOOTING TREE.
5-76. To use the tree start at 0, read stepOof Paragraph
5-77 and make the required check; the next step then
depends upon whether the first check was a PASS or FAIL.
Several of the FAIL branches split into sub-branches, take
the sub-branch which best fits the observations made. At
each step of the tree it is important to read the appropriate
step of Paragraph 5-77 as the tree itself does not give
sufficiently detailed information, in most cases, for the
check to be made. Refer also to the Block Diagram and
Schematics of Section VII when using the troubleshooting
tree. If you complete the tree and still have failed to
localize the problem area then refer to Paragraph 5-78 for
additional information.
5-77. This paragraph provides information for each step of
the troubleshooting tree.
NOTE
Make the Front panel checks, described
in Paragraph 5-74, if you have not already
done so.
Check with an oscilloscope at A2 Pin 4: there
should be a sine wave of between 5.5V and 7V
p-p. Sweep the FREQUENCY dial from 1 to 10
for all positions of the range switch. If the signal
appears, even momentarily, then the problem is
probably frequency calibration.
5-19