Model 8901B Service
NOTE
If the display
is
not zero,
it
is
important that all other tests be checked
(N=l
through N=5). Some
LO
faults cause more than one test to fail,
For
example,
if
the
HF
VC0+8 output fails, the following tests will fail:
N=l, Test
4,
N=2, Tests
5,
6,
and
7,
N=3, Tests
9
and 10,
N=4, Tests
17
and 18.
This
is
because a frequency measurement of the
LO
is made in these
particular tests.
If
the +40V Supply fails, the following tests will fail:
N=2, Tests 5 and
6,
N=3, Tests
9
and 10,
N=4, Tests 17 and
18.
Begin troubleshooting at the lowest-numbered test.
This test
is
run automatically at power-up. Results are not displayed, but
a pass-fail indication is made on internal
LEDs.
(See
Power-up Check,
paragraph
8-9.)
Counter Tests
N=l
Test
1.
Undefined.
N=l
Test
2.
Time Base Test. See Service Sheets
22
and
23.
Tests the 6.25 kHz TTL time base signal to determine
if
it
toggles within a reasonable
length of time. The Controller looks at A11U15D’s output
for
260
ps.
At
least one transition
(high-to-low
or
low-to-high) of the clock should occur during this time. If no clock transition
occurs,
2
will be displayed in digit position
3.
However,
if
a transition
is
detected, a second
(verification) check is made by the Controller.
N=l Test
3.
Counter Test. See Service Sheet
23.
Counts the selected Time Base, which should be exactly
1000000.
If
the result
is
not
1000000,
3
will be displayed in digit position
6.
N=l Test
4.
HF VCO and Divider Output. See Service Sheets
17
and
18.
Connects the DAC output to the HF VCO and counts the
LO
frequency
to
determine if
it
is
within certain limits. The Controller turns
off
the Sweep-Up and Sweep-Down Current
Sources and LF VCXO tune filter, allowing the HF VCO to free run. The Controller then
outputs the approximate center frequency code to the tuning DAC and checks if the
HF
VCO output is between 250 and
800
MHz. If the frequency
is
not within these limits,
4
will
be displayed in digit position
8.
DAC
and
HF
VCO
Range Tests
N=2
Test 5. HF VCO Top of Range Test. See Service Sheet
18.
Tests the DAC’s ability to drive the
HF
VCO to the top of
its
frequency range. The DAC is
programmed to output the highest tune voltage.
If
the HF VCO does not tune to between
655 and
800
MHz,
5
will be displayed in digit position
2.
N=2
Test 6.
HF
VCO Bottom of Range Test. See Service Sheet 18.
Tests the DAC’s ability
to
drive the
HF
VCO to the bottom of its frequency range. The DAC
is
programmed by the Controller to output the lowest tune voltage.
If
the HF VCO does not
tune
to
between 280 and
310
MHz,
6
will be displayed in digit position
4.
Special Functions 8B-9
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