TROUBLESHOOTING AND REPAIR
FREQUENCY SYNTHESIS
A proper signal at TP5, with TP4 shorted to ground, indicates that the RF circuits are
probably not faulty, and that the problem is in the audio section. The loop
amplifier/acquisition oscillator can be checked by shorting TP5 to ground and
measuring the waveform at TP1 with an oscilloscope, which should be a sine wave of
about 800 Hz and 14V pk-pk level. Next, measure the TP1 waveform with both TP5
and TP8 shorted to ground. The waveform should be a sine wave of about 13.6 kHz
and 8V pk-pk level. Failure of this test indicates a problem somewhere between TP5
and
TP1.
The programmable attenuator can be checked for proper operation as follows. First,
connect test points 7 and 9 to ground, and program the UUT to SPCL 943 (DACs set
to full scale). Measure the resistance from TP4 to ground. The reading should be about
240 ohms. Next, program the UUT to SPCL 941 (DACs set to zero). The resistance
reading should now be about 29.5 ohms. Failure of this test indicates a problem
somewhere between TP1 and TP4.
Status Code 245 indicates an unleveled condition in the leveling loop that controls the
signal amplitude at the RF input of mixer U1, and is triggered when the modulator
control voltage at Jl6 exceeds about 10V. This fault condition can be caused by either a
level problem in the RF path including the Sum Loop VCO and sum loop circuits
between J9 and CR1, or by improper signal frequencies within the sum loop.
Table 6C-11 presents the nominal characteristics of the signals at the various test points
on the A12 Sum Loop PCA. The normal range of the signals, along with specific values
for the instrument diagnostic state, SPCL 909, are listed.
SUM LOOP ASSEMBLY ADJUSTMENTS 6C-33.
The following procedures cover the five potentiometer adjustments on the A12 Sum
Loop PCA listed below:
R112, Steering Level
R121, Buffer Gain Match
R116, FM Null
R167, Loop Gain
R132, Acquisition Oscillator Level
NOTE
These adjustments are not routine and are required only when associated
components have been replaced or when the adjustment has been
changed.
Steering Level Adjustment, R112 6C-34.
TEST EQUIPMENT:
•
DVM
REMARKS:
The Steering Level Adjustment is normally required only when U103 or any associated
components are replaced or when the adjustment has shifted.
6C-41
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