TROUBLESHOOTING AND REPAIR
RF LEVEL/AM
premodulator operation and switched filter operation controlling that band as shown
in Table 6D-1. If the problem is not frequency dependent and if the level is accurate
above +7 dBm but inaccurate below +7 dBm, the Attenuator/ RPP Assembly is likely
at fault.
If the level problem is not in a particular frequency band, it is advisable to troubleshoot
at a low frequency where an oscilloscope is useful. Put the instrument in a known state
by selecting SPCL01, set the frequency to 88 MHz, and set the amplitude to +13 dBm.
The voltage at TP8 on the modulation control PCA should be +1.3 ± .5V DC. If this
voltage is correct, the problem is localized to the Output PCA following the detector
diode CR20 or the Attenuator/RPP assembly. The appropriate signal levels following
this point are:
R121
Q7 base
Q7 collector
CR23 cathode
Q9 base
Q9 collector
Q16 collector
0.9V
p-p
0.4V
p-p
0.85V p-p
0.82V p-p
0.6V
p-p
1.5V
p-p
3.0V
p-p
At 88
MHz.
These voltages are approximate and are as measured with a 10 megohms, 8 pF,
oscilloscope probe using a ground connection made at the probe tip with less than 1
inch of lead.
Table 6D-1. Band, Filter, and Frequency Programming Data
FREQUENCY (MHz)
.01- 14.999999
15- 21.999999
22- 31.999999
32- 46.999999
47- 63.999999
64- 127.999999
128- 179.999999
180- 255.999999
256- 349.999999
350- 511.999999
512- 624.999999
625- 729.999999
730-1056.000000
FFFFGGGG1 234MHH
1234TTTTHHHHIAE
522761 26AAAADOT
23464852FFFF C
2274065 T
000010000100101
100000000001100
010000000001100
001000000010100
000100000010100
000010000100100
000010001000100
000011001000100
000011101000110
000011101000100
000011101000000
000011111000000
000011111000010
6D-7
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