25
25A1104 (Rev.1)
emitter dc measurements are not correct replace Q603. If the voltages on Q603 were correct troubleshoot for correct
component placement and values back through the SAW filter FL600.
4. Verify pin 4 of IC610 (2nd LO input) measures the respective 2nd LO frequency at approximately -12dBm. If the 2nd LO
amplitude is low check the values and correct placement of all components leading back through the circuit path to the
collector of Q760. The approximate dc voltage measurements for Q760 are collector=4.6Vdc, base=1.5Vdc and emit-
ter=1Vdc. If any of these dc measurements are not correct verify all components are correctly placed around Q760.
Lastly replace Q760.
5. If the 2nd LO frequency is not present at pin 4 of IC610 verify that the tuning voltage of approximately 2.5Vdc is mea-
sured at TP750 in the 2nd LO VCO. If TP750 measures 0Vdc or near 5Vdc the 2nd LO VCO is not operating properly.
Verify that pins 1 & 16 of IC1 measure approximately 4.5Vdc. If not, troubleshoot back through Q430 and the 5Vdc-sup-
ply circuit. Verify the correct placement and values of all components leading back through the 2nd LO circuit path. If
none of the above is a problem possibly either IC1 is defective or IC300 is not properly programmed or defective.
Audio output troubleshooting from detector (IC610)
Set up:
Apply 1KHz modulation at 33Khz deviation to the external modulation input of the RF generator. Use A-weighting and
30Khz low pass filters for all audio measurements unless specified otherwise. Set the RF generator output to -50dBm.
Low audio out of detector output (pin7 of IC610):
1. First verify that L610 is tuned for maximum audio output at pin7 of IC610. Tune the core of L610 from top to bottom of
the core shaft and back again for max audio output. This is to verify that a double audio peak is not detected. A maxi-
mum audio level at pin 7 of IC610 should only occur at one core rotation position within the shaft of L610. If more than
one audio peak is detected replace L610. If not, measure the 10.7Mhz 2ndIF signal at pin 12 of IC610 using a FET
probe and spectrum analyzer centered at 10.7MHz with a span set to 100KHz (10KHz/division). Observed on the spec-
trum analyzer, the 1Khz modulated 10.7Mhz signal should occupy approximately 6 1/2 divisions across the grid (each
division is equal to 10KHz) which represents +/-33KHz deviation. This measurement should also be the same when
measured at pin14 of IC610.
2. If the proper 10.7Mhz signal response as described above, is observed on the spectrum analyzer and the audio output
at pin7 of IC610 is very low or not present replace IC610. If the proper 10.7Mhz signal response is NOT observed refer
to the RF troubleshooting sections above.
Poor THD measurement out of detector output (pin7 of IC610):
1. First verify that L610 is tuned for maximum audio output at pin7 of IC610 by adjusting L610. Also, verify that the
10.7MHz 2nd IF signal is properly modulated as described in section above.
2. If the THD is still poor verify that the 1st LO has not deviated more than 12
Parts Per Million (PPM). As an example, a
1st LO of 600 MHz (600,000,000 cycles per second) with a tolerance of 12 PPM could vary in frequency by +/- 7.2KHz.
Since there are 600 "one million" units in 600MHz, the 600 units multiplied by the tolerance +/- 12 PPM equals 7.2KHz.
The exact frequency offset could be between 600,005,400Hz and 599,994,600Hz. This offset outside of the 12PPM tol-
erance would cause an increased distortion reading throughout the audio chain. If this is observed replace Y707. If this
is NOT the case replace IC610.
Poor (SINAD) measurement out of detector output (pin7 of IC610):
Note: any external "ON Channel" interference in the frequency of operation under test can cause erratic and poor
SINAD measurements.
1. Verify using a spectrum analyzer that frequency of operation under test has no interference down to -100dBm. Verify
that the unit under test has been properly tuned up by performing the RF alignment procedures.
2. Set the RF generator to -105dBm. Measure the SINAD out of the detector at pin7 of IC610. The SINAD measurement
should measure approximately equal to or greater than 12dB at this level. If not refer to the RF troubleshooting sections
above to isolate where signal degradation is occurring.
TR100 trim does not change level
1.) Confirm audio signal at TP_R.
2.) Check for insufficient solder on the trimmer.
3.) Confirm 4.5Vdc at IC200 pin 14.
4.) Check that R100, R101, R102, C101 and C102 are placed and are the correct values.
Fails Audio at TPE
1.) Check IC200 pin 14 for audio
2.) Check for 9VDC supply at IC200 pin 4
3.) Check placement and values of R100, R110, R111, R112, R115, C111, C112, and C113.
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