
Model 8640B
TM 9-4935-601-14-7&P
ADJUSTMENTS
5-27. VARIABLE-FREQUENCY MODULATION OSCILLATOR ADJUSTMENT (OPTION 001) (Cont'd)
10. Set MODULATION FREQUENCY vernier to 20. The counter should read 2.00 ± 0.01 kHz and voltage level at
A11TP4 should be within 0.01 Vrms of level recorded in step 8. Repeat steps 8 and 9 until voltage level and
frequency are correct.
11. Monitor voltage at AllTP4 while using MODULATION FREQUENCY switch and vernier to tune oscillator from 2
kHz to 20 kHz. The voltage level at 2 kHz (on the X100 range) should be 1.6 ± 0.05 Vrms and level at all other
frequencies should be within 0.03 Vrms of level at 2 kHz.
At 2 kHz: 1.55---------1.65 Vrms
All frequencies:---------+0.03 Vrms
12. If level at A11TP4 is too high, reduce A11R28 by one standard value (10%); if level is too low, increase A11R28
by one standard value. Then repeat steps 8 through 11.
13. Set MODULATION FREQUENCY range switch to X3 K and vernier to 200 and adjust HIGH FREQ capacitor
A11C9 for a counter reading of 600 ± 3 kHz.
597------------------------------603 kHz
14. Connect DVM to AM OUT test point, A11TP5. Set MODULATION FREQUENCY range switch to X100 and the
vernier to 20. Adjust AM-FM adjustment, A11R35, for 840 ± 10 m Vrms at A11TP5.
830----------------------------850 m Vrms
15. Connect DVM to the FM OUT test point, AllTP3. It should read within 5 m Vrms of reading in step 14.
-----------------------±5 mVrms
16. Use MODULATION FREQUENCY range switch and vernier to tune oscillator across each range (except 400 and
1000 Hz FIXED FREQ). Monitor voltage level at AllTP3; the DVM should read within 10 mVrms of level noted at
20 on vernier dial from 200 Hz to 100 kHz. It should read within 20 mVrms of level noted at 20 on vernier dial
from 20 Hz to 600 kHz.
200 Hz to 100 kHz:------------------±10 mVrms
20 Hz to 600 kHz:-----------------±20 mVrms
17. Set MODULATION FREQUENCY range switch to X3 K and vernier to 20. Connect DVM to AM OUTPUT jack
through 600 ohm feedthrough. Adjust AUDIO LEVEL adjustment, A11R40, for 3.00 ± 0.03 Vrms at the jack.
2.97------------------3.03 Vrms
18. Set AM to OFF and FM to INT. Connect DVM to FM OUTPUT jack through the 600 ohm feedthrough. The DVM
should read 3.0 ± 0.06 Vrms.
19. Check that AUDIO OUTPUT LEVEL control indicates 3V when turned fully cw. If it does not, loosen its
setscrews and adjust it so that it does; then tighten setscrews.
See Tables 7-1 and 7-2 for backdating.
5-11
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