Model 8640B TM 9-4935-601-14-7&P
ADJUSTMENTS
5-27. VARIABLE-FREQUENCY MODULATION OSCILLATOR ADJUSTMENT (OPTION 001) (Cont'd)
PROCEDURE:
1. Check that modulation oscillator is installed with all of its covers in place.
2. If the knobs have been removed, turn MODULATION FREQUENCY vernier shaft full cw. Install frequency dial
on vernier shaft so that the gears mesh and number 200 on the dial is 10 to 20° to the left (ccw) of the cursor.
Turn MODULATION FREQUENCY switch shaft full ccw and install range knob on switch shaft so that 400 Hz
FIXED FREQUENCY position is at the cursor (top). Install vernier knob. (The knobs should not touch each
other.)
3. Turn trim capacitors A11C2 and C3 full cw.
NOTE
Turning C2 ccw decreases the output voltage while raising the frequency.
Turning C3 ccw increases the output voltage while raising the frequency.
4. Set Signal Generator's controls as follows:
AM................................................................................................................. INT
MODULATION FREQUENCY Switch...........................................................X100
MODULATION FREQUENCY Vernier................................................... Full ccw
FM ................................................................................................................OFF
AUDIO OUTPUT LEVEL...........................................................................Full cw
5. Connect DVM to OSC OUT test point, A11TP4. The DVM should read 1.6 ± 0.3 Vrms.
1.3------------------------------1.9 Vrms
6. Connect frequency counter to AM OUTPUT jack. The counter should read 1.8 ± 0.2 kHz.
1.6-----------------------------2.0 kHz
7. Set MODULATION FREQUENCY vernier full cw and adjust trim capacitors, A11C2 and C3, until voltage level at
Al11TP4 is within 0.1 Vrms of level read in step 5 and frequency at AM OUTPUT is 21 ± 1 kHz.
8. Set MODULATION FREQUENCY vernier for a frequency counter reading of 2.0 ± 0.01 kHz. Loosen setscrews
in gear that meshes with frequency dial gear (vernier). Rotate dial gear so that dial reads 20 (at the cursor) and
tighten setscrews in gear. The frequency counter should read 2.0 ± 0.01 kHz when dial reads 20 at the cursor.
Record voltage level at A11TP4.
-------Vrms
9. Set MODULATION FREQUENCY vernier to 200. Adjust A11C2 and C3 until voltage level at A11TP4 is within
0.01 Vrms of level recorded in step 8 and frequency is 20.0 ± 0.1 kHz.
5-10
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