Model 8640B TM 9-4935-601-14-7&P
CHANGE 16 (Cont'd)
Service Sheet 5 (RF ON/OFF Switch Modification):
Change to read as follows:
RF ON/OFF switch S2 (see Service Sheet 15) is normally wired to leave the RF Oscillator on while
switching only the modulator off (see Service Sheet 12). This allows the RF Oscillator to remain warmed
up, the Auxiliary RF Output to remain on, and the counter and phase lock to remain operating. The RF,
however, is not truly "off" but is reduced by an amount equal to the pulse on/off ratio (at least 40 dB down,
and dependent upon OUTPUT LEVEL Vernier setting).
For some applications it may be desirable to modify the switch to turn both the modulator and the RF
Oscillator off. To modify the RF ON/OFF switch, proceed as follows:
1. Remove bottom cover (see Service Sheet F).
2. Remove two nuts that secure Connector Board Assembly A3A4, and remove the board. The board is
located directly behind RANGE switch cam housing.
3. Remove the jumper wire between the two holes labeled "RF OSC ON/OFF INHIBIT".
4. Reinstall board and bottom cover.
5. Check RF ON/OFF operation by observing the counter or Auxiliary RF Output, which should stop
operating with RF ON/OFF switched to OFF.
Service Sheet 15 (Principles of Operation):
Change the RF On/Off Switch paragraph to read as follows:
RF ON/OFF switch S2 normally turns off only the amplitude modulator (by switching off the modulator
drive current).
If it is desirable to switch both the modulator and the RF Oscillator off (by also interrupting the +20V supply
to the oscillator) the RF ON/OFF function may be easily modified. See Service Sheet 5 for modification
instructions.
CHANGE 17
Page 6-23, Table 6-3:
Change A10A2U14 to 1820-1354.
Service Sheet 11 (schematic):
Change part number for A10A2U14 to 1820-1354.
7-42
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