
TM 9-4935-601-14-7&P
AM Preamplifier (A24, A36A2)
SERVICE SHEET 14
Model 8640B
SERVICE SHEET 15
PRINCIPLES OF OPERATION
RF On/Off Switch
RF ON/OFF switch S2 normally turns off both the RF Oscillator (by interrupting the +20V supply to it)
and the amplitude modulator (by switching off the modulator drive current). If it is desired to leave
the oscillator running when RF is OFF (e.g., to keep the AUX RF OUT on and the counter operating
or the phase locked, or to minimize turn-on drift), the RF ON/OFF function may be easily modified.
See Service Sheet 5 for modification instructions. Some RF, however, will then be present at the
output when RF is OFF.
TROUBLESHOOTING
Troubleshoot by checking switches and connectors for proper contact.
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