
Model 8640B
TM 9-4935-601-14-7&P
SERVICE SHEET 16
PRINCIPLES OF OPERATION
Vernier Attenuator (A1)
OUTPUT LEVEL vernier R1 attenuates the AGC reference voltage and the superimposed AM modulation signal
and drives the AGC Amplifier. The potentiometer has a precision 18 dB log taper. Resistor AlA1Rl limits the low
resistance end of the potentiometer. Resistor A1AlR2 is switched into the AGC amplifier input line by S1AR in all
but the highest OUTPUT LEVEL range. With R2 switched out, the AGC reference is effectively increased by 10
dB (a factor of 3.16) and the RF output is increased by 10 dB.
Meter Attenuator and Odd Range Code (A1)
The output of Meter Amplifier A26A4U1A (Service Sheet 12) is the RF LEVEL meter voltage. Resistor A1AlR3
attenuates the amplifier output by 1/3.5 in the highest or 16th OUTPUT LEVEL range. Resistor A1AlR4
attenuates the output by 1/1.1 on ranges 8 to 15. Resistors AlAiR5 and R6 adjust the meter output attenuation to
compensate the meter for cumulative errors in the output attenuator on the high attenuation ranges. Switching is
done on SIB. Switch S1AF gives a closure to ground on all odd numbered ranges for use by the lamp logic
circuits on A4 (Service Sheet 17).
TROUBLESHOOTING
Troubleshoot by checking switches, connector, and resistors for proper contact and resistance.
AM Interconnections and RF ON/OFF Switch (A9,A26)
SERVICE SHEET 15
8-50
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