Model 8640B TM 9-4935-601-14-7&P
SERVICE SHEET 5
PRINCIPLES OF OPERATION
General
The A3 RF Oscillator Assembly contains the main RF oscillator, a varactor
assembly, and two buffer amplifiers. The output of the RF oscillator is applied to the
RF OUT port through the A10 Divider/Filter Assembly, the A26 AM/AGC and RF
Amplifier Assembly, and the A1A1 and A19A1 Output Attenuators (see block
diagrams for schematic locations).
Oscillator Loop
The 230 and 550 MHz RF oscillator is a single transistor, cavity-tuned oscillator.
Integral with the oscillator assembly is a Varactor Head Assembly which provides
electrical tuning for FM. The high-frequency transistor is in a common-base
configuration. The emitter and collector loops couple into the cavity and to each
other to provide the positive feedback necessary for oscillation.
Tunable Cavity
The cavity is a foreshortened type which is essentially a length of coaxial
transmission line with a short at one end and a capacitive load at the other. The
shorted transmission line is less than 1/4 wavelength long at the frequency of
oscillation and its impedance is inductive. The cavity resonates at the frequency at
which the inductive reactance of the transmission line equals the capacitive
reactance of the load capacitor. The resonant frequency is varied by changing the
length of the cavity (a secondary effect) and by changing the load capacitance. The
varactor diodes are in parallel with the main load capacitance. The cavity is
mechanically fine tuned by protruding a small metal slug into the cavity. Signal is
coupled out of the cavity into two buffer amplifiers by loops which protrude into the
cavity.
Buffer Amplifiers (A3A1A2, and A3A1A3)
Operation of the Divider/Filter Buffer Amplifier and the Counter Buffer Amplifier is
essentially the same. The Divider/Filter Buffer Amplifier drives the Modulator
Preamplifier. The Counter Buffer Amplifier drives the counter input. The main
function of these amplifiers, however, is to isolate the RF Oscillator from external
circuits.
Transistors Q1 and Q2 are two common-emitter amplifier stages. The base of Q1 is
dc-grounded through the coupling loop T1. Emitter current is established by
resistors R3 and R4; capacitor C2 ac-bypasses R4. The gain of Q1 is set by R1, R2,
R3, and R6 (also C8, Divider/Filter Buffer Amplifier only). The collector of Q1 is
Counter/Lock Circuits Troubleshooting
SERVICE SHEET 4
SERVICE SHEET 5 (Cont'd)
ac-coupled to the base of Q2 by capacitor C4. Operation of transistor Q2 is similar to Q1. In the Counter Buffer
Amplifier only, resistors R10, R11, and R12 form a 10 dB pad to reduce the output level and increase the output-to-
input reverse isolation.
The amplifier board is secured through slotted holes by two screws. By loosening the screws and sliding the board,
the amount of coupling loop protruding into the cavity can be altered and the amplifier output level varied.
TROUBLESHOOTING
General
The oscillator transistor, buffer amplifiers, and external circuits of the A3 RF Oscillator Assembly may be repaired to
the component level. However, if a problem has been isolated to components in the RF Oscillator cavity, the
oscillator assembly should be returned to Hewlett-Packard for repair. Do not attempt to disassemble it because
proper reassembly depends upon specialized skills and procedures.
Buffer Amplifiers
Refer to Service Sheet B for access to the buffer amplifier assemblies. Check dc bias voltages to reveal a faulty
component. See Section V for adjustment.
RF ON/OFF Switch Modification
The RF ON/OFF Switch function may be wired to:
a. switch off both the RF Oscillator and Modulator leaving the RF output completely off but requiring a
stabilization period after turn on; or
b. switch off only the Modulator leaving the RF Oscillator on and warmed up, the Auxiliary RF Output on, and
the counter and phase lock operating. In this case, however, the RF is not truly "off" but is reduced by an amount
equal to the pulse on/off ratio (at least 40 dB down and dependent on OUTPUT LEVEL vernier setting).
Either configuration can be easily altered to the other as follows:
a. Remove bottom cover (see Service Sheet G).
b. Remove two nuts that secure A3A4 Connector Board Assembly, and remove board. The board is located
directly behind the Range Switch cam housing.
c. To modify the circuitry to leave the RF Oscillator on at all times, add a jumper wire between the two holes
labeled "RF OSC ON/OFF INHIBIT" as shown overleaf. To modify the circuitry so the RF Oscillator is switched off,
remove the existing jumper wire.
d. Reinstall board and bottom cover.
e. Check RF ON/OFF operation by observing counter or Auxiliary RF Output signal.
8-26
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