550 Chapter10
Synthesizer Section
Unlocked Fractional N PLL
7. VCO clamp troubleshooting: Q131, Q132 and the associated
components should limit the tune voltage at R240 to about −3.3 V to
+11 V (±11 V for 08560-60062 and below). If the integrator (its
output voltage is on TP13) tries to produce a voltage outside this
range, excess current is shunted through CR131 and Q131 for
positive excursions or CR132 and Q132 for negative excursions. The
base of Q131 should be at about +9.60 V, and the base of Q132
should be at about −2.09 V for proper operation. If troubleshooting
an earlier A14 frequency control assembly (08560-60062 and below),
the bases of Q131 and Q132 should be at about ±9.6 V for proper
operation.
8. VCO troubleshooting: Check the dc biases in the VCO function block.
The bias voltages, for some points in the VCO, are indicated in
Figure 10-8 on page 551 (or Figure 10-8a for earlier instruments).
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