TROUBLESHOOTING AND REPAIR
RF LEVEL/AM
FM Gain Adjustment, R82, on Mod Control PCA 6D-24.
See "Alignment of FM PCA" in Section 6E.
FM steer Gain, R101 on Mod Control PCA 6D-25.
See "Alignment of FM PCA" in Section 6E.
FM INV Balance, R102 on Mod Control PCA 6D-26.
See "Alignment of FM PCA" in Section 6E.
ATTENUATOR/REVERSE POWER PROTECTION (RPP) 6D-27.
The A20 Attenuator/RPP Assembly consists of the A21 Attenuator/RPP PCA, the
A7 Relay Driver PCA, and a metal housing. The Attenuator/ RPP PCA is mounted
inside the housing and the Relay Driver is attached on top of the housing. This
assembly is mounted to the Output Module, opposite the Output PCA. The output
signal of the Output PCA (at P1) is the input to the Attenuator/RPP PCA (at J1).
The Attenuator section of the Attenuator/ RPP PCA provides an attenuation range of
0 to 138 dB in 6-dB steps. This is accomplished by seven, independently cascaded,
50-ohm attenuation sections (K1 through K7). There are one 6-dB, one 12-dB, and five
24-dB sections. Each section consists of a DPDT relay and a pi attenuator pad. One
relay position (when DC power is applied to the relay), provides a low-loss through
path for the RF signal. The other position (no DC power applied to the relay), inserts
the attenuator into the RF signal path. Control of the sections is from the Controller
PCA through the Relay Driver PCA. Attenuation correction data for each attenuator
is stored in the compensation memory on the Controller PCA. Necessary correction is
applied via the leveling loop control voltage.
The RPP section of the Attenuator/RPP PCA protects the attenuator and the output
amplifier from excess applied DC voltage or RF power. C6 and C7 provide a DC
voltage block. K8, when in the protect position (no DC power applied to the relay),
protects against long duration excess RF power. The detector diode (CR1) senses
excess RF power and trips the latching comparator circuit (U1-A) on the Relay Driver
PCA. This change of state of U1-A passes through U1-D, Q8, and Q9 to remove the
DC power from K8. This puts K8 into the protect state. Diodes CR2 through CR9 on
the Attenuator/RPP PCA form an RF limiter circuit. This provides protection against
short duration excess power events or until K8 can change state. This may take up to 4
ms. When the latching comparator (U1-A), on the Relay Driver PCA, changes to the
tripped state, the positive voltage on U1-A pin 1 is applied to the inverting input of
U1-C causing the output of U1-C to go low (approximately 0V DC). This signal
(RPTRPL) informs the Controller that the RPP has been tripped which causes the
instrument to go into the RF OFF state and flashes the STATUS light. Diodes CR8
and 9 provide bias voltage for the limiter diodes to set the limiting threshold. The
excess power detection threshold (for CR1 on the Attenuator/RPP PCA) is set by the
resistor network at the input of U1-A.
6D-20
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