TROUBLESHOOTING AND REPAIR
RF LEVEL/AM
The amplitude modulator on the Output PCA consists of PIN diodes CR27 through
CR33 and associated components, and the modulator receives the 15 to 1056-MHz
signal from the Premodulator PCA through W1. The modulator is a voltage-controlled
variable attenuator that provides AM and output level control. Modulator control
voltage is determined by the leveling-loop circuitry. The leveling loop is described later
in this section.
Q5, U8, Q1, and associated components follow the modulator in the signal path and
form a three-stage, 17-dB gain, 15 to 1056-MHz amplifier. This gain stage is followed
by a 4-band switched filter to remove harmonics in the 15 to 27-MHz, 27 to 32-MHz,
32 to 47-MHz, and 47 to 64-MHz frequency bands. For frequencies below 64 MHz,
CR4 is on to direct these signals to the filter bank. The appropriate filter is selected by
CR6-CR13, and the result is returned to the signal path by CR16. Signals above 64
MHz proceed to this point via a high frequency, switched filter, (CR14, CR15, C38,
C39, and circuit traces). This is a LPF for frequencies below 625 MHz to further reduce
harmonics. The 64 to 1056-MHz signal is recombined with the 15 to 64-MHz signal at
C180. This signal then drives a 3-dB power splitter that consists of resistors R63, R30,
R31, and R32 and the associated transmission lines.
One power-splitter output drives the leveling loop detector diode CR20. The other
output goes to a 5.5-dB pad followed by a 7-dB amplifier, Q7 and associated
components. The HET band switch follows the buffer amp and consists of PIN diodes
CR18, CR21-24 and biasing components. In the 15 to 1056-MHz position, the signal
passes through diodes CR21 through CR24 to the pulse modulator, U5. A buffer
amplifier follows the pulse modulator and consists of Q9 and associated components.
This provides 8.5-dB of gain. This amplifier is then followed by the 6-dB final
amplifier, which is composed of Q16 and associated components. The final amplifier
provides at least +20 dBm output at low distortion.
For HET band operation (0.01 to 15 MHz), the signal from the power splitter is routed
through CR18 to the HET band circuitry. The RF signal passes through a 95-MHz
LPF, then an adjustable attenuator (R70 through R75), and then to the RF port of U3
(a double-balanced mixer). The signal frequency at the mixer RF port varies from
80.01 to 95 MHz. The 80-MHz local oscillator (LO) signal for the mixer comes from
the A2 Coarse Loop PCA through J3 and is amplified by U1. This signal is then
amplified by class C amplifier Q10, which is followed by a band-pass filter and 3-dB
pad to provide +18 dBm at the mixer LO port.
The mixer 0.01 to 15-MHz output signal is passed through a diplexing low-pass filter
(C99 through C104, R76) that suppresses unwanted mixer spurious products while
maintaining a 50-ohm load at the mixer IF port. The filtered IF signal is amplified by a
two-stage IF amplifier Q13, Q14, and associated components.
The IF amplifier gain is nominally 20 dB. The signal then is filtered to remove
remaining LO and RF signals before being recombined at CR24 with the main signal
path. The +5V power supply for the LO amplifier is switched off by Q3 so that spurious
signals are not introduced when the instrument is operating in the 15 to 1056-MHz
bands.
6D-4
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