TROUBLESHOOTING AND REPAIR
RF LEVEL/AM
Level Control 6D-6.
The instrument output level is set by the level-control circuit. Inputs to this audio
signal processing circuit are the internal and external modulation signals, a DC
reference voltage, and the digital control commands. The circuit output is the leveling
loop control voltage that provides vernier level control and amplitude modulation
control of the signal generator output. Digitally encoded level, modulation depth, and
temperature-compensation information are provided by the A11 Controller PCA.
External AM signals are cabled to J13, pin 1 on the Modulation Control PCA. This
point is monitored by an AC peak detecting voltmeter composed of comparator U16
and U17 and associated parts. A similar circuit is present to monitor external FM
signals, and they share a common reference circuit, R70 through R74 and CR12. These
components provide voltages of 1.02 at U16 pin 8 and .98V at U16 pin 10. When these
voltages are exceeded these comparators trip and trigger monostable multivibrators
U17 A and U17B to provide indication to the controller that the peak AC voltage is not
1V.
Analog switch U5 selects the internal or external DC- or AC-coupled modulating
signal or selects no modulation. The selected modulation signal is buffered by U21 and
is applied to pin 19 of U6, a multiplying 12-bit DAC. U6, with amplifier U8-A, acts as a
digitally programmed variable attenuator and controls AM depth. The AM signal (at
TP6) is summed by op-amp U8-B with a DC-reference current provided by CR7. The
output at U8-B pin 8 is called the 1+AM signal. This signal, with additional scaling, is
the basis for level and AM depth. AM depth adjustment is provided by potentiometer
R10 and AM DAC offset by R8.
The instrument RF output amplitude is temperature compensated in a frequency-
dependent manner. The 1+AM signal is applied to the reference input, pin 15, of an
8-bit multiplying DAC, U11, and to one input of summing op-amp U8-D. The DAC
output, at U8-C pin 1, is the 1+AM signal scaled by a factor that is generated from
stored constants. This voltage is applied to a resistor/ thermistor network that includes
R15, R16, R18, and RT17. This signal is also applied to summing op-amp U8-D. The
voltage at U8-D pin 14 is the temperature compensated 1+AM signal.
This signal is applied to the reference input of level DAC U12. This 14-bit multiplying
DAC, with op-amp U4, generates the leveling loop control voltage (at TP7). The
leveling loop control voltage is the temperature compensated 1+AM signal multiplied
by a factor proportional to the 14-bit level control number provided by the Controller
PCA. The signal generator RF output level adjustment is provided by potentiometer
R20, and DAC offset voltage adjustment is provided by potentiometer R23.
RF LEVEL TROUBLESHOOTING 6D-7.
If the signal generator level is inaccurate or an unleveled condition exists, the Output
assembly (A8 + A10 + A11), or the A20 Attenuator/RPP Assembly is probably at
fault. If an unleveled condition exists, the problem should be in the RF circuitry prior
to the detector, the detector circuitry, or the DC part of the leveling loop circuitry. Go
to the heading "Unleveled Condition" later in Section 6D.
If there is no unleveled condition, the problem is likely in the circuitry following the
detector which includes the buffer amp Q7, the heterodyne circuit, the pulse
modulator, the output amplifiers Q9 and Q16, and the A20 Attenuator/RPP
Assembly. If the level problem exists only below 15 MHz, troubleshoot the heterodyne
circuitry. If the level problem exists only in a specific frequency band, check
6D-6
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