TROUBLESHOOTING AND REPAIR
FREQUENCY AND PHASE MODULATION
MODULATION CONTROL CIRCUIT DESCRIPTION 6E-8.
The following description applies only to the FM modulation circuitry on the
Modulation Control PCA (A11), which is covered in three parts:
• FM input voltage processing.
• FM STEER and SUN STEER voltage generation.
• FM control signals generation.
FM Input Voltage Processing 6E-9.
The circuits in the list below serve to select and amplify the external FM input signal
and the internal mod oscillator signal from a level of 1V AC-peak to a level of 4V
AC-peak at the top of each FM range (4 MHz, 1 MHz, etc.), and to provide a vernier
output within each range as the multiplying DAC is programmed by the controller.
• Op Amp U27
• Associated input resistors, capacitors, and CMOS switches, U39
• DAC p/o U34 and Op Amp U9A
• Inverter/amp U9B
The selection of combinations of EXTAC FM or EXTDC FM and INT FM inputs is
made with the CMOS switches in U39 with its associated resistors and capacitors at the
input of Op-Amp U27. The resistor R82 sets the gain so that a 1 V AC-peak signal is
amplified to 4V AC-peak. The FM DEV DAC p/o U34-l,2 is set to 3600 counts (out of
4096 at full scale). The FM DEV DAC and op amp U9A produces 4V AC-peak to the
inverter/amplifier circuit U9B, which in conjunction with CMOS switch Q6 either
amplifies directly or inverts the signal to produce the proper output polarity. This
accommodates the instrument action of either over or under programming at the sum
loop. The multiplying FM DEV DAC U34 (p/o) is under controller operation to
produce a vernier output within each range, or over-range in fixed range, or variation
of nominal reference of 3600 counts (out of 4096) for closed-case calibration.
The comparators U16C and U16D with associated resistors serve to trigger one-shots
U26A and U26B to provide information that the applied external level has a peak
amplitude centered around 1V AC-peak. The controller responds to deviation from 1V
AC-peak to alert the operator with front panel indicators for a "HI" or "LO"
indication.
FM Steer Voltage Generation 6E-10.
The FM Steer signal is derived in DAC, p/o U32, and op amp U36B, and ranges
between 0 and 10.2 V DC (nominally 5.1V DC). The variable resistor R101 is used to
adjust this range. The level is under control of the controller for zeroing the frequency
offset in DCFM.
FM Control Signals Generation 6E-11.
The control signals for the FM OSC PCB (A14) are sent by the controller and latched
in U35. The signals are:
• Three Range switches: FMRN2H, FMRN1H, and FMRN0H.
• Four controls: DCFMH, LOWFMH, PMODH, and HRPMH.
6E-10
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