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15
Section V
CIRCUIT DESCRIPTIONS
This section details circuitry of the Inovonics Model 540 FM Subcarrier
Monitor/Demod, and also includes procedures for the comparatively few
calibration adjustments. Circuit descriptions refer to four pages of
Schematic Diagrams contained in the Appendix, Section V, Pages 25, 26,
27 and 28.
INTRODUCTION
Navigating the
Schematics
Schematic component reference designations have not been assigned in
as haphazard a manner as they might at first appear. Instead of
annotating the
schematics
in a logical sequence, we have instead chosen
to designate the
components on the circuit board
following their
physical placement, top-to-bottom, left-to-right. We expect this practice
will prove useful when troubleshooting, making it easier to locate the
physical part following analysis of the diagram.
The Model 540 schematic diagram consists of four sheets. Sheets 1 and
2 cover the main circuit board. Sheet 3 shows components on the
separate front-panel circuit assembly, and Sheet 4 is a diagram of the
optional RDS data-recovery plug-in module. Main-board components
begin with the number “1”; i.e.: R1, C1, IC1. Front-panel components
are in the five-hundred series; i.e.: R501, S501. Since it is a separate
circuit altogether, the RDS plug-in has its own, very short series of
numbers, again beginning R1, C1, etc.
The front-panel circuit assembly interconnects with the main board
with short ribbon-cable jumpers. J503 on the front-panel assembly
mates with J3 on the main board, J504 to J4, and J505 to J5.
The “PIC” and
Front-Panel
Logic
The Model 540 employs a device called a “PIC,” or Peripheral Interface
Controller. This is a single-chip microcontroller of limited intelligence,
but ideal for elementary logic and simple control functions. It is
factory-programmed to perform certain bonehead routines, most of
which deal with the front-panel buttons and indicators.
INPUT CONDITIONING AND FIRST TWO CONVERSIONS (Schematic Sheet 1)
Input Stage and
Band-Pass
Filter
The composite/MPX input from the station’s mod-monitor is adjusted
with
INPUT LEVEL
control R14 to calibrate the unit at full carrier
modulation. Buffer IC7A drives a high-pass L/C filter to attenuate
main-channel program frequencies below 50kHz, and following a
second buffer stage, IC7B, an L/C low-pass function attenuates out-of-
band noise above 100kHz. The selected band of frequencies is buffered
