Instruction manual

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26
Section V
CIRCUIT DESCRIPTIONS
This section details the circuitry of the Inovonics Model 708 FM Stereo
Generator. Circuit descriptions refer to the three pages of Schematic
Diagrams contained in the Appendix, Section VI, Pages 38, 39 and 40.
Component
Annotation
The schematics may appear to be annotated in a somewhat haphazard
manner, insofar as component reference designations are concerned.
Rather than annotate the schematic in a logical sequence, we have
instead chosen to designate the components on the circuit board, top-to-
bottom, left-to-right, following the physical placement of the parts in
their vertical rows. It is our expectation that this practice will aid any
required troubleshooting, making it easier to locate the physical part or
test point from an analysis of the circuit diagram.
Left and right program channel circuits are identical, so only the left
channel will be described here. A skilled technician will be able to
extrapolate for right channel operation with little effort.
INPUT CIRCUITRY
Line Input
Balancing
IC42B is an “active-balancing” stage for the left channel program line
input, affording rejection of unwanted common-mode signals. When
jumper JMP3 connects R202 into the network (H), the Line Input
accepts “High” program levels between +5dBu and +20dBu. With
R202 jumpered out (L), “Low” program levels from –10dBu to +5dBu
are accommodated. Input range setup is described on Page 9.
The left channel INPUT GAIN control, R198, is in the feedback path of
voltage-amplifier stage IC43B. This affords the 15dB variable gain
adjustment within each jumpered range.
Pre-Emphasis IC45B provides the additional gain required for transmission pre-
emphasis. C77 or C78, jumper-selected to parallel the input resistor to
this stage impart the 50µs or 75µs characteristic, respectively. CMOS
switch IC44D, controlled by the front-panel PRE-EMPHASIS switch, S6,
turns the pre-emphasis on and off.
FILTER OVERSHOOT COMPENSATOR
Sources of
Overshoot
All low-pass filters exhibit a certain amount of overshoot and ringing
when presented with complex input waveforms. Generally, the sharper
the cutoff, the more pronounced the effect. Overshoots result from the
elimination of higher-order input signal components which, prior to
filtering, helped define the signal peak amplitude. Even a fully phase-