13. PCB Layouts and Schematics
Note: The printed circuit board layouts and schematics in this section are
intended for use only by qualified personnel.
Caution: These servicing instructions are for use only by qualified personnel. To
avoid electric shock do not perform any servicing other than that contained in the
operating instructions portion of this manual unless you are qualified to do so.
Refer all servicing to qualified service personnel.
These schematics and layouts are provided for reference use only, for use by qualified service
personnel and for use in answering certain technical questions that are beyond the scope and
intent of this manual. The schematics and layouts in this manual were accurate at the time
that this manual was written. Your actual product may contain changes not shown on these
drawings. The inclusion of this material in this manual in no way obligates Symetrix to
provide updated information or to inform users of any changes, past or pending.
A complete service manual is available from the factory for a nominal charge. Please contact
the Symetrix Service Department at the address listed in Section 11 of this manual.
13.1 Troubleshooting Hints
If you are attempting repair of your 425, the following tips and hints may be useful, especially
if you are not familiar with operational amplifiers.
The first thing that you should check are the power supplies. Their nominal voltage should be
within 5% of the noted value on the schematic. The IC regulators used are current limited and
short-circuit protected; their output voltage drops under excessive load (like something
downstream that draws excessive current).
In audio amplifiers that utilize operational amplifiers as their active gain element, the two
feedback resistors establish the working gain of the circuit. In Symetrix equipment, and other
equipment using bipolar (separate plus and minus) power supplies, the nominal DC output
voltage of each stage should be at or very near zero volts.
If an opamp's output is at or near one of the power supply rails, this usually means that the
opamp has failed. The exception to this rule is when the circuit configuration uses the opamp
as a DC amplifier or as a comparator.
When used as a DC amplifier, the output should follow the input signal (modified by the
circuit's gain equation).
When used as a comparator, the opamp's output swings between the two supply rails and
there should be no intermediate output state.
Units using substantial amounts of digital circuitry are probably best serviced at the factory.
13.2 Additional Reading
You can find additional information on the design and applications of operational amplifiers in
the following text:
"Audio IC Op-Amp Applications," Walter G. Jung, Copyright 1987, Howard W. Sams &
Company, Indianapolis, IN.
13-1
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