Fisher 205 Service Manual

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THI5§MANUAL ALSO COVERS
THE'215 RECEIVER

WORLD LEADER IN HIGH QUALITY STEREO

SERVICE MANUAL

PRICE $1.00

205

SERIAL NUMBERS
BEGINNING 10001

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CONTENTS and SERVICE TIPS

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TEST EQUIPMENT 2
MECHANICAL DISASSEMBLY 3
DIAL STRINGING 3
HARMONIC DISTORTION TEST 3
SIGNAL FLOW 4
CHASSIS LAYOUT 5
TUNER ALIGNMENT 6

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AM RF 8
FM RF 9
AM-FM IF 10
MULTIPLEX 12
PREAMP AND SELECTOR SWITCH 14
CONTROL AMP 16

POWER AMP-POWER SUPPLY 18

CAUTION i

This precision high-fidelity instrument should be serviced only by qualified personnel,
trained in the repair of transistor equipment and printed circuitry.

Many of these items are included only as a reminder 7 they
are normal procedures for experienced technicians. Shortcuts
may be taken, but these often cause additional damage to
transistors, circuit components, or printed circuit boards.

SOLDERING: A well-tinned, hot, clean soldering iron tip
will make soldering easier, without causing damage to the
printed circuit board or the components mounted on it.
Regular use of a sponge cleaner will maintain a clean
soldering surface. The heat available at the tip, (not the watt-
age of the iron) is important. Some 50~watt irons reach tem-
peratures of 1,0000 F, while others will hardly melt solder.
Small-diameter tips should be used' for single solder con-
nections, pyramid and chisel tips for large areas.

Always disconnect the AC power cord from the line when
soldering. Turning the power switch OFF is not sufficient.
Power-line leakage paths, through the heating elements of the
iron, may destroy transistors.

PARTS REMOVAL: If a part is not being returned for in-
warranty factory replacement, it may be cut in half (with
diagonal cutting pliers) to make removal easier. Multiple
terminal parts, such as IF transformers, or electrolytic capaci-
tors, should be removed using special de-soldering tips made
especially for this purpose. Removing solder from terminals,
reduces the possibility of breaking the printed circuit board
when the part is removed.

ACCIDENTAL SHORTS: A clean working area, free of metal
particles, screws, etc., is an important preventive in avoiding
servicing problems Screws, removed from the chassis during
servicing, should be stored in a box until needed. While a set
is operating, it takes only an instant for a base-to-collector
short to destroy a transistor (and others directcoupled to it).
In the time it takes for a dropped screw. washer, or screw-
driver, to Contact a pair of terminals (or terminal and chassis),
a transistor can be ruined.

SOLID-STATE DEVICES: lntegrated Circuits contain the
equivalent of many circuit parts, including transistors, diodes,
resistors, and capacitors. The preferred troubleshooting pro-
cedure requires isolating the trouble to one stage using AC
signal tracing methods. Once the suspected stage is located
the DCvoItages at the input and output leads are measured to
give an accurate indication of the operating conditions of the
IC. DO NOT use an ohmmeter. to check continuity with the
IC mounted on the printed circuit board. Forward biasing the
internal junctions within the It) may burn out the transistors.
Do not replace a defective IC until all external resistors,
capacitors, and transformers are checked first, to prevent the
replacement K: from failing immediately due to a defect in
the connecting components. Solder and unsolder each lead
separately using a pliers or other heat sink on the lead to

prevent damage from excessive heat. Check that the leads are
connected to the correct locations on the printed circuit
board before turning the set on.

Whenever possible, a transistor tester should be used to de-
termine the condition of a transistor or diode. Ohmmeter
checks do not provide conclusive data, and many even de-
stroy the junctionls) within the device.

Never attempt to repair a transistor power amplifier module
until the power supply filter-capacitors are fully discharged.

If an output or driver transistor becomes detective (opens or
shorts), always check ALL direct-coupled transistors and
diodes in that channel. In addition, check the bias pot., and
other parts in the bias network, before installing replacement
transistors. All output and driver transistors in one channel
may be destroyed if the bias network is defective. After parts
replacement, check bias for specified idling current.

In some applications, replacement of transistors must be
made from the same beta group as the original type. The
beta group is indicated by acolored marking on the transistor.
Include this information when ordering replacements.

When mounting a replacement power transistor, be sure the
bottom of the flange, mica insulator, and the surface of the
heat sink, are free of foreign matter. Dust and grit will pre-
vent flat contact, reducing heat transfer to the heat sink.
Metallic particles can puncture the insulator, cause a short,
and destroy the transistor.

Silicone grease must be used between the transistor and the
mica insulator and between the mica and the heat sink for
best heat transfer. Use DowACorning DC-3, or an equivalent
compound made for power transistor heat conduction.

Use care when making connections to speakers and output
terminals. To reduce the possibility of shorts, lugs should be
used on the exposed ends, or stranded wire should be tinned
to prevent frayed wire ends.Current in the speakers and out-
put circuitry is quite high - poor contacts, or small wire,
can cause significant power losses. For wire lengths greater
than 30 feet, 16 AWG, or heavier, should be used.

VOLTAGE MEASUREMENTS: All voltages are measured
with the line voltage adjusted to 120 volts. All measured
voltages are t 20%. DC voltages are measured to chassis
with a VTVM. with no signal input unless otherwise noted.
AC signal voltages are measured under the conditions speci»
fied on the schematic.

ALIGNMENT PROCEDURES: DO NOTattempt realignment
unless the required test equipment is available, and the align-
ment procedure is thoroughly understood.

© COPYRIGHT 1973 FISHER RADIO D All Rights Reserved.