Fisher 250 TX Service Manual

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PRICE $1.00

SERVICE MANUAL 250-TX

SERIAL NUMBERS
FROM 73396

The Fishen

25O-TXW

24RX CONSOLE CHASSIS

WORLD LEADER IN HIGH QUALITY STEREO

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THE FISHER 250 -TX

2

ST EQUIPMENT AND SERVICE TIPS

Line Voltage Autotransformer or Voltage Regulator

DC Vacuum Tube Voltohmmeter

Accurately Calibrated AC Vacuum Tube Voltmeter
Oscilloscope ((Flat to 100 kHz Minimum)
Low-Distortion Audio (Sine Wave) Generator
lntermodulation Distortion Analyzer

Harmonic Distortion Analyzer

2 - Load resistors, B-Ohms, 100 Watt (Minimum Rating)
AM/FM Signal Generator

The following equipment is required to completely test and align modern high-fidelity amplifiers, tuners, and receivers.

107 MHZ Sweep Generator

Multiplex Generator
(preferably with RF output)

455 kHz Sweep Generator

Ferrite Test Loop Stick

2 - Full Range Speakers for Listening Tests

Stereo Source - Turntable, Tape Recorder, etc.

Soldering Iron with Small Tip, Fully Insulated from AC Line

Suction Desoldering Tool

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 50Awatt irons reach
temperatures of 1,0000 F, while others will hardly melt
solder. Small-diameter tips should be used for single solder
connections, 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 element 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
capacitors, 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
direct-coupled to it). In the time it takes for a dropped
machine screw, washer, or screwdriver, to contact a pair of
socket terminals (or terminal and chassis), a transistor can be
ruined.

SOLID-STATE DEVICES: Integrated 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 DC voltages 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 IC 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 IC 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

C UTION: This precision highafidelity instrument should be serviced only by qualified
personnel, trained in the repair of transisto equipment and printed circuitry.

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 transnstor or diode. ohmmeter
checks do not provide conclusive data, and may even destroy
thejunctionls) 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 defective (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, always check the bias adjustment 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 a colored marking on the transistor. Be sure
to include this information when ordering replacement
transistors

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
prevent 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 Dow-Corning D03, 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 sized
wire, can cause significant power losses in the system. 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 110%. DC voltages are measured to ground
with a VTVM, with no signal input unless otherwise noted.
AC signal voltages are measured under the conditions
specified on the schematic.

ALIGNMENT PROCEDURES: DO NOT attempt realignment
unless the required test equipment is available, and the
alignment procedure is thoroughly understood,

BECAUSE ITS PRODUCTS ARE SUBJECT TO CONTINUOUS IMPROVEMENT, FISHER RADIO RESERVES THE RIGHT
TO MODIFY ANY DESIGN 0R SPECIFICATION WITHOUT NOTICE AND WITHOUT INCURRING ANY OBLIGATION.