Fisher 125 Service Manual

Page 2

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CAUTION: This is a FISHER precision high-fidelity in-
strument. it should be serviced only by qualified personnel -
trained in the repair of transistor equipment and printed circuitry.

TEST EQUIPMENT REQUIRED

The following are needed to completely test and align modern highvfidelity instruments
such as amplifiers, tuners and receivers.

Vacuum-Tube Voltohmrneter
(lOO-mV DC scale)

Audio Vacuum-Tube Voltmeter
(lO-mV AC scale)

Oscilloscope (Flat to 100 kHz Minimum)
Audio (SineWave) Generator
lntermodulation Distortion Analyzer
Harmonic Distortion Analyzer

AM/FM Signal Generator

Multiplex Generator (preferably with RF
output - FISHER Model 300 or equal)

10.7-MHz Sweep Generator
455-kI-Iz Sweep Generator
Line Voltage Autotransformer
or Voltage Regulator
2 - Load Resistors, 4 or 8 Ohm, 50 Watt
2 _. Full Range Speakers for Listening Tests

Stereo Source -- Turntable or Tape Recorder
for Listening Tests

Soldering Iron with Small Tip
Fully Insulated from Power Line

Suction Desoldering Tool

- PRECAUTIONS -

Many of these items are included just as a reminder-they
are normal procedures for experienced technicians. Short-
cuts can be taken but often they cause additional damage
-to transistors, circuit components or the printed-circuit
board.

Soldering-A well-tinned, hot, clean soldering iron tip will
make it easier to solder without damage to the printed-
circuit board or the many circuit components mounted on
it. It is not the wattage of the iron that counts -- it is the
heat available at the tip. Some 50-watt irons reach tem-
peratures of 1,000 F-others will hardly melt solder.
Small-diameter tips should be used for single solder con-
nections --larger pyramid and chisel tips are needed for
larger areas.

I When removing defective resistors, capacitors, etc., the
leads should be wt as close to the body of the circuit com-
ponent as possible (If the part is not being returned for
in-warranty factory replacement it may be cut in half-
with diagonal-cutting pliers -to make removal easier.)

e Special de-soldering tips are made for unsoldering mul-
tiple-terminal units like IF transformers and electrolytic
capacitors. By unsoldering all terminals at the same time
the part can be removed with little chance of breaking the
printed-circuit board.

a Always disconnect the chassis from fire power line when
soldering. Turning the power switch OFF is not enough.
Power-line leakage paths, through the heating element,
can destroy transistors.

Transistors-Never attempt to do any work on the transis-
tor amplifiers without first disconnecting the AC linecord
and waiting until the power supply filter-capacitors have
discharged.

0 Guard against shorts~ it takes only an instant for a
base-to-collector short to destroy that transistor and pos-
sibly others direct-coupled to it. [In the time it takes for a
dropped machine screw, washer or even the screwdriver,
to glance off a pair of socket terminals (or between a
terminal and the chassis) a transistor can be ruined]

a DO NOT bias the base of any transistor to, or near,
the same voltage applied to its collector.

0 DO NOT use an ohmmeter for testing transistors. The
voltage applied through the test probes may be higher
than the base-emitter breakdown voltage of the transistor.

Output Stage and Driver- Replacements for output and

driver transistors, it necessary, must be made from the
same beta group as the original type. The beta group is
indicated by a colored dot on the mounting flange of the
transistor. Be sure to include this information, when order-
ing replacement transistors.

a If one output transistor burns Ont (open or shorts),
always remove ALL output transistors in that channel and
check the bias adjustment, the control and other parts in
the network with an ohmmeter before inserting a new tran
sistor. All output transistors in one channel will be de-
stroyed if the base-biasing circuit is open on the emitter end.

I When mounting a replacement power transistor he sure
the bottom of the flange, the mica insulator and the sur-
face of the heat sink are free of foreign matter. Dust and
grit can prevent perfect contact reducing heat transfer to
the heat sink. Metallic particles can puncture the insulator
and cause shorts - ruining the transistor.

0 Silicone grease must be used between the uallsistor and
the mica insulator and between the mica and the heat sink
for best heat conduction. (Use Dow-Corning DC-3 or
C20194 or equivalent compounds made for power transis-
tor heat conduction.)

I Use care when making connections to speakers and out
put terminals. Any frayed wire ends can cause shorts that
may burn out the output transistors - they are direct-
coupled to the speakers. To reduce the possibility of shorts
at the speakers, lugs should he used on the exposed ends or
at least the ends of the stranded wires should be tinned to
prevent frayed wire ends The current in the speakers and
output circuitry is quite high. Poor contacts or small size
wire can cause power losses in the speaker system. Use 14
or 16 AWG for long runs of speaker wiring.

Voltage Measurements-Voltage measurements are made
with the line voltage adjusted to 117 volts and all readings
are i 10%. All voltages are DC, measured with a VTVM
to ground, with no signal input unless otherwise noted.
l: indicates 1-kHz audio voltages, measured with
an audio AC VTVM to ground at various points from the
phone input to the power amplifier output.

Alignment Procedures-Replacement of transistors and
components in the front end, IF amplifier and multiplex
decoder will normally not require realignment of these
circuits. Realignment of these circuits, unless absolutely
necessary, is not recommended. Do not attempt a realign-
ment unless the required test equipment is available and
the alignment procedure is thoroughly understood.

BECAUSE US PRODUCTS ARE SUBJECT TO CONTINUOUS IMPROVEMENT. FISHER
RADIO CORPORATION RESERVES THE RIGHT TO MODIFY ANY DESIGN OR
SPECIFICATION WITHOUT NOTICE AND WITHOUT INCURRING ANY OBLIGATION.

© oopmcm r955 mum RADIO DURVDRANON in am. new