Fisher TX 300 Service Manual

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Serial bigsegsnnsng 1,0000

ervice Manua

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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.

EQUIPMENT AND TOOLS NEEDED
The following are needed to completely test and align this high-fideiity instrument.

Test Instruments

Vacuum~Tube Voltohmmeter DC VTVM

Audio (AC) Vacuum-Tu be Voltmeter (AC VTVM)
Oscilloscope (Flat to 100 kc minimum)

Audio (Sine-wave) Generator

lntermodulation Analyzer

Miscellaneous

Adjustable-Line-Voltage Transformer or
line-voltage regulator

Load Resistors (2) -8-ohrn, 50-watt (or higher)

Stereo source (Turntable with stereo cartridge
or Tape Deck)

Speakers (2) Full-range, for listening tests

Soldering iron (with small-diameter tip).
Fully insulated from power line.

_ PRECAUTIONS -

Many of the items below are included just as a re»
minder-they are normal procedures for experienced
technicians. Shortcuts can be taken but often they
cause additional damage~to transistors, circuit com-
ponents or the printed~circuit board.

SolderingiA well-tinned. hot. clean soldering iron tip
will make it easier to solder without damage to the
printedrcircult board or the many many circuit com-
ponents mounted on it. It is not the wattage of the
ivon that countsiit is the heat available at the tip.
Low~wattage soldering irons will often take too long to
heat a connection-pigtail leads will get too not and
damage the part. Too much heat, applied too long, will
damage the printed-circuit board. Some 50»watt irons
reach temperatures of 1,000" Fiothers will hardly
melt solder. Small-diameter tips should be used for
single solder connections-larger pyramid and chisel
tips are needed for larger areas.

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

0 Special desoldering tiplets are made for unsolder~
ing multiple-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.

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

TransistorsfiNever attempt to do any work on the
transistor amplifiers without first disconnecting the
AC-power linecord-wait until the power supply filter-
capacitors have discharged.

0 Guard against shorts-it takes only an instant for
a baseeto-collector short to destroy that transistor and
possibly 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 transrstor
can be ruined]

0 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, if 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 in-
formation, when ordering replacement transistors.

o If one output transistor burns out (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 insert-
ing a new transistor. All output transistors in one
channel will be destroyed if the base~biasing circuit
is open on the emitter end.

a When mounting a replacement power transistor be
sure the bottom of the flange, the mica insulator and
the surface of the heat sink are free of foreign matter.
Dust and grit can prevent perfect contact. This re-
duces heat transfer to the heat sink. Metallic particles
can puncture the insulator and cause shorts-ruining
the transistor.

- Silicone grease must be used between the transist»
or and the mica insulator and between the mica and
the heat sink for best heat conduction. Heat is the
greatest enemy of electronic equipment. It can shorten
the life of transistors, capacitors and resistors. (Use
DoweCorning D03 or C20194 or equivalent compounds
made for power transistor heat conduction.)

0 Use care when making connections to speakers and
output terminals. Any frayed wire ends can cause
shorts that may burn out the output transistors-they
are direct-coupled to the speakers. There is no output
transformer- nothing to limit current through the trane
sistors except the fuses. To reduce the possibility of
shorts at the speakers, lugs should be used on the
exposed ends-at least the ends of the stranded wires
should be tinned to prevent frayed who ends. The
current in the speakers and output circmtry is quite
high. Any poor contact or small-size wire, can cause
power losses in the speaker system. Use 14 or 16 AWG
for long runs of speaker-connecting wiring.

DC-Voltage MeasurementsiThese basic tests of the
transistor circuitry are made without the signal gener-
ator. Without any signal input measure the circut volt-
agesgas indicated on the schematic. The voltage
difference between the base and the emitter should
be in the rnillivolt range-a sensitive DC meter is
needed for these readings. A Iowwoltage range of 1 volt,
full scale-or lower-is needed.

Audio-Voltage (gain) MeasurementsiThe schematic
and printed-Circuit board layout diagrams are used.
Input Signals are injected at the proper points -found
most quickly by using layout of the printed-circuit
board instead of the schematic. An AUDIO (AC) VTVM
connected to the test points should indicate voltages
close to those values shown in the boxes on the sche-
matic. Many of the signal levels in the input stages
are only a few millivolts-they can not be read on the
AC ranges supplied on most Vacuum-Tube AC/DC Volt»
ohmmeters (VTV_M_s). Even with a 1-volt range a signal
level of 100 millivolts (.1 volt) will be the first 1/10
of the meter scale. A reading of 1 millivclt (.001 volt)
will hardly even move the meter needle.