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C AUT ION:
strument.
trained
The following are needed to completely test and align modern high-fidelity instruments
Test
Instruments
Vacuum-Tube Voltohmmeter DC VTVM
Audio (AC) Vacuum-Tube Voltmeter (AC VTVM)
Oscilloscope (Flat to lOOkc
Audio (Sine-wave) Generator
Intermodu lat ion Ana Iyzer
Sweep (FM) Generator (88 to 108 mc)
Marker Generator
Multiplex
Generator
FISHER Model 300 or equal).
Many of the items below are included just as a re-
minder-
they are normal
technicians.
Shortcuts
can be taken but often they
cause additional
damage - to transistors, circuit com-
ponents or the printed-circuit
Soldering-A
well-tinned,
will make it easier to solder without damage to the
printed-circuit
board or the many many circuit
ponents mounted on
it.
iron that counts - it is the heat available at the tip.
Low-wattage soldering irpns will often take too long to
heat a
connection -
pigtail leads
damage the
part.
Too much heat, applied too long, will
damage the printed-circuit
reach temperatures
of
melt solder. Small·diameter
single solder connections -larger
tips are needed
for
larger areas.'
• When removing defective resistors, capacitors, etc.,
the leads should be cut as close to the body of the
circuit component as
possible.
returned for in-warranty factory replacement it maybe
cut in half - with diagonal-cutting
removal
easier.)
• Special de-soldering tiplets are made for unsolder-
ing multiple-terminal
units like IF transformers
electrolytic
capacitors. By unsoldering all terminals at
the same time the part can be, removed with
chance of breaking the printed-circuit
• Always disconnect the chassis from the 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
transistor
amplifiers
without
-
~
AC-power linecord - wait
capacitors have discharged.
• Guard against shorts - it takes only an instant for
a base-to-collector
short to destroy that transistor and
possibly others direct-coupled
takes for a dropped machine screw, washer or even
the screwdriverrto
glance off a pair of socket terminals
(or
between
a terminal
can be
rllined.]
• DO NOT bias the base of any transistor to, or near,
the
same
voltage
appl ied
• DO NOT use an ohmmeter for testi ng 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
and driver transistors, if
necessary,
the same beta group as the original
group is indicated
by a colored dot on the mounting
flange of the transistor.
formation, when ordering replacement transistors.
This is a F ISH ER precision high-fidelity in-
It should be serviced only by qualified personnel -
in the
repair of transistor
such as amplifiers, tuners and receivers.
minimum)
(preferably with RF
output-
procedures,
for experienced
board.
hot, clean soldering iron tip
com-
It is not the wattage of the
will
get too hot and
board. Some 50-watt irons
1,000°
F - others will
hardly
tips should be used for
pyramid and chisel
'
(If the part is not being
pliers
-
to make
and
little
board.
to do any work on the
first disconnecting
the
until
the power supply
filter-
to
it.
[In the time it
and the
chassis)
a transistor
to
its
collector.
for output
must be made from
type.
The beta
Be sure
to
include this in-
equipment
and printed
Miscellaneous
Adjustable-line-Voltage
line-voltage regulator
Load Resistors (2) -8-ohm,
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.
• If one output transistor
always remove all output transistors
and check the bias adjustment, the control and other
parts in the network with an ohmmeter before insert-
ing a new transistor.
All output
channel will be destroyed if the base-biasing circuit
is open on the emitter end.
• When mounting a' replacement power transistor
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 prelent
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.
grea,test enemy of electronic
the life of transistors,
capacitors and resistors. (Use
Dow-Corning DC-3 or C20194 or equivalent compounds
made for power transistor
• 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
transformer - nothing to limit current through the tran-
sistors except the fuses. To reduce the possibility
shorts at the speakers, lugs should be used on the
exposed ends
-
at lea,st the ends of the stranded wires
should be tinned
to prevent frayed wire ends. The
current in the speakers and output
high. Any poor contact or small-size wire,:,~~n cause
power losses in the speaker system. Use 14 ojl16 AWG
for long runs of speaker-connecting
DC-Voltage
Measurements-These
transistor circuitry are made without the signal gener-
ator. Without any signal input measure the circuit
ages
-
as indicated
on the schematic.
difference
between the base and the emitter
be in the
mi
llivolt
range
needed for these readings. A low-voltage range of 1 volt,
full scale - or lower - is needed.
Audio-Voltage
(gain)
Measurements-The
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
matic.
Many ot the
signal
are only a few millivolts-they
AC ranges supplied on most Vacuum-Tube AC/DC Volt·
ohml!'eters (VTVMs).
Even with a l-volt range a signal
level of 100 millivolts
(.1 volt) will be the first 1/10
of
the
meter scale. A reading of 1 millivolt
will
hardly even move the meter needle.
circuitry.
Transformer or
50-watt (or higher)
burns out (open or shorts),
in that channel
transistors
in one
be
perfect contact. This re-
Heat is the
equipment.
It can shorten
heat conduction.)
speakers.
There is no output
of
circuitry:
is quite
wiring.
basic tests of the
volt-
The voltage
should
-
a sensitive DC meter is
schematic
sche-
levels in the input stages
can not be read on the
(.001 volt)
'
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