Fisher FM 90 B Service Manual

Page 2

CAUTION: This is a Fl SH ER precision high-fidelity instrument. 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 modern high-fidelity instruments
such as amplifiers. tuners and receivers.

Test Instruments
Vacuum»Tube Voltohmmeter DC VTVM
Audio (AC) Vacuum-Tube Voltmeter (AC VTVM)
Oscilloscope (Flat to 100 kc minimum)
Audio (Sine-wave) Generator
lntermodulation Analyzer
Sweep (FM) Generator (88 to 108 mc)
Marker Generator

Multiplex Generator (preferably with RF output-

FISHER Model 300 or equal).

- 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 comv
ponents or the printed-circuit board.

Soldering-A well-tinned, hot, clean soldering iron tip
WlII make it easier to solder without damage to the
printed-circuit board or the many many circuit com-
ponents mounted on it. It is not the wattage of the
iron that counts-it 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 hot and
damage the part. Too much heat, applied too long, will
damage the printed- c-ircuit board. Some 50- watt Irons
reach temperatures of 1,000 F-others will hardly
melt solder. Small-diameter tips should be used for
single solder connections-larger pyramid and chisel
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. (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.)

u Special desoldering tiplets are made for unsolder-
ing multiple terminal units like lF 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.

0 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.

Miscellaneous

Adjustable-Line-Voltage Transformer or
line»voltage regulator

Load Resistors (2)-8-ohm, 507watt (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.

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 a nothing to limit current through the tran»
sistors except the fuses. To reduce the possibility of
shorts at the speakers, lugs should be used on the
exposed endsfiat 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 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 Measurementszhese basic tests of the
transistor circuitry are made without the signal gener-
ator.Without any signal input measure the circuit volt-
ages-as indicated on the schematic. The voltage
difference between the base and the emitter should
be in the millivolt range-a sensitive DC meter is
needed for these readIngs. A low-voltage range of 1 volt,
full scale-or lower-is needed.

Audio- Voltage (gain) Measurements-The 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- c-ircuit
board instead of the schematic An AUDIO (AC) VTVM
connected to the test points shouid 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 (VTVMs). Even With a l-volt range a signal
level of 100 millivolts (.lvolt) will be the first 1/10
of the meter scale. A reading of 1 millivolt (.001 volt)
WIII hardly even move the meter needle.

GND.

Wiring of Tape Jack Connector

CONNECT A TO HOT TERMINAL OF LEFT CHANNEL
HIGH-LEVEL INPUT; B TO HOT TERMINAL OF
RIGHT CHANNEL HIGH-LEVEL INPUT;

(3ND TO GROUND SIDE OF BOTH INPUTS

COVER

IstIa

© COPYRIGHT 1965 FISHER RADlO CORPORATION All Rights Reserved