Fisher FM-190, Service Manual

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C AUT ION:
This is a F ISH ER 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 comp l ete l y test and align modern high-fidelity instruments
such as amplif i ers, tuners and receivers.
Miscellaneous
Adjustable-line - Voltage Transformer or
l i ne-voltage regulator
Load Resistors (2) -8 -o hm, 50 - watt (or higher)
Stereo source (Turntable with stereo cartridge
or Tape Deck)
Speakers (2) Ful l -range, for listening tests
Soldering i ron (with small-diameter tip).
Fully insulated fr o m power l i n e .
Test Instruments
Vacuum-Tube Voltohmmeter DC VTVM
Audio (AC) Vacuum - Tube Voltmeter (AC VTVM)
Oscilloscope (Flat to lOOkc min i mum )
Audio (S i ne - wave) Generator
Intermodu lat ion Ana Iyzer
Sweep (FM) Generator (88 to 108 mc)
Marker Generator
Multiplex Generato r (preferably with RF output -
FISHER Model 300 or equal).
Many of the items below are included just as a re-
minder - they are normal procedures , for exper i enced
technic i ans. Shor t cuts can be taken but often they
cause addit i onal damage - to transistors, circuit com-
ponents or the printed - circuit board .
Soldering-A well-tinned, hot, clean soldering iron tip
will make i t eas i er to solder without damage to the
printed-circuit board or the many many c i rcu i t com-
ponents mounted on i t . It is not the wattage of the
iron that counts - i t is the heat available at the tip.
Low-wattage soldering irpns will often take too long to
heat a connect
i o n - pigtail leads wil l get too hot and
damage the part . Too much heat, appl i ed too long, will
damage the printed-c i rcu i t 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 connect i ons -larger pyramid and chisel
t i ps are needed fo r l arger areas.' '
• When removing defective resistors, capacitors, etc.,
the leads should be cut as c l o se to the body of the
c i rcu i t component as poss i ble . (If the part is not being
returned for in-warranty factory replacement it maybe
cut in half - with diagona l- cutting p l iers - to make
remova l easier . )
• Spec i al de - so l dering tip l ets are made for unsolder-
ing m u ltip l e-terminal units like IF transformers and
electrolytic capacitors. By unsoldering all terminals at
the same time the part can be, removed with li ttle
chance of break i ng the printed - circuit board.
• Always d i sconnect the chass i s from the power line
when soldering. Turning the p o wer s witch OFF is not
enough. Power-line leakage paths, through the heating
e l ement, can destroy transistor s .
Transistors - Never attempt to do any work on the
transistor amplif i ers without f i rst d i sconnecting the
- ~ AC - power linecord - wa i t unt il the power supply filter -
capac i tors have discharged.
• Guard against shorts - i t takes only an instant for
a base - to - collect o r short to destr o y that transistor and
poss i bly others direct-co u pled to i t . [In the time it
takes for a dr o pped machine screw, washer or even
the screwdriverrto glance off a pair of socket term i nals
(or betwee n a terminal and the chass i s ) a transistor
can be rll i n e d .]
• DO NOT bias the base of any transistor to, or near,
the sam e voltage app l ied to i t s c o l lec tor .
• DO NOT use an ohmmeter for testi ng transistors.
The vo l tage app l ied thr o ugh the test probes may be
higher than the base - emitter breakdown vo l tage of the
transistor.
Output Stage and Driver-Rep l a c ements for output
and driv e r transistors, i f ne c essary , must be made from
the same beta gro u p as the orig i nal type . The beta
gro u p is indicated by a co l ored dot on the mounting
flange of t h e trans i st o r. Be sure t o include this in-
format i on, when ordering rep l acement transistors.
• 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.
•
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 prelent perfect contact. This re-
duces heat transfer to the heat sink. Metallic part i cles
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
grea , test enemy of electronic equipment . It can shorten
the life of transistors, capacitors and resistors. (Use
Dow - Corning DC-3 or C20194 or equivalent compounds
made for power transistor heat conduction.)
• Use care when making connections to speakers and
output terminals. Any frayed w i re ends can cause
shorts that may burn out the output transistors - they
are d i rect-coupled to the speakers . There is no output
transformer - nothing to lim i t current through the tran-
sistors except the fuses. To reduce the possibility of
shorts at the speakers, lugs should be used on the
exposed ends
- at lea,st the ends of the stranded wires
shou l d 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, : , ~~n cause
power losses in the speaker system. Use 14 oj l 16 AWG
for long runs of speaker-connecting wiring.
DC-Voltage Measurements - These basic tests of the
trans i stor circuitry are made without the signal gener-
ator. W i tho u t any signal i nput measure the circuit volt-
ages - as i ndicated on the schematic. The voltage
difference between the base and the emitter should
be in the m i llivolt range - a sensitive DC meter is
needed for these read i ngs. A low-voltage range of 1 volt,
full sca l e - or lower - is needed.
Audio-Voltage (gain) Measurements - The schematic
a n d pr i nted-circuit board layout diagrams are used.
Input signals are injected at the proper points - found
most quickly by us i ng layout of the printed - circuit
board instead of the schematic. An AUDIO (AC) VTVM
connected to the test po i nts should indicate voltages
close t o those values shown in the boxes on the sche -
matic . Many ot the signa l levels in the input stages
are on l y a few millivolts-they can not be read on the
AC ra n ges supp l ied on most Vacuum - Tube AC/DC Volt·
ohm l !'eters (VTVMs). Even with a l - volt range a signal
l evel o f 100 m i l l ivolts ( . 1 volt) w il l be the first 1/10
of th e meter sca l e. A reading of 1 millivolt (.001 volt)
wil l hard l y even move the meter needle. '