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.
'