Rider BEN CRISSES Operating instructions

REPAIRING
No.
224
by
BEN
CRISSES
and
DAVID
GNESSIN
a
RIDER
publication

$2.75
BEN
CRISSES
DAVID
GNESSIN
REPAIRING
PORTABLE
AND
CLOCK RADIOS
John
F.
Rider
Publisher,
Inc.,
116
West
14th
Street
New
York
11,
N.
Y.
-

Copyright
June
1958
by
JOHN
F.
RIDER
PUBLISHER,
INC.
All
rights
reserved.
This
book
or
any
parts
thereof
may
not
be
reproduced
in
any form
or
in
any
language
without
permission
of
the
publisher.
Library
of
Congress
Catalog
Card
Number
58-12136
Printed
in
the
Uhited
States
of
America

preface •
Because of
requirements
of
mobility, compactness, etc.,
the
de-
sign
of
portable
and
clock radios presents special
repair
problems
to
the
technicians,
many
of
them
different from
the
usual
radio
re-
pairs.
The
authors
have emphasized these problems
in
this book.
No
attempt
is
made
or
intended
to create the impression
that
port-
able
and
clock radios are separate
and
distinct from conventional
ac-dc types. Instead, the
authors
have assumed
that
the
reader
has
a knowledge of basic
radio
repair
and
theory,
and,
for this reason,
have concentrated
their
information
for use
in
the
servicing
of
port-
able
and
clock radios.
Theory
and
other
basic
information
have
been
kept
to a mini-
mum
so
that
practical
information
be emphasized.
The
organiza-
tion
of this book
is
such
that
it
enables
the
user to find all his
required
information
on
a subject
in
one
section,
with
cross-refer-
encing
not
required. Finally, the examples given are of
equipment
or
circuits
that
enjoy wide
popularity.
It
should be emphasized
that
the
units discussed are chosen because
their
high
reliability
has
made
them
so very
popular,
and
not
because they
are
unusually
susceptible to breakdown.
y

vi
PREFACE
The
book begins
with
an
introduction
to typical
portable
radio
circuits. Emphasis
is
placed
on
the
filament circuitry
and
how
the
major
problem
of
current
drain
is
handled.
It
then
deals
with
trans-
istor connections
and
circuits.
The
portable
radio
power supply
is
examined
at
length,
with
observations
on
battery testing
and
storage.
Three
chapters are devoted to repair, replacement,
and
align-
ment,
including
a
detailed
discussion of
probable
sites of mechanical
troubles, a full list of all
repair
equipment
needed, notes
on
new
parts
and
substitution
of old,
and
step-by-step analysis of replace-
ment
procedures
and
short-cuts.
The
text
also presents
an
ample
discussion of design
improvement
-
adjustments
that
go
beyond
mere
repairing
of
minor
troubles
that
result
in
long-term improve-
ment
of performance
or
extension of life
of
the
set.
A final
unique
chapter
is
devoted to clock radios.
It
discusses
the circuitry
and
switching
of
these receivers,
the
varieties
of
clock
movements
and
steps
in
their
disassembly
and
adjustment,
clues to
probable
defects
in
clock mechanisms,
and
tips
on
cleaning
and
lubricating.
The
authors
wish to
extend
their
appreciation
to
the
following
organizations for
making
available
much
of
the
data
and
material
required
for the book:
Admiral
Corp.; Crosley Div., Avco Mfg. Co.;
Electronic Industries Association (EIA) ; Electronic Technician;
National
Electronic Distributors Association
(NEDA);
Motorola,
Inc.;
RCA
Service Co.,
Inc.;
Raytheon
Mfg. Co.; Sessions Clock
Co.;
Telechron
Div. of G.E.; Zenith
Radio
and
Television
Corp.
The
authors
also express
their
appreciation
to Mr. I. Remer,
for his many suggestions;
and
last
but
not
least to
their
families,
who
showed such
wonderful
patience
and
understanding
while this book
was being written.
New York, N.
Y.
June
1958
Ben
Crisses
David
Gnessin

contents
•
1,
introduction
to
portable
and
clock
radios,
1
2.
power
supplies,
13
3.
mechanical
troubles
and
repairs,
36
4.
general
troubleshooting,
47
5.
repair
and
replacements,
68
6.
alignment,
90
7.
design
Improvement,
98
8.
clock
radios,
106
vii

1. introduction
to
portable
and
clock
radios
•
In
recent years there has been a steadily increasing
demand
for
radios
that
are
not
only light
enough
and
compact
enough
to carry
anywhere,
but
also capable of playing wherever they may be taken.
There
has been,
at
the same time, a growing
market
for radios
that
indicate the time, wake one
in
the morning,
and
lull
one
to sleep
at
night.
This
has resulted
in
radios
that
are more complicated,
both
electronically
and
mechanically,
than
those of former years.
They
pose
unusual
problems to the
men
who
repair
and
maintain
them.
No
longer can a service technician look
at
the
underside
of a
chassis
and
make
routine
repairs.
He
must
trace,
or
at
least
examine
more
closely, a complex circuit before he takes action.
Manufacturers
of
portable
and
clock radios have
taken
conven-
tional
superhet
circuits
and
modified
them
to satisfy
the
demands
of
the public. For
portable
radios, the circuits have been changed so
that
sets may play
on
battery only,
or
on
battery,
on
a-c
and
on
d-c
lines.
These
sets are commonly referred to
as
three-way portables.
In
addition, I-volt
miniature
tubes,
subminiature
tubes, transistors,
printed
components,
and
printed
wiring are used more
and
more

2 REPAIR.ING PORTABLE
AND
CLOCK
RADIOS
as
manufacturers
strive for smaller,
more
portable
radios.
Further-
more, there are three-way
portable
radios
that
can
operate
from
117-volt
or
230-volt ac-dc power sources
and
cover a frequency
range
greater
than
the
broadcast
band
(535-1605
kc).
Clock radios use conventional I17-volt
a-c
radio
circuits, to
which a clock mechanism has been added.
The
clock includes a
mechanical switch
that
can
turn
the
radio
on
or
off
at
preset times.
Clock-portables are also available,
in
which a conventional electrical
control clock
can
be plugged
into
the
portable
radio
when using
house current,
and
unplugged
when
the
portable
is
operated
on
batteries.
To
clarify
the
portable
radio
picture,
the
differences between
ac-dc radios
and
portable
radio
receivers will be shown
in
this chap-
ter
and
in
Chap.
2.
These
differences lie largely
in
the
filament
and
power supply circuits.
TYPICAL TUBE-TYPE PORTABLE
RADIO
CIRCUITS
The
great
problem
in portables
is
current
drain.
A special set
of tubes has been designed to overcome this difficulty.
Table
1-1
lists
the
tube
lineup
(in typical sets
A,
B, C,
and
D)
in
the
majority
of portables (except transistor types)
being
manufactured
today.
Table
1-1
Stage Set A Set B Set C Set D
R-F
or
I-F
IU4
IAH4
INS
IAH4
Converter
IRS
or
IV6 IA7 IV6
IL6
Det-Ampl
IU5
IAJ5
IH5
IAJ5
Output
3V4
none
used
or
IAG4
3Q5 (a transistor
3S4
is
used)
Rectifier 11723
none
used 117Z6
none
used
or
(battery only) (battery only)
selenium

INTRODUCTION TO PORTABLE
AND
CLOCK RADIO$ 3
LINE
CORD
SHOWN
IN
POSITION
FOR
BATTERY
OPERATION
VOL
CONT
a ON·OFF
SW
"B"
BATTERY
90V
TOP a BOTTOM
455
KC
Ill
I F
TOP
6
BOTTOM
455KC2!1>1F
"A'
BATTERY
7.
5 V
Fig. 1-1. Typical
tube
layout
of
miniature-tube
set.
1620
KC
osc.
Fig. 1-2. Typical
tube
layout
of
miniature/submlniature-tube
set.
CONVERTER
e-------
B A
IF,
AF,
DETECTOR
OUTPUT
e+......±l1!1l1----e----'-t+
111-
________________
____,
Fig. 1-3. Battery
supply
circuits;
note
the
filaments
In
parallel.

4 REPAIRING PORTABLE
AND
CLOCK RADIOS
Typical
tube
layouts. Figures
1-1
and
1-2
show the
tube
layout
and
tube-type
combinations
used
in
many
portable
radios.
Figure
1-1
shows a typical set
using
miniature
tubes,
and
Fig.
1-2
a typical
set using
subminiature
tubes
with
a
miniature
power
amplifier tube.
D-c filament supplies.
For
battery
economy, tubes used
in
port-
able
radio
receivers
are
of
the
filament type,
and
require
direct
current
for filament
operation.
Figure
1-3
shows
the
supply
circuits
of
a set
operating
on
battery
power.
The
required
direct
voltages
for
B+
and
filaments
are
supplied
in
parallel
because
the
total
drain
is small.
Figure
1-4
shows
the
filament
circuit
popular
in
three-way sets.
This
set uses a 9-volt
battery
for
the
filament circuits, for
portable
DETECTOR
RF
CONVERTER
IF
1.4V
Cl
C3
R4
AUDIO
OUTPUT
POWER
SWITCH
BATTER:
CDC
9V
=A BATTERY
R6
Fig. 1-4. The
filament
circuits
in
a
typical
three-way
portable
radio.
operation.
Note
that
the
series .connection
of
filaments places the
audio
output
stage
near
the positive
end
of
the
d-c source.
(The
diagram
shows the
power
switch
in
BATTERY
position.)
Placing
the
output
tube
at
the
positive
end
of
the d-c source provides a means
of
securing
the
required
d-c bias voltage for the
output
stage
in
the
following
manner:
The
audio
output
control
grid
is
returned
through
R4
to pins
I
and
5
of
the r-f stage.
This
point
is
I
.4
volts positive
with
respect

INTRODUCTION TO PORTABLE
AND
CLOCK RADIOS S
to
B-.
It
would
seem
that
the
audio
output
control
grid
has a
positive
d-c
bias,
but
of
course this
is
not
so.
The
filament voltage
across
the
audio
output
tube
is
2.8 volts,
with
the
center
tap
(con-
sidered the
cathode
point)
at
7 volts positive
with
respect to
B-.
Since
grid
bias
is
the
value
of
grid
voltage
with
reference
to
the
cathode, a bias
of
-5.6
volts
is
placed
on
the
grid
of
the
output
stage.
The
other
tubes
are
biased
in
a
similar
manner
(see typical
portable
radio
schematics
in
Chap.
2)
by
returning
the
control
grids
to different
points
in
a
network
formed by
the
tube
filaments.
Note
also
the
shunting
resistors
(RI,
R2, R3,
and
R5) across
the
filaments.
These
resistors
shunt
plate
and
screen
currents
around
the
filament
circuits; otherwise, the relatively heavy
plate
current
for
the
output
stage
would
probably
damage
the
other
filaments.
Thus,
R5
is
the
filament
shunt
resistor for
the
audio
output
stage, R3
performs
the
same function for
the
i-f stage,
R2
does
similar
work
for
the
con-
verter
stage, while
RI
serves as filament
shunt
resistor
for
both
de-
tector
and
r-f tubes
in
series.
The
three
capacitors shown
in
Fig.
1-4
(CJ,
C2,
and
CJ) serve
as
bypass capacitors,
preventing
the
signal
variations (intermediate-frequency
and/or
audio-frequency)
in
the
filament
return
lines from
upsetting
the
filament bias. Because
the
bypass capacitors have a lower reactance to
ground
(or
B-)
than
the
filament
path,
the signal bypasses
the
filaments
and
returns
through
the capacitors, leaving the filament voltages
(and
conse-
quently
the filament bias values) stable.
Figure
1-5
shows
another
type
of
popular
filament
arrangement.
To
use a 3-volt battery,
and
thereby reduce size,
the
filaments
are
switched
into
a series-parallel
arrangement
during
battery
operation.
In
ac-dc
operation,
the
filament
string
is
switched
into
a series ar-
rangement
as follows:
Ganged
switches SWJ
and
SW2,
operating
together,
are
placed
in
the
ac-dc position, as shown
in
the
diagram.
The
8.6-volt ac-dc
line
feeds the voltage
through
SW
1,
through
the
two series filaments
of
the
3S4
tube,
through
the
1U4,
through
R5,
through
the
1U5,
through
the 1R5
(and
R6
in
parallel),
and
out
through
SW2
to
the
ac-dc
return.
The
filaments themselves
require
about
7 volts which, to-
gether
with
the 1.6-volt
drop
across R5, makes
up
the
8.6-volt supply.
When
the ganged switch is placed
in
the
BATTERY
position,
the
8.6-volt ac-dc
line
is
opened
and
a series-parallel
battery
filament
arrangement
is
employed.
Note
that
the
2.8-volt filament
of
the
3S4
with
the 0.2-volt
drop
across R4
in
series takes
up
the
whole 3 volts
from
the
battery.
The
1U4 filament (1.4 volts)
and
the
1.6-volt

6
REPAIRING
PORTABLE
AND
CLOCK RADIOS
67l/2V
a++±J1li11I-
TO
354
GRID
--
r-
1
I
I
I
I
TO
AC/DC
8.6V
LINE
354
I
U4
L_
-GANGED
---
IU5
Fig.
1-5.
Series-parallel
switching
for
filament
circuits.
IR5
drop
across
R5
make
up
a second
parallel
3 volts from
the
battery.
The
2.8-volt filam~nt
requirement
of
the
IU5
and
IR5
tubes
in
series
is
met
by
the
3 volts from
the
battery. Resistors
RJ
and
R6
are
filament
shunt
resistors to
protect
the filaments from heavy
plate
current
surges. Resistors
RI
and
R2
provide
d-c
grid
bias, as covered
in
the
description of Fig.
1-4.
Figure
1-6
shows a
standard
ac-dc filament
circuit
with
limiting
resistors.
The
series
and
parallel
resistors
in
the
filament circuits
provide
the
different voltages
and
current
drains.
When
necessary,
they
must
be
replaced
with
identical units.
The
resistors can be
briefly described as follows:
RI
is a
current-limiting
resistor to pro-
tect
the
rectifier diode;
R2
drops
the
filament voltage to
the
low
voltage
required
(the
point
between
RI
and
R2
is
the
normal
take-
off for the
higher
positive voltage for the
plate
circuits) ;
R5
is a
series
current-limiting
resistor to
protect
the
filament
circuit
from
current
surges;
RJ
is a filament
shunt
resistor
that
protects
the
3S4
filament from
plate
current
surges; R4
is
really
in
shunt
with
the
other
three
tube
filaments,
acting
as
a filament
shunt
resistor to pro-
tect those filaments from
plate
current
surges.
B+
voltages.
B+
voltages for
portables
present
no
special prob-
lems
and
therefore
are
not
discussed
in
this
chapter.
Where
the

INTR.ODUC1ION
TO PORTABLE
AND
CLOCK R.ADIOS 7
plate
voltages are
supplied
by batteries, there
is
only
the
matter
of
switching the battery
in
or
out.
Where
the
plate
voltage
is
supplied
by rectification (as
in
three-way
portables),
the
problem
resolves
into
the
familiar ac-dc
B+
supply.
It
was stated
that
the
differences between ac-dc radios
and
port-
able
radio
receivers lie largely
in
the filament
and
power-supply
117V
AC.OR
DC
J
B+
Fig.
1-6.
354
IU4
IR5
IU5
R5
A
standard
ac-clc
filament
circuit.
circuits.
The
circuits described here illustrate this.
There
is
another
type
of
portable
radio
receiver
that
differs
in
much
more
than
fila-
ment
circuitry (it has
no
filaments
at
all) .
This
is
the
transistor
portable
radio.
TRANSISTOR
CIRCUITS
If
a service technician
is
to
repair
a defective transistor set,
he
must
understand
that
it
presents problems different from those en-
countered
in
tube
sets.
(As
yet, there
is
little
standardization
of
tran-
sistor
portable
radio
circuits.)
Cautions in initial testing.
Probably
all technicians have
at
one
time
or
another
checked a vacuum-tube
audio
amplifier by
touching
finger
or
screw
driver
to
the
center
contact of
an
audio
potentio-
meter
to get a buzz
in
the
familiar
"circuit
disturbance"
test.
The
high
input
impedance
of
the
audio
tube
picks
up
hum
and
noise
from
the
capacitance of a finger
or
screwdriver
and
amplifies
it
to
create the buzz showing
normal
operation.
Touch
the center-contact
of
an
audio
potentiometer
of
the
transistor
audio
amplifier
and
you'll
get
nothing
-even from a good stage.
(The
low
input
im-

8 R.EPAIR.ING POR.TABLE
AND
CLOCK R.ADIOS
pedance of the transistor amplifier ignores the
familiar
circuit dis-
turbance.)
Testing
transistors involves the use of special transistor testers.
Because the transistor's active life
is
considerably
greater
than
equi-
valent tubes, they are usually the last components to be suspected
in
troubleshooting.
Transistor
circuitry
is
generally
much
smaller
and
more
fragile
than
equivalent
tube
circuitry,
and
the
trend
is
toward
more
printed
or
packaged circuits.
In
such sets,
entire
circuits
are
replaced,
rather
than
individual
elements.
P-N-P
and
N-P-N
recognition.
In
many
cases,
the
technician
will
not
be advised
whether
he
is
dealing
with
p-n-p
or
n-p-n type
transistor
in
the
circuit. However,
it
is
important
to
know
what
type of transistor
is
being
used, so
that
proper
biasing voltages will
be
applied
during
testing.
By
noting
the
direction
of
the
arrowhead
in
the symbol (Figs.
1-7,
1-8,
and
1-9)
on
the
schematic,
the
transis-
tor
type can be
determined.
The
arrow points away from
the
cen-
ter
in
the n-p-n,
and
toward
the
center in
the
p-n-p type.
Typical
transistor connections.
Depending
upon
which termi-
nals are used for
input
circuits
and
which for
output
circuits, a
transistor can be connected
in
any one of
three
ways:
I.
Grounded-base connections.
2.
Grounded-emitter
connections.
3.
Grounded-collector connections.
Figures
1-7,
1-8,
and
1-9
show these
three
connections.
The
grounded-base connection (Fig.
1-7)
is
roughly
equivalent
to the
grounded-grid amplifier.
The
grounded-collector connection (Fig.
1-9)
is
roughly
equivalent
to the cathode follower.
In
both
of
these
connections,
there
is
no
phase inversion of signal passing
through
the
transistor amplifier.
The
grounded-emitter
connection (Fig.
1-8)
is
similar to the
normal
grounded
cathode
amplifier,
with
the
usual
phase inversion of signal.
The
following are
important
points
to
remember:
I.
The
polarities
on
all transistor elements
are
shown
in
the
figures.
2.
The
grounded-emitter
is
the
only
circuit
that
produces a
phase reversal between
output
and
input
signals.
3.
Compared
to tubes, transistor amplifiers have extremely low
input
impedances
and
rather
low
output
impedances,
the
exact values
depending
on
the
circuitry used.

INTRODUCTION
TO
PORTABLE
AND
CLOCK
RADIOS 9
INPUT
OUTPUT
INPUT
OUTPUT
B
-1,J
B
·,1J
-
E=
EMITTER -
(
A)
C=COLLECTOR (
B)
B=BASE
Fig. 1-7. Transistors with
grounded
base
connections:
(A)
n-p-n
type;
(I)
p-n-p
type.
N PN
PNP
B B
INPUT
OUTPUT
INPUT
OUTPUT
~-
-1,J
L.
·,1J
E
EMITTER
-C= COLLECTOR -
(
A)
B•BASE
(
B)
Fig. 1-8. Transistors
with
grounded
emitter
connections:
(A)
n•p-n
type;
(8) p•n•p
type.
NPN
PNP
B B
INPUT
OUTPUT
INPUT
OUTPUT
~-
·,1J
L.
-1,J
E= EMITTER
-C= COLLECTOR -
(A)
B•
BASE (
B)
Fig.
1-9.
Transistors with
grounded
collector connections:
(A)
n-p-n
type;
(B)
p-n-p
type.

10
REPAIR.ING
PORTABLE
AND CLOCK RADIOS
Circuit analysis
of
a typical transistor portable. Figure
1-10
is
a schematic
diagram
of a typical transistor
portable.
Note
that
n-p-n transistors are used.
This
means
that
all collector voltages
are positive
with
respect to the base
and
emitter. Also
note
that
grounded-emitter-type circuits are used
throughout.
(A simplified
n-p-n
grounded-emitter
circuit
was shown
in
Fig. I-SA.)
In
the
oscillator (Fig. 1-10), the collector bias
is
obtained
through
the
decoupling
network
composed of
R5
and
CJO,
and
through
T6.
The
base
is
biased
through
the
voltage
divider
circuit
composed of
R23
and
R24. Resistor R25
is
the stabilizing resistor, which
is
by-
passed by CJ2 to
prevent
oscillator degeneration.
The
oscillator
tank
is
tuned
by varying CJD.
The
oscillator
output
is
coupled
to
the
mixer
base
through
C4.
The
received signal
input
is
also
coupled
to
the
mixer
base
through
the
lower
winding
of
LI,
since
both
are
wound
on
the ferrite-core
antenna.
The
mixer
collector
is
biased
through
the
decoupling
network
of
R3
and
CJ. Resistors
RI
and
R2
are stabilizing resistors for
the
mixer
emitter.
Capacitor
C2
is
a bypass to
prevent
signal degenera-
tion.
The
output
is
developed across
the
primary
of T
1.
The
pri-
maries of the i-f transformers are
tuned
while
the
secondaries are
not.
This
is
done
to
match
the high collector
impedance
to
the
low
input
impedance
of the base of the following stage.
The
collector of the first i-f stage
is
biased
through
the
decoup-
ling network composed of
R9
and
C6.
The
base
is
biased
through
R6
and
C9.
Network
RB
and
C5
form the stabilizing
network
for
the
emitter. Network
R7
and
C19 form a feedback
network
to
neu-
tralize the
internal
capacitance of the transistor.
It
should
be
noted
that
it
may be necessary to change R17
and
C19 if a new transistor
is
installed.
The
second i-f stage
is
very
similar
to
the
first; hence
it
does
not
require
separate discussion.
The
signal
is
detected by a IN295 diode,
with
the
volume
con-
trol
as
its load.
An
ave voltage from
the
detector stage
is
applied
to
the
base
of
the
mixer
and
first i-f stages
through
a filter
network
com-
posed of
RIO,
C9,
and
R4.
The
signal to the driver base
is
coupled
through
C14;
the
base
is
biased
through
the
voltage divider composed
of
R17
and
RI
8.
Network
R16
and
C16 form the
emitter
stabilizing network. Col-
lector bias
is
applied
through
the
primary
of
T4,
bypassed by CJ7.
The
output
stage
is
a common-emitter
push-pull
amplifier.
Resistors
R20
and
R22
form
the
voltage divider
required
for biasing

LI
C4
.05
2N94
MIXER
R4
'"
2N94
1ST
IF
I
I
I
R23
2N94
2NDIF
~~
NOTES
2N35
DRIVER
RIB
...
'°"
~C15
i_
SO
MFO
-=-
6V
ALL
11:ESISTOflS
1/3
WATT,
CAfllON,!20"1,
UNLESS
OTHERWISE
SPECIFIED.
100
ALL
VOLTAGES
AIIE
DC
UNLESS
OTHERWISE
S,CCIFIEO
ALL
CONDENSERS
AIIE
IN
MICll:OFAll:AOS
UNlESS
OTHERWISE
SP'ECIFIED
DC.
YOLT.I.GES
SHOWN
HE
MEASURED
WITH
NO
SIGNAL
USING
A AC·OC
Ofl
VACUUM
TUBE
YOLTMETE•.
-½
DENOTES
CHASSIS
2N35
OUTPUT
~
~
~
H
~.~
SWITCH
ON
1
~
VOLUME
CONTll0l
+ ITI
~,v
-T
Z-8
IATTUUES
*••[a'D
I.F.
ITT
EMITTE•_j f J
IASE
COLLECTO•
osc
b]
0
,0
t©~
IASEJ
EMITTE•_J
COLLECTO•
TRANSISTORS
Fig.
1-10.
Circuit
of
a
typical
transistor
portable
radio.
Zenith R.adlo
and
Television Corp.

12
REPAIRING POR1ABLE AND
CLOCK
RADIOS
the bases. Resistor
R21
is
a stabilizing resistor,
and
Cl
8 cuts
out
the
high
frequencies.
Network
Cl5/Rl9
provides additional filtering
and
decoupling
action for the battery supply.
When
the earphone jack
is
inserted
in
fl,
the
output
stage
is
eliminated from the circuit.
TRENDS
IN
PORTABLE
AND
CLOCK
RADIOS
The
most consistent trend
in
portable
and
clock radios is toward
miniaturization.
Printed
wiring
and
printed
components are being
used more
and
more. Rectifier tubes are being replaced by selenium
rectifiers; loop-type antennas are being replaced by ferrite-core an-
tennas. Subminiature tubes are being replaced by transistors,
and
all
miniature
tubes may eventually be replaced by transistors, when
transistor costs decrease.
In
short, all portables will be completely
transistorized.
This
trend
toward miniaturization
and
the use of
transistors will require special servicing techniques; these will be dis-
cussed later.
The
technician can also expect to see new types of batteries
that
are smaller
and
more efficient
than
those now
in
use. Solar
and
atomic batteries have been developed; only their
high
cost precludes
their use
at
the present time.
Proper
servicing
and
testing techniques
will be developed
as
the frequency of usage
of
these newer items
is incrased.
Clock radio circuits, because of the power requirements
of
the
clock portion, are
not
changing much. However, the
trend
toward
miniaturization will result
in
smaller radios, which will increase
the complexity of servicing.
In
spite of all these changes, servicing techniques will still
fol-
low accepted good practice, providing even
broader
sources
of
rev-
enue to the practicing service technician.

2.
power
supplies •
PORTABLE
POWER
SUPPLIES
The
portability of its power supply distinguishes the portable
radio from the
other
types. Therefore, an understanding of the
power supply
is
a must for the technician. Among power supply
considerations, the filament
and
bias circuit characteristics were
discussed
in
Chap.
l.
As
previously noted, there are two types of portables: the three-
way ac-dc-battery type
and
the battery-only variety.
The
three-way
portables can be divided
into
those using a vacuum-tube rectifier
and
those using a selenium rectifier.
The
three-way usually has
filaments
in
series
or
series-parallel to facilitate switching. Battery
sets can be found
that
have filaments
in
either series
or
parallel.
Figure
2-1
shows a typical three-way power supply,
with
a sele-
nium
rectifier (DJ) supplying the power when batteries are
not
in
use. Note
that
two ganged switches are used. Switch
SWJ
selects
ac-dc
or
battery operation.
The
switch
is
ganged to connect the
three basic supply lines -filament,
B+,
and
ground
-to either the
battery supply
or
the plug-in primary power.
13

14
REPAIRING
PORTAIU
AND CLOCK
RADIOS
When
SWJ
is placed
in
BATI
position
it
connects
the
A
and
B
battery
and
ground
to
the
lines.
When
placed
in
Ac-nc position,
the
line
power
is
applied
as
follows:
The
lower
plug
conductor
serves
as
common
ground
for
the
power
supply
through
the
lowest switch
terminals
on
SW
1.
The
upper
plug
conductor
feeds rectifier
line
cur-
rent
through
selenium
rectifier
DJ,
current-limiting
resistor
RI
••
IOOA
•••
IU4
IL6
IU5 IU4
•
..~
12m
-~----,,11~_-
-i----•-+
--~
nr
\
--r;,·
O
i~
GROUND
_
~
A BATT
•8.$Y
(APPROX}
B
BATT•
IOOV {APPROX}
Fig.
2-1.
The
typical
power
supply
of
a
three-way
portable
radio
using
a
selenium
rectifier.
(which protects
DJ
from line surge currents, to a
point
between
RI
and
R2.
Here
the
rectifier line
current
divides
the
higher
voltage (100 volts)
going
through
B+
filter resistor
R3,
through
the
middle
switch terminals of
SWJ
to
the
B+
lines
of
the
set; while
the
other
line
at
the
junction
of
RI
and
R2
goes
through
the
large
voltage-dropping resistor
R2
(where
the
potential
drops
to
about
8.5 volts)
and
passes
through
the
top terminals
of
SWJ
to supply
the
filaments of
the
set.
The
filter capacitors are
meant
to remove
hum
from
the
lines.
Capacitor
CJ
is
a small .047-µf
unit
meant
to bypass
line
noise from
the
"hot"
lead
of
the
power plug. As a safety measure,
the
low
end
of CJ
is
returned
through
another
capacitor
in
series
to
ground.
Thus,
even if CJ
should
possibly short-circuit,
the
worst
that
might
happen
would
be a
loud
hum.
(If
CJ were
returned
directly to
ground,
sparks
would
fly
when
it
shorted.)
Capacitor
C2
is
an
electrolytic filter from
the
100-volt
point
to
ground
(also using
another
capacitor
in
series to
ground
as a safety
measure).
Capacitor
CJ
is
the
second electrolytic filter
that
keeps
hum
out
of
the
filament lines.
Capacitor
C4
is
the
safety series
capacitor
to
ground.
Switch
SW2
is
the
ON-OFF
switch.
When
turned
off,
it
opens
the
filament circuit, thus
opening
the
primary
ac-dc
line
or
filament
This manual suits for next models
1
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