11
Case Histories
11
of
Receiver Troubles and their Remedies
Sec.
1
PHILCO 1,,
1'B,
1'X,
1'RX-(Cod•
1%1,
12%)-(Ce,it'i)
Inoperatin
___
1)
wire
from
first
section
of
tuning
con-
(r-f
dead, f-f
denser
pulled too
tight
and
grounding
O.K.)
against
chassis
Inoperative
-------1)
(no
i-fj
high-voltage lead (
black
wire)
from
i-f
transformer
(first
facing
front
of
chassis) grounded.
This
does
not
effect
audio
2)
compensators in
top
of
i-f
can
grounded
to
can
Inoperative,
___
l)
Low !emsitivity
(possibly
short
wave
recep-
tion
comes
in
on
broad-
cast
band
setting)
check
contacts
on
band
switch.
These
contacts
may
shift
or
drop
out
complete-
ly.
This
m1:1.y
be
indicated
by
short
wave reception on
the
broadcast
band,
and
by
improper
tracking
of
the
oscil-
lator
check
for
intermittently
"open"
0.003-
mfd. condenser
No.
£0
Inoperative
at
__
l)
low
end
of
short-wave
2) check
for
intermittently
"open"
0.0007-
mfd. condenser connected
to
76
tube
band
Inoperative
_____
.:._1)
from
11
to
26 me.
Inoperative
be-
..
1)
, low 15
me
replace
condenser
from
lug
1 on
bottom
of
waveband
switch
to
ground
marked
with
orange
and
yellow
dots
with
one
of
the
"lug"
type
(value
700-mmfd.).
A
poor
connection
internally
in
the
pig-
tail
condenser stops oscillation
high
resistance
tuning
condenser
gang
rotor
contacts. Bond
rotor
to
tuning
condenser
frame
with
flexible
wire
pig-
tails
2)
tighten
screw
holding
the
bottom solder-
ing
lug
to
the
tuning
assembly
lnoperafr,e
at
___
1)
if
this
is due
to
failure
of 76
oscillator
to
function
at
the
high
frequencies,
first
try
a
new
76
oscillator
tube.
If
this
does
not
help,
change
the
500-ohm oscillator
cathode
resistor
to
a 300-ohm
unit
hich-freq.
end
of
broad-
cast
band
Intermittent
---1)
reception
(Tolume
drops
"open"
250-ohm
resistor
in
plate
circuit
of second
i-f
stage.
This
resistor
is
con-
tained
within
the
case
of
the
small
black
bakelite
condenur
for
this
stace
and
is
located
adjacent
to
the
2nd
i-f socket
to
very
low
level,
no
plate
voltage
on i-f 78
tube,
and
,:,:
..
"shadow-
..
, z-raph"
indi-
cation
wid-
ens)
Intermittent
____
l)
check 0.01-mfd.
tubular
audio diode-
coupling condenser connected between
bottom
terminal
of
volume
control
and
the
junction
of
the
two
diode load
re-
sistors
reception
(shadow-
.,
-·"~aph
oper-
atH
normal-
ly-)
2)
replace volume
control
3)
faulty
78
tube
(even
though
it
checks
O.K.)
4) replace coupling condenser
No.
59
(0.05-
mfd.) connected
from
first
a-f
to
second
a-f
tubes
5) check
transformer
No.
46
Intermittent
__
1)
reception,
check compensator condensers
No.
30,
39, 45 located
in
first, second
and
third
i-f
transformer
cans.
Fault
may
be
due
to
loose
rivets
or
dirt
between
plates.
Tap
cans
to
locate
trouble,
and
replace
assembly
Low 1en1itivity
(Cont'd)
154
PHILCO
1,,
1'B,
1,x,
URX-(Codes
121,
1%%)-(Cont'li)
Bass
distortion
-1)
(
treble
harsh)
(77
first
det.
and
two
i-f
check
for
reduction
in
value
of
10,000-
ohm
resistor
No.
83 connected
in
network
between noise
potentiometer
and
No.
91
15,000-ohm
resistor.
Replace
with
20-
w~tt
wire-wound
resistor.
Realign
re-
ceiver
78
screen
voltages
above
normal
after
set
warms
up)
Distortion
at
______
l)
replace 0.05-mfd. first
to
second
stare
low
volume
audio coupling condenser
No.
51
2)
check 10-mfd. electrolytic condenser No.
7S connected
from
high
voltage center-
tap
to
chassis
Distortion
at
___
l)
hich
volume,
Weak
"tinny-"
sirnal
check
for
breakdown
between sections of
condenser block
No.
62
(capacities
2-2-
mfd.,
2-0.05-mfd.,
1-1-mfd.)
Distortion
and
....
1)
check
grid-bias
voltage
for
the
driver
Hum
and
output
stages.
Lack
of
normal
bias
on
either
of
these
stages
usually
results
in
the
control
grids
of
the
42 tubes glow-
ing
red
(a
low-emission 42
tube
will
produce
the
same
result)
. Check
for
a
"leaky"
or
"shorted"
0.15-mfd.
grid
filter
?Y-pass con~ens~r
in
the
driver
stage
2)
1f
control-grid
bias
on 42
tubes
is low
or
absent,
check
secondary
return
of
push-pull
input
transformer
for
ground-
mg
to
the
case
or
the
laminations
of
the
transformer
internally.
Replace
trans-
former
3)
l.akage
of
the
paper
which
is
supposed
to
insulate
the
electrolytic
filter
con-
denser
from
the
chassis.
Either
one,
or
both, condensers
must
usually
be replac-
ed
4)
"leaky"
electrolytic condenser
No.
75. Re-
place
with
new
unit
Weak,
choked
----1)
reception
check
screen-grid
voltage
applied
to
77
first
a.f
tube
(should be
approximately
50
volts).
If
off,
replace
the
1-meg.
car-
bon
resistor
which
supplies
this
voltage
(with
tb
noise
sup-
pressor
tube
in
or
out
of
the
circuit)
How
to
tell
_______
1)
difference
b.-
tween
"fad-
ing"
and
"fre-
quency
drift"
Fa.dine
----------------1)
(
condensers
and
resistors
check
O.K.)
Fadinc-
--------1)
in all
cases
of
"apparent
fading"
of
sig-
nal,
shift
1·eceiver
tuning
5
or
10 kc plus
-then
minus.
If
this
restores
the
sig-
n~!,
the
trouble
is
not
due
to
fading
but
to
frequency
drift
which
has
occur-
red in some
part
of
the
receiver
( com-
mon causes
and
remedies
for
frequency
drift
in
various
models
of
receivers
list-
ed
further
on)
intermittent
open-circuit
in
the
third
i-f
transformer.
Replace
with
new
unit
if
defective
bakelite which holds
stator
of
oscillator
tuning
condenser
expands
or
contracts,
allowing set-screws
and
rivets
to
loosen.
This
causes
stator
to
shift
its
position
slightly
thereby
changing
the
oscillator
frequency. Remove
gang
condenser
and
tii-hten all
set-screws
and
rivets.
Realign
receiver
Intermittent
__
l)
secondary
of
first
r-f
transformer
has
fadinr
hirli-rHi1tance
connection
(Cont'd}.
To Next Page
Loading...
