Technical information

O
RADIO
RETAILING'S
",
e)de
zENtTH
5s-t27,
5s-t50
After
a
few
minutes
of
operation
all
signals
but locals
fade
completely.
Re-
place
the
6A8G
cathode
resistor.
This
is mounted
directly
on the
tube
socket
near
the front
of the
chassis.
Replace
with 400
ohm
I
watt
resistor.
ZENITH
70.
Internrntcnt
reduction
in volume
-accompanied
by sharp
click
rnd
ragged
tone.
Replace
first
audio
P"late.
blocking
conderrsir
with
300
volt,
.J
mtd.
paper
tvne,
'ZENITH
70.
Fdding
or intermittent
reception.
Look
for
open
or shorted .25
mfd.
dual
ptate
by-pass
condenser to-
cated
on the first
and
second
r.f,
stages,
or .03
mfd.
by-pass
condenser
located
between
the
plate
of the first
a.f.
tube
and
the
primary
of the first
a.f. trans-
former.
If signals
vary
from
weak
to
normal
suspect
the
first. If
they are
always
weak
check
the second.
ZENITH
70.
Volume
dies down after
set
is in
operation for
a few
seconds, re-
quiring
readjustment
of
volume
control.
Replace
detector
(white)
plate
resistor.
Correspondents
report
other
troubtes
tr-aceable
to resistors,
some replacing
alt
10.
ZENITH
70.
Where
trouble
is ex-
perienced
in
cutting
down
the
volume
of
loud
locals
conncct
the
unused
..ofi"
position
lug
of the
r.f.
gain
control
re-
sistor
(not
the
antenni
resistor
sec-
!i9r.r)
t"
onc
of
the
2Z first
audio
gridr.
This
providcs
antcnna,
r.f.
end-
e.f.
volume
controt.
ZENITH
P.7I
Dead
.
'.
shorted
filter
section
on
either
side
of choke
coil.
Cut shorted
lead,
connect
8 mfd
electrolytic
with
negotiae
side
to
choke;
positive
lead
goes
to
connector
strip.
ZENTTH
9t.
92
Poor bass
response
and low
scnsi-
1_vity
. . - look
for
open
.5 nrfd (part
22-113)
in
the
cathode
of
the
second
detector.
zENtTH
9t,
92
Inoperative
until 24A
a.v.c.
is re-
nroved
open
a.v.c.
voltage
divides
connect
15,000
ohm
I watt
wire
wound
resistor
across
that
section
which
is be-
trveen
cathode
and
sdreen
grid
tops.
ZENITH
I31F
Noisl', weak
. .
.
clreck lroth
i.f.
tlaus-
forlrrers
rvith krrv
range
ohnrmeter.
If
lrrorc
tlutn l0
ohnrs,
repl:rcc
tr.arrsfor.urer.
ZENITH
.t2O
Motorboats
check
.0005
rnfd
plate
bypass
on
56
second
detector
for
open.
ZENITH
,t2O
Rapid
motorboating,
no
reception
.
open
500 mmf.
condenser
across
plate
and
cathode
of the
56
second
de-
tector.
ZENITH
666
Noise
when
car is in
motion
which
disappears
when
set heats
up
look
for broken
stranded
tead
between
the
mixer grid
cap
and
condenser
gang.
As
the
tubes
heat,
this
brings
the
connection
nrcre
Frnrly
together.
ZENITH
705
Broad,
insensitive
.
.
. remove
cathode
resistor
and
bypass
on
ZA7
mixer.
Ground
cathode,
then
repeak
I.F.
and
R.F.
stagcs.
zENtTH
7t5, 755,
756
Frying
noise
at low volume
levels
.
.
.
If this
condition
still exists
when
the
antenna is
removed
and the
an-
tenna
post grounded
replace
the
55 sec-
ond
detector
and
a.v.c. tube. In'sorne
cases
the same
tube
causes
a
peculiar
popping
noise
which
is often
attributed
to local
disturbances.
ZENITH
730
SERIES
Intermittent
or inoperative
shorted
I.F.
bypass
f rom primary
to
secondary
or
shorted
I.F.
windinc.
Also
to
improve
performance
,..oi"
fine
stranded
enameled
wires
of
LF.
coils
whe-re
they
pass
through
eyelets
at
base
of coil
form
and
runlxteinallv
through
spaghetti,
sharpening
I.F.
reso_
nance peak
due
to reduced
coupling,
realign.
LF.
frequency
is
.485
K.C.
-
ZENITH
750.
Set inoperativc,
i.f.
tube
grids
heat
red
hot
. . . Look
for
rhorted
i.f.
transformer.
This
is caused
vcry.
often-
by
the leads
of
one
winding
touching
those
of the
other
side
of thl
winding.
ZENITH
755
Fades
out
after
few
minutes
of
operation
defective
heavv
dutv
resistor
connected
from
filtei
con--
denser
to
push-pull
transformer.
ZENITH
755.
Oscillator
fails
to
gpcrate
below
850
kc.
. .
, Antcnna
coil
frequently
absorbs
moisture
"na
,froufl
be rewou-nd
.or-reptaced,
carefully
dopiJ
with.good
coil
dope
to avoid
repeiition
of
troublc.
ztNtTH
760.
Hum,
after
regular
vatues
of filter
condensers
have
been
installed
.
.
.
Mav
be
reduced
by
bypassing
the
.5 bypass
condenser
next
to
primary
of input
transformer
with
8 mike
-electrolyiic.
Distortion
.-
.
Bypass
center-tap
of
votume
control
to ground
with
.00015
mica
unit.
This
removes
the
r.f.
load
on
the
grid
of
the
6rst
a.f.
tube
and
al_
lows
greater
a.f.
amplification.
ZENITH
870
Dead
below
700 kc.
. check
for
shorted
or leaky
.01
mfd 400
volt
con-
denser
in
the
oscillator plate
circuit.
ZENITH
880.
Distortion
and
avc
blocking
Shorted
.(X)04
condenser, part
n-n|
in
6rst
r.f.
coil can.
zENtTH
t205.
tso3
Improper
action
of volume
control
.
suspect
6J5
audio
tube.
Poor
radiorgan
action
is
often
caused
by
defective
6F8G.
ZENITH
I5O2
Wont log
loose
Parker-Kalon
screws in gang
hub gear.
65
f
I
I
I
lz-
l@
I
I
ZnNffH
S0
Scricr.
To
simulate
I
r,v.c.
, .
In re-sctting thc
automatic
J
tuning
levers
first throw
the antenna
|
tsimming
controt to the
extreme
left
I
rnd leave
it
there. Tune
the
weakest
I
rtrtion of
the
selection
as sharpty
as
I
gossible
and
set the
volume
control
I
until the volume
is
barcly
audible,
about
I
one
or
two
db. Leave
the volumc
con-
I
trol atone from
then on. In
setting
each
I
of the
other
stations,
dctune
them
so
I
that the volunre
is the
same
as it
was on
I
the
weakest
station
and
lock
the tever
at
I
this
point.
In
this way,
as each tever
is
J
pressed
all stations
come
in with equat
J
volume,
the volume
equalizing
being
I
done
by
the lever
automatically
detun-
I
ing the corrcct
amount.
Do not
try
I
this on supers
of
a later
series
as
serious
I
distortion
will
result.
In
the
tuned r.f.
I
sets
the selectivity
is not
so
grcat
as to
I
cause
appreciable
distortion.
I
zrxtrx
so
I
I
Loud
hum,
all
parts
ancl
tubes
check
I
g.K .,
Connect
a.25
meg.re-
I
slstor
between
grids
of
second
audio
!
rtace.
I
I
ZENITH
sO,
52.
Loss
of
sensitivity
at
I
one
end
of dial.
Removc
cen
from
ion-
I
denser
gang,
remove
screlvs
holding
I
ttators rn
place
at
eech
end,
place
lock_
I
wesher
under
thesq
and tighten.
Whcu
I
rcrews looscn
re-aligning
only
unbal-
I
rnccs
set at
other
end
of
dial.
I
ZENITH
50,
60,
70. Persistent
hum.
I Py-p".r
detector
cethode
to
ground
with
I
l. or two mfd.
condenser,
supplementing
J
the one
already
in
thc
circuii
I
I
zENtTH
s2
J
Loud
hum
which
increases
as
set
I
operates
replace
36 mfd
wet
6lter
J
condenser
with
a wet
or
dry
unit.
I
I
ZENTTH
52
I ..
Xoi.y
at intervals
replace
J
bleeder
resistor.
apply
small
arnount
of
I
vaseline
to
volume
control.
I
I
zENtTH
s2
I
Hunr
replace
500,000
ohm
I
reslstor
across
secondary
of
pushpull
I
rnput
transtornler.
I
I
zrNrrx
5s-t27,
s5-r50
I
fraing
after
a
felr'
minutes
opera-
tion
replace
6A8G
cathode
re-
sistor.
This
is
a 400
ohm
unit.