Technical information

r
r
i
O
RADIO
RETAITING'5
,,,
"htti@
'hnlslilhtt
l
SPARTON 600
X{otorboating
Check
the bias
resistor
on the
183
output tubes.
The
resistance
should
be
1,250
ohms
but
changes to 700
ohms
often
a
few
min-
utes operation.
Replace
with
a wire
rvound
unit.
SPARTON
5OO
Strong hunr
when set
is
tuned to res-
onance rvith
a signal
replace .006
mfd
condenser
connected
fronr
one
side
of
porver
transforruer
to
ground
with
a
.5 or I
mfd
rrnit. It is
importarrt
that
this
condenser
is
of the non-inductive
type.
SPARTON 740
Fading
chcck
for le:Lky
cathode
bypass in
preselector
stirge. To
test
this,
disconncct volunre
corrtrol
leads
frour
thc
tcrnrinal
strip.
lf sct
oper-
atcs
irt
this
contlitiorr. r'cplace
cox-
rlettscr.
SPARTON
870
Noisy
. .
terminal board
on side of
power
unit for connection
of external
choke
breaking down.
Replace with
new terminal.
SPARTON 9OO
Hum
and
noisy reception
this
trouble
is nearly
always
caused by
poor
contact
between
the can of the electro-
l1'tic
condensers
and chassis. Some-
times
corrosion at
the bottom of
the
copper can is the reason for
poor
con-
tact. Often
paint
on
the
chassis
pre-
vents good
contact from
being
made.
Clean the
electrolytic can
and chassis.
SPARTON 930.
Fading
and
cutting
out. . . Replace
detector
plate
chokc
coil.
SPARTON
930
Oscillation
check
counections
to
r.f.
coils
for
corrosion.
SPARTON
930
I{olvl
at lorv
volume
.
rcolace
r'.f.
cht-rke
in
plate
cilcrrit
r,f
srcond
tletcctor.
sparton 930, 931,
5E9. When
plates
of
the
,185's
show low
or
no
potential
remove
screw at
side tab of
cylindrical
condenser
rt rear left
of amplifier
.
ln. Remove
re-
sponsible
condenser by unscrewing
with up-
ward
twist. Replace.
Job
can
be done in
l0 minutes
without removing
chassis from
cabinct.
SPARTON 930, 931. Replacing
483
power
tubes
with 45's
The usual
method
is to rewire the filaments in
series to reduce voltage from
5 to
2r.
This usually
results in hum. A
better
method
is
outlined
here:
Remove the
bias
resistor from
the center lead of
the
power
transformer. Transfer
one
of
the
filament
wires from
the
outer
tap to
which
it is normally
connected to
the
center-tap. Remove
the old
hum
balancing
potentiometer
and install
a
20
ohm
center-tapped
resistor
across
the
filament
circuit. Connect the opened
end
of the
bias resistor
to the midpoint.
58.
SPARTON
931,
301. H*m
is
gciqall!
coused
by rothing
more
serio*
thar
|oorly
matched
push-PuJl
187s.
SPARTON
930, 931. Reception
onty
betweerr
1500
and 850
kc.,
voltages,
con-
densers,
tubes ok.
Check
for
cold
soldered
joint
at lst r.f.
ptate
choke.
While the set is
open check value
of
15,000
ohm bleeder resistor. They
some-
times creep up
as high as 67,000 ohms
when heated
and if found in
this condi-
tion
should
be
replaced.
SPARTON
931. Distortion,
inability
to
handle
polver
. . . Look for
shorted
cathode
by-pass
condenser
in block
mounted in
base of
external
audio unit.
Quick
repair
can
be made
without rc.
moving
block
by cutting shortcd
con.
denser
lead and connecting
a
midget
l/10
mfd., 200 volt
condenser
across
the
cathode
resistor,
tucking it
in
the
bot-
tom of
the base. If
slight
attenuation
of
low frequencies
is caused
and customer
objects
mount
a 2 mld.,
200 volt non-
inductive
replacemcnt
externally,
SPARTON 931. Thin
tone and oscilla-
tion accompanied by high
plate
voltage
readings
and
low
speaker field
current.
15,000
ohm
bleeder
has increased in
value
due
to
age. Replace
with l0 to 25
watt
unit.
SPARTON AIRPLANE DIALS.
Microphonic
trouble , See that there
is clearance between diat and
cabinet. It
may also
be
advisabte
to
put
a coat of
clear lacquer
on wires
passing
through
oscillator coil so
that
these
will
not
vibrate.
SPARTON
MODELS
Dial
does
not track .
.
renrove
chassis
Irom
cabinet,
set
dial
pointer
to
correct
setting.
Pointer is
nterely
pressed
on shaft
and lvorks
loose from
vibration.
SPARTON WITH PRESELECTOR
Facling
.
. all tl-resc
nroclels
have
die cast
tuning
gang
on a steel shaft.
Iiading
was
tr:rcecl to
rotors shrinking
loose from shaft.
Drill
hole
throuqh
casting ancl insert setscrew into
shaft
or
clean encl of each
gang
section and
solcler
to
sha{t.
SPARTON
EQUASONNE
A.C.
MODELS.
Routine
trouble
tests
from
specialist's
notebook
.
Tem-
porarily
short
terminals I
and 2
with
tip of
screwdriver.
If
pronounced
click
is not
heard
from
speaker check
speaker
connections
and main filter
condenser
block.
If
power
unit is
ok
loosen
and
remove
terminal
screw
on .25 mfd.
con-
denser
shown in
diagram. If
set
con-
tinues
to operate
after removal
of screw
replace
it.
On some models
in which
original
condenser is
still in
place
the
following
test
may
be used
instead
of
above.
This
older
style
condenser
has
m-etal
top
with
screw
through
ccntcr.
It'Ionrentarily
short
top
of coidenser
to
side
of
amplifier.
If
spark
occurs
the
unit
_is
ok.
If
not, replace,
This
may
usually
be done
without
removing
ampti-
fier
from
cabinet
bv removing-bott'om
screw
going
through
terminal on side
of condenser
and
bending
terminal up
against
side of condenser,
then turning
condenser
counter-clockwise.
To
install
unit reverse
this
procedure.
Turn volume
control completqly oft.
If noise similar
to static
results check
detector
plate
choke before
suspecting
control.
If new choke
cannot be ob-
Front
of
Rodio Chqss;s
Nole.-
Aslerix
-
*
marks
localrbo
of
wrts
wh;ch
mosl
oft'en become defec
lrve
'
tained remove all wire from old choke
form and scramble-wind
the
form full
of No. 36 enamelled
wire. Solder ends
to terminals and
do
not use anything but
pure
rosin
flux.
Impregnate
choke with
commercial r.f. coil dope or something
similar.
If
set operates but volume is
tow
and decreases after
passing
half-way
point
on
control
trouble
is usually due
to open in 110-ohm
wire-wound resistor
connected between terminals 5 and
7.
By
placing
volume control full on
(if
set
does
not operate) and touching an-
tenna to coupling
pin
between
selector
can and r.f. amplifier can the set should
pick
up
some signals. If so, trouble is
indicated in
selector can,
either a defect
or bad alignment.
The
pin
referred to
is
directly
under the
back of the drum dial.
Without the selector stations
will
naturally be
jumbled
together.
After above
routine
tests
and with
volume
control
full
on
pull
extra
485
tube used on selector
can as amplifier
almost out of
socket
and
work
it up
and down so that
it makes and breaks
contact
with
'
the socket terminals.
There should
be loud clicks. If not,
chcck.for
open
plate
choke or condenser
beneath
socket.
Location of tubcs in amplifier
unit for
best reception and freedom from
oscil-
lation at 900
kc.
. .
.
After
testing tubes
place
best one in lst r.f,
position,
next in
Znd r.1., third in
detector socket then, in
order,
3rd r.f., 4th
r.f. and
finally
the
poorest
tube in Sth r.f.
When
aligning
select
some frequency
above
1,000
kc.,
about
1,300
is
prcferablc.
With oscilla-
tor connected to antenna
posts
adjust
trimmers
as follows No. 3 first,
then No.
2 and
finally No. l.
Check back second
time.
Now connect antenrta
to
selector
unit and tune in some station
around
1,300 and adjust antenna trimmer. If
antennas are changed,
re-adjust
this
trimmer each time.
STERLING
STERLING MODEL F
Hunt insert 1.500
olrnr
1(J rv:Ltt
rcsistor in
series with 6elcl
to
ircrcase
bias on
,15
output.